Johns Hopkins APL

TECH N ICAL DIGEST July-September 1980, Vol. 1, No. 3

The Magsat issue Editorial Board Walter G. Berl, Chairman Frederick S. Billig Billy D. Dobbins Morton H. Friedman Robert W. Hart Samuel Koslov Vincent L. Pisacane Gary L. Smith Robert J. Thompson, Jr.

Ex Officio Edward L. Cochran M. B. Gilbert Vernon M. Root

Editorial Staff

M. B. Gilbert, Managing Editor Jerome W. Howe, Associate Editor Stephen G. Smith, Art Director Daryl L. George, Staff Artist David W. Sussman, Staff Photographer

The Johns Hopkins APL Technical Di­ gest (ISSN 0001-2211), established in 1961 as the APL Technical Digest, is published quarterly under the auspices of The Johns Hopkins University Ap­ plied Physics Laboratory (JHU/ APL), Johns Hopkins Road, Laurel, Md. 208lO. The objective of the publication is to provide a summary of unclassified individual programs under way at JHU/ APL. Requests for free individual copies, free subscriptions, or permission to reprint the text should be submitted to the Managing Editor. Postmaster: Send address changes to that address. Second-class postage paid at Laurel, Md. © 1980 by The Johns Hopkins Univer­ sity Applied Physics Laboratory. fohns Hopkins APL TECHNICAL DIGEST

July-September 1980, Volume 1, Number 3

The Magsa[ issue

TECHNICAL ARTICLES 162 The Geomagnetic Field and Its Measurement: T. A . Potemra, F. F. Mobley, L. D. Eckard Introduction and Magnetic Field Satellite (Magsat) Glossary 171 Overview of the Magsat Program G. W. Ousley 175 Magsat Performance Highlights F. F. Mobley 179 The Magsat Power System W. E. Allen 183 The Magsat Telecommunications System A . L. Lew, B. C. Moore, J. R. Dozsa, R . K. Burek 188 The Magsat Attitude Control System K. J. Heffernan, G. H. Foun/ain, B. E. Tossman, F. F. Mobley 194 The Magsat Attitude Determination System G. H. Foun/ain, F. W. Schenkel, T. B. Coughlin, C. A. Wingate 201 The Magsat Magnetometer Boom System J. F. Smola, 205 The Magsat Scalar Magnetometer W. H. Farthing 210 The Magsat Precision Vector Magnetometer M. H. Acuna 214 Magsat Scientific Investigations R . A . Langel 228 Studies of Auroral Field-Aligned Currents with Magsat T. A. POlemra SPECIAL TOPIC 233 China - As Viewed by an Aerospace Engineer F. S. Billig DEP ARTMENTS 240 Publications, Presentations, APL Colloquia, The Authors

Front Cover: Conceptual painting by S. G. Smith of the Digest staff. The earth, showing the magnetic anomalies mea­ sured by Magsat and earlier satellite vehicles, is silhouetted against the frontispiece of Gilbert's seminal trea­ tise on geomagnetism (De Magnete, second edition, 1628). A line drawing of Magsat IS in the foreground. THOMAS A. POTEMRA, FREDERICK F. MOBLEY, and LEWIS D. ECKARD

THE GEOMAGNETIC FIELD AND ITS MEASUREMENT: INTRODUCTION AND MAGNETIC FIELD SATELLITE (MAGSAT) GLOSSARY

The earth's magnetic field, its measurement by conventional methods, and the specific objec­ tives and functions of the Magsat system to obtain precise absolute and directional values of the earth's magnetic field on a global scale are briefly described.

EARLY HISTORY The directional property of the earth's magnetic field has been appreciated by the Chinese for more than 4500 years. Records indicate I that in 2634 B.C. PKYSIOLOGIA NOVA the Chinese emperor Hoang-Ti was at war with a D£MAGNETL local prince named Tchi-Yeou and that they fought MAGNETICI SQVE CORPO' RIBVS ET MAGNO MAG~ETE a great battle in the plain of Tcho-Iuo. Tchi-Yeou tc\1ure Sox Iibris. comprehcn[us raised a dense fog that produced disorder in the Guiliclmo Gilbcrl~ colcef1rcnG , imperial army - a forerunner of the modern Modico Londincnri. . ill '1U1b"; (a ,,!U{( ad ~an( malrrinmfpcc/an!.",!,, · smokescreen . As a countermeasure, Hoang-Ti con­ mrs dar:.qumm!tsat (Xff'nmmf!s (~ac1!fslm(' structed a chariot on which stood the small figure a':f'lu1!ftlmr~ ~r4dal171lr d 'Y!t ( Il ~fu r . Omnia nuncdihgcntcr rccognlta lk ernen, of a man with his arm outstretched. This figure, dalius quam ank '" lucom

to'~man6 / 1 . U . D . always pointed to the south, allowing the emperor .:J & Mathernali. . Ad calcrm librilldjunciu; fS llndrxCapl ,. to locate the direction of his enemy's retreat. Tchi­ fum Rfrnm rl Vrrborum IOCllflrliftlmus ExCVSVS SEDIN I Yeou was captured and put to death. Typis G6tzianis Sumpfibus The first systematic and scientific study of the ../fufhorlS dnnoM.DC.XXVlII . earth's magnetic field was conducted by William Gilbert, physician (later promoted to electrician) to Queen Elizabeth, who published in 1600 his procla­ mation "Magnus magnes ipse est globus ter­ restrius" (the earth globe itself is a great magnet) in his De Magnete. 2 This treatise was published nearly a century before Newton's Philosophiae Naturalis Principia Mathematica (1687), and it has been suggested that Gilbert invented the whole pro­ cess of modern science rather than merely having Fig. 1-The engraved title page from the second Latin discovered the basic laws of magnetism and of edition of Gilbert's De Magnete. It shows lodestones, static electricity.3 Gilbert's efforts may have been compasses, and a terrella (a small spherical magnet sim­ inspired by the need for Her Majesty's Navy to im­ ulating the earth, in the upper left corner). In a vignette at prove (if not understand) the principal means of the bottom is a ship sailing away from a floating bowl compass with a terrella at the center. The first edition of navigation - the magnetic compass. This fact is De Magnete was published in 1600, and copies have evident from the frontispiece of the second Latin become extremely rare. edition of De Magnete, (Fig. 1) published in 1628. An understanding of the earth's magnetic field magnetic field also plays an important practical and its variations is still of great importance to role in searching for possible resources beneath the navigators. (More recently the U.S. Navy has "in­ earth's crust and in stabilizing artificial satellites. spired" APL to develop and improve a more ad­ Major disturbances to the geomagnetic field - vanced satellite system for navigation.) The geo- called "magnetic storms" - induce large, un-

162 Johns Hopkins APL Technical Diges( ______TECHNICALARTICLES

wanted effects in long-distance telephone circuits to that done for electric fields by E, for example. and sometimes cause widespread power blackouts. This unit of induction, ii, is 1 weber per square The geomagnetic field and its interaction with the meter (1 Wb/m2); it is the magnetic induction of a continuous flow of ionized gas (plasma) from the field in which 1 coulomb of charge, moving with a sun (the solar wind) provide the basic framework component of velocity of 1 m/s perpendicular to for the complicated space environment of the the field, is acted on by a force of I newton. In SI 2 earth, including the Van Allen radiation belts and units, 1 Wb/m = 1 tesla. auroral zones. The distorted configuration of its In studies of planetary fields, where very small geomagnetic field is called the "magnetosphere." fields are involved, the nanotesla (nT), formerly the Many APL-built spacecraft have made major con­ "gamma" ('Y), is used where 1 nT = 10.9 tributions to an understanding of the geomagnetic tesla = 10-9 Wb/m2 = 1 'Y. (The cgs unit of field and associated magnetospheric phenomena magnetic intensity is the gauss, where 1 during the past 15 years. Magsat is the latest one to tesla = 10 4 gauss.) The intensity of the surface do so. geomagnetic field varies from about 30,000 nT at the equator to more than 50,000 nT at high GEOMAGNETIC FIELD DESCRIPTION latitudes near the magnetic poles. The geomagnetic field can be thought of as being produced by a huge bar magnet imbedded in the SECULAR VARIATION earth, with the axis of the magnet tilted away It has been known for over 400 years that the slightly from the earth's rotational axis. The poles main geomagnetic field is not steady but experi­ of this magnet are located near Thule, Greenland, ences global secular variations. In fact, from a and Vostok, Antarctica (a U.S.S.R. research sta­ study of the paleomagnetic properties of igneous tion). To a good approximation, the geomagnetic rocks, it has been determined that the geomagnetic field can be represented by a simple dipole, but field has reversed polarity several times over the there is a significant contribution from nondipole past 4.5 million years (Fig. 2).5 components and from a system of complicated cur­ The behavior of the geomagnetic field over a rents that flow in the magnetospheric regions shorter time scale is shown in Fig. 3. That figure surrounding the earth. The most accurate represen­ shows the positions of the virtual geomagnetic pole tation of the geomagnetic field is provided by a since 1000 A.D. based on the assumption that the series of spherical harmonic functions. 4 The coef­ geomagnetic field is a centered dipole. 6 ficients of such a series representation are evaluated The following five features of the secular varia­ from an international set of spacecraft and surface tion have been determined: 7 observations of the geomagnetic field and are pub­ 1. A decrease in the moment of the dipole field lished for a variety of practical uses in navigation by 0.05070 per year, indicating that the present and resource surveys. A principal goal of Magsat is geomagnetic field may reverse polarity 2000 to provide the most accurate evaluation of the geo­ years from now. Preliminary analysis of Mag­ magnetic field model in this manner (see the article sat data has revealed that this variation may by Langel in this issue). be more rapid than was suspected from previ­ ous observations, and that the field may re­ MAGNETIC UNITS AND TERMINOLOGY verse polarity in only 1400 years; A wide variety of units and symbols are currently 2. A westward precessional rotation of the di­ in use in the many scientific and engineering fields pole of 0.05 0 of longitude per year; involved with magnetism. The following definitions 3. A rotation of the dipole toward the geograph­ are offered in hope of clarifying some of these for ic axis of 0.020 of latitude per year; a better understanding of the following discussions. 4. A westward drift of the nondipole field of Classic experiments have shown that the force 0.20 of longitude per year; acting on a charged particle moving in a magnetic 5. Growth and decay of features of the nondi­ field is proportional to the magnitude of the pole field with average changes of about charge. A vector quantity known as the "magni­ 10 nT per year. tude induction" is usually denoted by ii which Although these secular variations necessitate con­ characterizes the magnetic field in a manner similar tinual corrections to magnetic compasses they pro-

Volume I, Number 3,1980 163 ~ Normal field

~ Reversed field

Epoch Brunhes Matuyama Gauss Gilbert

II I I Fig. 2-The polarity of the 2.13 2.43 2.80 1.68 1.85 2. 11 2.90 geomagnetic field for the past 2.94 3.06 4.5 million years deduced 0.93 3.32 from measurements on ig­ 0.87 neous rocks dated by the 0.69 3.70 potassium- argon method and 0.350 3.92 from measurements on cores 0.330 4.05 from ocean sediments (from 0.114 Ref. 5). 4.25 .108 4.38 0.02 Millions of years before present 4.50 vide some clues to the internal source of the geo­ velocity is about 20 km per year. This is a million magnetic field. times faster than the large-scale motions of the solid part of the earth deduced from geological ob­ GEOMAGNETIC FIELD SOURCES servations and considerations. Seismological evi­ If the average westward drift of the dipole field dence reveals a fluid core for the earth that can in item 2 above is representative of the rate of mo­ easily experience large-scale motions, and it is pre­ tion of the field, then the corresponding surface sumed that the geomagnetic secular variation - and indeed the main field itself - is related to this fluid core. Furthermore, geochemical and density considerations are consistent with a core composed mainly of iron - a good electric and magnetic con­ ductor. Therefore, the study of the earth's internal magnetic field draws in another discipline - mag­ netohydrodynamics, which involves moving fluid conductors and magnetic fields. Modern theories of the geomagnetic field are based on the original suggestion of Larmor that the appropriate internal motion of a conducting fluid could cause it to act as a self-exciting dynamo. S To visualize this, assume the moving core to be an in­ finitely good conductor. Any primordial magnetic field lines, outside the core, for example, will be dragged around by the currents within the core as if they were "frozen" into the core. If the core rotates nonuniformly with depth, the field lines will become twisted around the axis of rotation in a way that opposes the initial field. The twisting ac­ tion packs the magnetic field lines more closely, causing the field intensity to grow. This growth can neutralize the original field and produce an even larger reversal field. The concept of magnetic field amplification by the differential rotation of con­ ductors has been used by astrophysicists to explain the magnetic fields of stars (including the sun), Jupiter, and Saturn. Many theories exist, but the Fig. 3-The virtual geomagnetic pole positions since 1000 precise generation mechanisms for the internal A.D ., which correspond to the secular variations at London geomagnetic field are still unknown. 8 if one ascribes the geomagnetic field entirely to a centered dipole. The London variations were deduced from magnetic field orientations of samples obtained MAGNETOSPHERIC CURRENTS from archeological kilns, ovens, and hearths in the southern half of Britain (from Ref. 6). The present virtual When viewed from outer space, the earth's pole is located near Thule, Greenland. magnetic field does not resemble a simple dipole

164 Johns Hopkins APL Technical Diges{ but is severely distorted into a comet-shaped netic storms can occur every few weeks during the configuration by the continuous flow of plasma peak of the II-year solar cycle (the peak of the (the solar wind) from the sun (depicted in Fig. 4). present cycle is thought to have occurred in 1980), This distortion demands the existence of a com­ whereas "super" magnetic storms that so severely plicated set of currents flowing within the distorted distort the geomagnetic field as to move the entire magnetic field configuration called the "magneto­ auroral zone to lower latitudes are a much rarer sphere." For example, the compression of the geo­ event (the last super storm occurred on August 2, magnetic field by the solar wind plasma on the day 1972, when an aurora was observed in Kentucky). side of the earth must give rise to a large-scale Besides the evaluation of models for the internal current flowing across the geomagnetic field lines, geomagnetic field, Magsat, launched in October called the Chapman-Ferraro or magneto pause cur­ 1979, provided the most sensitive measurements yet rent (see Fig. 4). of the magnetospheric current system. The magnetospheric system includes large-scale currents that flow in the "tail"; "Birkeland" cur­ MAGNETIC FIELD MEASUREMENTS rents that flow along geomagnetic field lines (see The technique of using airplanes for magnetic the article by Potemra in this issue) into and away field surveys for geological prospecting became well from the two auroral regions; the ring current that established in the 1950's. Airplanes make their sur­ flows at high altitudes around the equator of the veys at altitudes of 1 to 5 km, whereas satellites or­ earth; and a complex system of currents that flow bit the earth at 200 km or higher. Thus it was completely within the layers of the ionosphere, the somewhat of a surprise when scientists discovered earth's ionized atmosphere. The intensities of these from the data of the Orbiting Geophysical Obser­ various currents reach millions of amperes and are vatory satellites in 1972 that useful information closely related to solar activity. They produce mag­ about the structure of the earth's crust could be netic fields that vary with time scales ranging from derived from satellite data - information that a few seconds (micro pulsations) to 11 years (corre­ would be very difficult to detect in airplane survey sponding to the solar cycle). data. Ideas for a satellite devoted to this objective Widespread magnetic disturbances sometimes ob­ were discussed for a number of years, finally lead­ served over the entire surface of the earth are ing to the Magsat program, which had the addi­ known as magnetic storms. These storms are asso­ tional objective of measuring the "main" field for ciated with major solar eruptions that emit X rays, making new magnetic charts. ultraviolet and extreme ultraviolet radiations, and particles with energies from I keV to sometimes over 100 MeV. The solar plasma accompanying so­ MAGSATSPACECRAFT lar eruptions causes a magnetic storm when it col­ Preliminary discussions among APL, NASA, and lides with the earth's magnetosphere. Minor mag- the U.S. Geological Survey (USGS), commencing in the mid-1970's, culminated in conceptual studies of a spacecraft dedicated to the task of completing a global survey of the earth's geomagnetic field. NASA and the USGS subsequently entered into an agreement to conduct such a program on a cooper­ ative basis. The Goddard Space Flight Center (GSFC) was selected by NASA as the lead labora­ tory for this endeavor. Numerous trade-off design studies were undertaken, with emphasis on flying an adaptation of an available spacecraft design, launched from an early Space Shuttle, as against flying a small spacecraft on a NASA/DoD Scout launch vehicle. However, in view of the uncertain­ ties surrounding the availability of the Shuttle, and in light of the desire of USGS to incorporate satel­ lite magnetic field data into their 1980 map up­ dates, the decision was made by early 1977 to pro­ ceed with a Scout-launched spacecraft. In April 1977, after a successful preliminary de­ Solar wind sign review, APL was funded to proceed with the Fig. 4-The configuration of the earth's dipole magnetic Magsat design and development effort with the field distorted into the comet·like shape called the mag· goal of launching the spacecraft by September 21, netosphere. The various current systems that flow in this 1979, at a projected cost of about ten million complicated plasma laboratory are labeled. The interplan· etary magnetic field is the magnetic field of the sun, dollars. which has a modulating effect on the processes that oc­ The Small Astronomical Satellite (SAS-3) had cur within the magnetosphere. been designed and built by APL and launched in

Volume 1, Number 3,1980 165 1975. Many of the features of SAS-3 seemed ide­ could be placed on the dark side of the satellite to ally suited to the magnetic field satellite mission. It avoid direct sunlight. was a small spacecraft capable of being launched Even in this case, as the sun approaches the by the inexpensive Scout rocket, it had the world's highest latitudes of + 23 0 on June 21, the orbit most precise tracking system (i.e., positi on determi­ would be shadowed in the south polar region. nation) in its Doppler tracking system (a derivative Shadowing was expected to begin in April so a of the APL Transit system), it had two star launch date of September 1979 was chosen, which trackers that could provide attitude determination would allow six months of fully sunlit orbits. to 10 arc-s (I arc-s = 0.00028) accuracy, and its Launch actually occurred at the end of October attitude control sys tem used an infrared earth­ 1979, so 5 Y2 months of fully sunlit orbits were ob­ horizon scanner/ momentum wheel assembly that tained. was ideally suited for Magsat. A critical problem, which was quickly id entified , was the excessive THE SPACECRAFT weight of Magsat. Tape recorders with a larger Magsat was intended to measure the vector com­ capacity for data storage were needed, a nd new S­ ponents of the earth's field to an accuracy of band transmitters were required for the hi gh data 0.01 070; this meant that the orientation of the vec­ rate during tape recorder playback. Compromises tor sensors must be known to 15 arc-s accuracy. in the solar cell array were necessary to keep the The star cameras were good to an accuracy of 10 weight down to 182 kg, the maximum that the arc-s, but they had 2 kg of essential magnetic Scout rocket could launch into a 350 by 500 km or­ shielding that would distort the magnetic field . An bit. extendable boom was needed to put the vector sen­ sors 6 meters away from the magnetic disturbance caused by the star cameras. But it was not possible MAGSAT ORBIT for the boom to be mechanically stable to 5 arc-so A n orbit was needed that would give full earth A system was needed to measure the orientation of coverage and as little shadowing by the earth as the vector sensors relative to the star cameras. This possible. A polar orbit would be ideal for earth system, the Attitude Transfer System (ATS), used coverage, but because the orbit plane would remain an optical technique involving mirrors attached to fixed in space, the motion of the earth about the the vector sensor to make the necessary measure­ sun would cause shadowing of the satellite within ment (see the article by Fountain el 01. in this 30 to 60 days after launch. Also, it would be diffi­ iss ue). cult to find star camera orientations that would not The elements of the A TS and the two star present problems with direct sunlight. However, for cameras had to be tied together mechanically in an orbit inclination of 97", the orbit plane some permanent and extremely stable fashion. The precesses at the rate of 10 / day, just the right structure to achieve this was the optical bench, a amount to make the orbit plane follow the sun. built-up assembly of graphite fiber and epoxy resin (This precession is due to the bulge in the earth's that provided a near-zero coefficient of thermal ex­ gravity field at the equator.) This sun-synchronous pansion. The bench was attached to the satellite at orbi t (Fig. 5) gives nearly 100 070 earth coverage and five points, two of which were released by pyro­ many months of full sunlit orbits. The star cameras technic devices after the satellite was in orbit. The

Fig. 5-The precession of the Magsat orbit plane with time. The Magsat orbit plane makes an angle of 9r with the earth's equatorial plane. At this inclination (and at the altitude of Magsat), the equa­ torial bulge of the earth causes the orbit plane to roo tate above the polar axis at 1 '/day, just the right amount to turn the orbit plane toward the sun as the earth proceeds in its orbit about the sun. Un· fortunately, the 23' tilt of the earth polar axis adds to the r tilt of the orbit plane in June, causing shadowing of the southern portion of the orbit by the earth. This shadowing began in mid-April for Magsat.

166 Johns Hopkins APL Technical Digest three remaining support points did not apply stress But in orbit, the satellite continuously experiences to the bench. Heaters and temperature sensors at zero g, these distortions would disappear, and our eight places stabilized the bench temperature at ground calibrations would be invalidated. To solve 25°C. the problem, we made a second calibration with the At the end of the 6 meter extendable boom were satellite upside down, thereby reversing the direc­ the vector magnetometer sensor and a scalar mag­ tion of the weight force (i.e., -I g) and producing netometer sensor (see the articles by Acuna and distortions equal and opposite to those of the ini­ Farthing). The vector sensor consisted of three tial calibration. We then presumed that the zero-g small toroidal cores of highly permeable magnetic calibration must be exactly midway between the material with platinum wire windings used to sense two results. This technique has been confirmed the components of the field. They were mounted with our flight results. on a very stable ceramic block, and the tempera­ The spacecraft was returned to APL where the ture was controlled at 25°C. The scalar two modules were separated so that the magnetom­ magnetometer measured the field magnitude very eter boom assembly could be installed. A series of accurately, but not its direction. it used Zeeman boom extension tests was performed to verify ATS splitting of energy levels in cesium-133 gas as a performance and alignment and to calibrate the technique for measuring the field. The scalar data boom deployment telemetry channels. In June provided redundancy and an independent check on 1979, the spacecraft was reassembled and, after a the calibration of the vector magnetometer. preliminary weight and balance determination, was Magsat was the latest and most complex of the returned to GSFC for initial magnetics tests and satellites built by APL. The command system fea­ radio frequency interference tests aimed at verify­ tured its own dual computers, which permitted ing that all subsystems could operate in the orbital storage of 164 commands, to be implemented at configuration without interfering with one another. desired times (see the article by Lew et al.). This Upon its return to APL, there followed detailed was very helpful because the low altitude of Magsat electrical performance tests, establishing the base­ meant that a ground station had only 9 to 10 line for future reference. minutes in which to send commands, receive the During August 1979, the spacecraft was exposed data played back by the tape recorders, and make to launch phase vibration and shock excitation tests decisions about managing the satellite's health. The followed by two weeks of combined thermal vac­ command system was also designed to accept com­ uum and thermal balance testing. In September the mands from another on-board system, viz., the at­ spacecraft was once again taken to GSFC for final titude control system. The attitude control system weight, center-of-gravity location, and moment-of­ also used a small computer to manage the satellite inertia determinations; final magnetic tests; and attitude. When it decided that commands were post-environmental verification of the optical align­ needed, a request was sent to the command system, ment of the star cameras and ATS. Upon its return which then implemented the command. to APL, a final vibration exposure (single axis) was performed to ensure that all components were TESTING secure. This was followed by a short electrical test. Fabrication and test of components and subassemblies commenced during the winter of LAUNCH AND POST-LAUNCH 1977-78 and were completed in 1979. The instru­ ment module was assembled in December 1978 and EXPERIENCES exposed to a thermal balance test in vacuum. The The spacecraft, ground station, and supporting base module was assembled in January and Febru­ equipment were trucked from APL to Vandenberg ary 1979, and a critical test of the attitude control Air Force Base, arriving on the morning of Octo­ system was performed to verify various design and ber 8, 1979 . Intensive field operations followed, in­ performance parameters. Development difficulties cluding electrical tests, assembly to the fourth stage delayed availability of the magnetometer boom rocket, and final spin balance. The spacecraft assembly until May 1979. The base module and in­ fourth stage rocket assembly was then mounted on strument module were assembled without the boom the main rocket assembly, the heat shield was in­ and taken to GSFC for the star camera and ATS stalled, and all-systems tests were performed. On alignments. October 27, 1979, a dress rehearsal was conducted, The alignment and calibration of all the optical leaving all in readiness for launch, planned for Oc­ elements mounted on the optical bench was an es­ tober 29 at dawn. The countdown began on the pecially difficult task. It was done with the fully as­ evening of October 28 but had to be suspended just sembled satellite mounted inside an aluminum cage, prior to terminal countdown because of extremely usi ng the optical test laboratory at GSFC. Since the high winds at about 10,000 ft altitude. The launch calibration was done in the gravity field of the operation was resumed on the evening of October earth (i.e., 1 g), the weight of the star camera and 29 and culminated in a successful launch at 6: 16 ATS components would distort the optical bench. A.M. PST, October 30, 1979. All stages fired cor-

Vulume I, Number 3.1980 167 rectly and the spacecraft was injected into a 352 by Subsequently, details of the spacecraft components 578 km sun-synchronous orbit. are discussed. The concluding articles describe the Data were recorded until the satellite burned up scientific results and on-going studies. at low altitude on June 11, 1980. A large amount of vector and scalar magnetometer data was col­ REFERENCES lected, and scientific results are beginning to IS. Chapman and J. Bartels, Geomagnelism, Oxford Press, p. 888 (1940). become available. We experienced some operational 2W. Gilbert, On the Magnet; The Colfeclor's Series in Science (D. J. problems because of earth shadowing in the latter Price, ed.) Basic Books, Inc., New York (1958). portion of Magsat's life, primarily caused by an 3Ibid, pp. v-xi. 4A. J . Zmuda (ed.), World Magnetic Survey, 1957-1969, International unexpected loss of battery capacity that forced Association of Geomagnetism and Aeronomy Bulletin No. 28, Paris some compromises in data collection. The sun­ (1971). shades of the star cameras showed light leaks, SF. D. Stacey, Physics of lhe Earlh, John Wiley and Sons, New York (1969). which caused the loss of some data. On the whole, 6M. J . Aitken and G. H. Weaver, "Recent Archeomagnetic Results in however, the Magsat satellite has been very suc­ England," J. Geomag. Geoelecl. 17, p. 391 (1965). 7T. Nagata, " Main Characteristics of Recent Geomagnetic Secular cessful, and all mission objectives should be ac­ Variation," J. Geomag. Ceoelecl. 17, p. 263 (1965). complished when the data are fully processed. 8S ee reviews of W. M. Elsasser, "Hydromagnetic Dynamo Theory," The articles that follow describe the develop­ Revs. Mod. Phys. 28, p. 135 (1956); D. R. Inglis, "Theories of the Earth's Magnetism," Revs. Mod. Phys. 27, p. 212 (1955); and T. ments that led to the Magsat program, and the mis­ Rikitake, Eleclromagnetism and the Earlh's Inferior, Elsevier, Amster­ sion objectives, and summarize early flight events. dam (1966).

A GLOSSARY OF MAGSAT COMPONENTS

Aerotrim Boom - A motorized ex­ from the ground, and routed the words from the earth in the infrared (IR) at 15 tendable boom consisting of a pair of to the destinations designated by the ad­ micrometers. The field-of-view was a silver-plated beryllium-copper tapes, dress codes contained in each word. At narrow beam rotated to form a 90· cone 0.002 inch thick, rolled on a pair of destination, the word was further decod­ as the wheel spun. When the beam inter­ spools. When extended the tapes formed ed and the specific element of the satel­ sected the earth, the IR radiation was a tube 0.5 inch in diameter up to 12 lite addressed was placed in the mode detected; an electronic system derived meters long. The air drag on the boom designated by the word. the pitch and roll angles of the satellite was used to balance the aerodynamic Data Formatter - The portion of the from this information. torques in yaw. telemetry system that took the various Magnetic Coils - Magsat had X-, Y-, Attitude Control System - The science and housekeeping digital data and Z-axis coils for torquing by interac­ system that controlled the satellite at­ bits and arranged them in a predeter­ tion with the earth's magnetic field. A titude; in Magsat it held the satellite mined sequence for modulation onto the coil consisted of many turns of alumi­ properly oriented with respect to the carrier frequency of the transmitter as num wire mounted on the outer skin of earth and the orbit plane. It consisted well as for recording by the tape re­ the satellite. When energized with a primarily of a momentum wheel with an corders. The predetermined sequence steady electric current, the coils ex­ integral infrared earth horizon scanner, permitted decoding of the signals by perienced torques from the earth's mag­ magnetic torque coils, gyro system, and ground-based computers. netic field that were used for attitude associated electronics. Despin/Separation Timer - One of a control. Attitude Transfer System (ATS) - An pair of devices mounted on the head cap Magnetometer Boom - A collapsible electronic and optical system for mea­ of the fourth-stage rocket motor. I twas structural element composed of seven suring the orientation of the vector intended to initiate despin followed by pairs of links in a scissors or "lazy­ magnetometer sensor relative to the star spacecraft separation at predetermined tongs"-type arrangement intended to cameras. Two optical heads of the ATS times following the completion of firing move the sensor platform from its were mounted on the optical bench near of the fourth-stage rocket motor. caged, launch-phase position to a posi­ the star cameras. One of the heads Horizon Scanner - The momentum tion 6 meters away from the instrument transmitted a beam of light to a plane wheel had within its structure an optical module. mirror on the back of the vector magne­ system capable of detecting radiation Magnetometers - The scientific in- tometer. The beam was reflected back into the same head where its angular Fig. A-The orientation of Magsat in deviation was measured and two angles +y, orbit, as determined by the attitude of the plane mirror were determined. determination system. The second head sent a beam of light to a dihedral mirror also on the back of Direction of flight the vector magnetometer. The light was reflected to a dihedral mirror on the op­ ~ tical bench, and then via the first dihe­ dral mirror back to the optical head. The position of the reflected beam was used to measure the twist angle of the +B vector sensor. To the sun Command System - The apparatus aboard the satellite that accepted the digital bit stream from the receiver por­ -ct tion of the transponders, decoded it to \ To the earth -y ----tOil axis) recover the command words transmitted -A

168 Johns Hopkins A PL Technical Digesl Fig. 8-The configuration of Magsat.

Tape recorder

ATS electronics Temperature control electronics Solar aspect sensor electronics

ATS roll and pitch yaw heads Star cameras B-axis (Z) magnetic coil

Aerotrim boom

I R horizon scanner and momentum wheel

Base module containing: Attitude control system Command system Data formatter Nutation damper Oscillator Power supply Telemetry Transponder Solar cell array

Dipole antenna

Solar aspect sensor

Vector magnetometer ATS pitch/ yaw mirror struments of Magsat consisted of a damped. In Magsat, damping was ac­ spacecraft to avoid developing spurious three-axis vector magnetometer and a complished in two ways: by a pendulous beat frequencies that could be a source scalar magnetometer for measuring the mechanical damper that used magnetic of interference to the various electronic magnetic field of the earth. The vector eddy currents for damping, and by the devices. The stability of the oscillator magnetometer sensor had three small closed-loop attitude control that was achieved by placing the crystal in­ magnetic elements, each sensitive to one modulated the momentum wheel speed side a double-oven arrangement, provid­ component of the earth's field . The to damp nutation. ing a high degree of thermal isolation scalar magnetometer measured only the from the fluctuations experienced by the Optical Bench - A structural plat­ magnitude of the field by optical pump­ base module, and by using a cut quartz form constructed of a graphite fiber­ ing of atomic excitation states in cesium- crystal selected so that its turnover epoxy-laminate honeycomb that was de­ 133 gas . temperature and the oven temperature signed to provide a very stable surface were precisely matched. This permitted Momentum Wheel - Magsat had an for mounting the star cameras and ATS operation with virtually no temperature internal tungsten wheel that spun at components. The properties of the mate­ effect on the oscillation frequency. about 1500 rpm. This rotation provided rial were used to ensure that the exact Power Supply - The system con­ angular momentum that gave the satel­ angular relationship was maintained be­ sisted of the following elements: solar lite a form of gyroscopic attitude stabil­ tween the star cameras and A TS compo­ cell arrays to generate electricity; a bat­ ity. This was a key feature of the atti­ nents irrespective of instrument-module tery mounted in the base module to tude control system. temperature fluctuations. store the electrical energy for use during Nutation Damper - When a distur­ Oscillator - An ultrastable quartz any shadowed portions of the orbits and bance torque is applied to a gyro-stabi­ crystal oscillator producing a 5 M'Hz to meet peak power demands; battery lized satellite such as Magsat, the atti­ output used as the source for the 162 voltage limiter devices to control battery tude motion includes nodding or wob­ and 324 MHz Doppler signals. The 5- charging; and DC-DC converter regulators bling. After the torque is removed, this MHz signal was also used to synchronize to condition power to the voltages need­ nodding ("nutation") persists unless the various DC-DC converters aboard the ed by each user.

Volume I, Number 3, 1980 169 Solar Aspect Sensors - Several types was found, the magnetic coils "locked" same ground stations. This NASA Stan­ were included in Magsat. Of special in­ onto it for a few seconds and the posi­ dard Near-Earth Transponder could also terest was the "precision" solar aspect tion was recorded. be used as a range/range rate trans­ sensor, mounted near the vector magne­ Tape Recorder - A device used to ponder for satellite tracking and orbit tometer sensor, which measured the store telemetry data until the satellite determination. Magsat, however, used angles to the sun with an accuracy of 5 was over or near a ground station. The the much more precise Doppler beacons to 10 arc-seconds. signals were recorded magnetically on in conjunction with the DMA tracking Star Cameras - Two star cameras iron-oxide-coated Mylar tape running network. were mounted rigidly on the optical between a pair of coaxially mounted bench. Each camera had a 4-inch-diame­ reels. Two tape recorders were mounted Vehicle Adapter - The conically­ ter lens that focused the stars on the on the deck between the base module shaped transition section bolted to the front end of an "image-dissector" elec­ and instrument module. fourth-stage rocket to which the space­ tronic tube. Inside the tube, in the Telemetry - The process by which craft was clamped. The two halves of vacuum, was a very sensitive surface the scientific (magnetometer) data and the clamp were fastened together at each that emitted electrons wherever starlight information concerning the satellite atti­ end by bolts passing through pyrotechni­ fell upon it. These electrons were tude, load currents, bus voltages, tem­ cally operated cutters. Separation of the directed by magnetic coils to pass peratures, and other "housekeeping" spacecraft from the launch vehicle was through a small hole into an electron data were transmitted to the NASA achieved by actuating the bolt cutters by multiplier where a cascade of electrons STDN ground stations. a stimulus from the spacecraft battery was generated, finally accumulating Transponder - A combined radio initiated by the despin/ separation enough effect to be a measurable electric receiver and transmitter operating at S­ timers. When the bolts were cut, the two current. With magnetic coils driven in a band, used for receiving command sig­ clamp halves moved apart, allowing predetermined manner, the surface of nals transmitted from the NASA STDN small springs to force the spacecraft the tube was searched for sources of ground stations and for transmitting the away from the adapter/fourth-stage electrons (i.e., starlight). When a source telemetry signals from Magsat to the assembly.

170 Johns Hopkills A PL Techllical DiMesl GILBERT W. OUSLEY

OVERVIEW OF THE MAGSAT PROGRAM

The Magsat project, the background leading to its inception, and its mlSSlOn objectives are briefly described, followed by summaries of plans for studies of regional geology and geophysics, geomagnetic field modeling, and the inner earth.

PROJECT DESCRIPTION Satellite measurements of the geomagnetic field began with the launch of Sputnik 3 in May 1958 The purpose of the Magsat project was to and have continued sporadically in the intervening design, develop, and launch a satellite to measure years. To date only the OGO-2, -4, and -6 satel­ the near-earth geomagnetic field as part of NASA's lites, which are the Polar Orbiting Geophysical Ob­ Resource Observation Pro~ram. It was intended to servatories (Pogo's), have provided a truly accurate provide precise magnetic field measurements for global geomagnetic survey. These satellites operated geoscientific investigations by: between October 1965 and June 1971, and their • Accurately describing the earth's main magnet­ alkali-vapor magnetometers provided global mea­ ic field for charting and mapping applications surements of the field magnitude approximately and for investigating the nature of the field every half second over an altitude range of 400 to source. This will provide more accurate mag­ 1500 km. netic navigation information for small craft, The satellite geomagnetic field measurements will trace more accurately the magnetic field were intended for mapping the main geopotential lines for the near-earth magnetosphere, will field originating in the earth's core, for determining separate the magnetic field into crustal and ex­ the long-term temporal or secular variations in that ternal field contributions, and will permit field, and for investigating short-term field pertur­ study of the earth's core and core-mantle bations caused by ionospheric currents. Analysis of boundary; and data from the Pogo satellites disclosed that the • Mapping, on a global basis, the fields caused lower altitude contains separable fields related to by sources in the earth's crust. This will con­ anomalies in the earth's crust, thus opening the tribute to our understanding of large-scale door to a new class of investigations. Several geo­ variations in the geologic and geophysical magnetic field models and crustal anomaly maps characteristics of the crust, and will in turn af­ based on the Pogo data have been published. fect the future planning of resource explora­ NASA established the Magsat program to pro­ tion strategy. vide the United States Geological Survey (USGS) of the Department of the Interior with data to be used in making magnetic field maps for the 1980 epoch. BACKGROUND The program is managed by the Goddard Space Magsat is one of several integrated elements of Flight Center (GSFC) in cooperation with the the NASA Resource Observation Program that use USGS. Responsibility for spacecraft design, devel­ satellites to study the earth's nonrenewable opment, and testing was assigned to APL. resources. One facet of this program is the use of The Magsat data are expected to resolve direc­ space techniques to improve our understanding of tional ambiguities in both field modeling and mag­ the dynamic processes that formed the present netic anomaly mapping. Increased resolution, geologic features of the earth and how these pro­ coupled with higher signal levels from anomalous cesses relate to phenomena such as earthquakes and fields, will overcome some of the shortcomings of to the location of resources. the Pogo data and provide improved anomaly One part of the Resource Observation Program maps. is the measurement of the near-earth geomagnetic Magsat data will be correlated with other geo­ field. Before the satellite era, magnetic data from physical measurements. For example, the highly ac­ many geographic regions were acquired over peri­ curate determination of the earth's gravity field ods of years by a variety of measurement tech­ and geoid accomplished by both laser tracking of niques. However, for many regions such as the satellites and altimetry studies with geodetic satel­ oceans and poles, data were either sparse or nonex­ lites provides information about the density distri­ istent. bution within the earth's crust.

Volume I, Number 3, 1980 171 MISSION OBJECTIVES age with no capability of averaging or statistical The USGS, principal user agency for this mis­ analysis. To provide a workable sample, data that sion, and other users from both U.S. and foreign are three times as dense are needed. governments, universities, and industry will pursue Before launch the desired lifetime of the mission the following objectives: was established as approximately 150 days. The ac­ tual lifetime was 225 days. This additional time • Obtain an accurate, up-to-date, quantitative provided a more than adequate statistical sample description of the earth's main magnetic field. and increased the probability of obtaining "quiet Accuracy goals are 6 nanoteslas (l nT = 10-9 day" coverage. weber per square meter) root sum squares (rss) Data at all local times except for those between in each direction at the satellite altitude and 20 0900 and 1500 are useful for anomaly studies. A nT rss at the earth's surface in its representa­ dawn launch was chosen to obtain useful anomaly tion of the field from the earth's core at the data on all portions of the orbit and to maximize time of the measurement; power. • Provide data and a worldwide magnetic field Originally, a 325 by 550 km orbit was proposed model suitable for USGS use in updating and to provide an orbital lifetime of between 4 and 8 refining world and regional magnetic charts; months. Because of the very high solar flux that • Compile a global scalar and vector map of was subsequently predicted during late 1979 and crustal magnetic anomalies. Accuracy goals are early 1980 (this flux increases the height of the at­ 3 nT rss in magnitude and 6 nT rss in each mosphere and leads to higher drag forces), the final component. The spatial resolution goal of the selected orbit was 350 by 550 km. A four-stage anomaly map is 300 km; and Scout vehicle injected Magsat into a sunlit (dawn to • Interpret the crustal anomaly map in terms of dusk) sun-synchronous orbit. It was launched from geologic and geophysical models of the earth's the Kennedy Space Center Western Test Range, crust for assessing natural resources and deter­ Vandenberg AFB, Calif., on October 30,1979. mining future exploration strategy. INVESTIGATION PLANS MISSION PARAMETERS To achieve the Magsat mission objectives, inves­ tigations are being carried out at GSFC and the The altitude of the Magsat measurements was USGS and by competitively selected principal in­ dictated by a trade-off between two requirements: vestigators. the lowest possible altitude is necessary for increas­ The GSFC investigations are those required for ing the anomaly signals and their spatial resolution; rapidly producing the principal mission products - a minimum satellite lifetime is necessary for obtain­ spherical harmonic models of the main geopoten­ ing adequate data distribution. On the basis of tial field and magnetic anomaly maps, analytic these requirements, the nominal perigee was about tools needed by other investigators (e.g., equivalent 350 km. The apogee was selected to provide the re­ source models of the anomaly field), and develop­ quired lifetime. Although higher altitude data are ment of preliminary crustal models of selected less useful, data up to 500 km could be used in areas not being studied by other investigators. anomaly studies. Accuracy goals are determined by The USGS will produce charts of both the the fact that, based on experience with Pogo, the United States and the entire earth; will develop so­ anomaly amplitudes at 300 km are expected to be phisticated models of the main geopotential field in the range of 0 to 50 nT. To obtain the maximum using Magsat data along with appropriate correla­ anomaly resolution and signal strength, data were tion data, with particular emphasis on secular vari­ acquired as the orbit decayed below 300 km during ation studies; and will develop crustal models of the reentry phase. selected regions. The main field and anomaly measurements must be made in the presence of the perturbing fields REGIONAL GEOLOGIC AND from ionospheric and magnetospheric sources. Since the mission was conducted during a period of GEOPHYSICAL STUDIES very intense solar activity, only approximately 20% Magnetic anomaly data are useful for regional of the data obtained at latitudes below 50 0 will be studies of crustal structure and composition, the sufficiently free of these perturbations to yield the usefulness including possible correlation with the required accuracy. At latitudes higher than 50 0 emplacement of natural resources and guidance for even fewer data are usable. To achieve a 300 km future resource exploration. Magnetic anomaly global anomaly resolution, coverage is needed maps based on aeromagnetic surveys are standard along grid lines that are no more than 150 km tools for oil and mineral exploration. A magnetic apart. The time required for achieving this anomaly anomaly, as the term is used in this overview, is the resolution is estimated to be 10 days for perfectly residual field remaining after the broader-scale quiet data or 50 days during periods of maximum earth's field is removed. The removed field is solar activity. This time represents minimal cover- usually computed from a spherical harmonic

172 Johns Hopkins APL Technical Digest model. The primary purpose of the regional studies geology, such as structure, lithology, and depth of is to identify subsurface geologic structures that exposure. For example, copper deposits such as may extend over distances and depths of a few ki­ those of the southwestern United States are associ­ lometers and to help target specific areas for drill­ ated with a class of young granite intrusion that is, ing or mining. Because of incomplete coverage and in turn, related to the Basin and Range Province, a because of large temporal changes in background large zone of block-faulting. In contrast, chromium fields between the times when adjacent local occurs in large intrusions of iron-rich igneous rock surveys were made and when large local variations of much greater age and completely different tec­ occurred, available aeromagnetic anomaly maps tonic setting. A general factor that strongly affects cannot effectively probe broad regional geological both the formation of mineral deposits and the features. With the advent of the theory of plate eventual discovery of their location is the crustal tectonics, interest in identifying and mapping these level at which they occur. broad regional features is increasing. Because of these relationships, a sound knowl­ The anomalies measured by Magsat will reflect edge of regional geology and geophysics is essential such important geologic features as composition, to the planning of mineral exploration and, gener­ temperature of rock formation, remanent magne­ ally, to the assessment of the mine'ral reserves of a tism, and geologic structure (faulting, subsidence, region or country. Magsat is providing new infor­ etc.) on a regional scale. Magsat is therefore pro­ mation on crustal structure and composition over viding information on the broad structure of the very large areas, including those in which bedrock earth's crust, with near global coverage. is poorly exposed. Therefore, the data will be Continental coverage will be most important for helpful in selecting the most promising areas for immediate economic applications. Magsat data will future mineral exploration and ill determining the help to delineate the fundamental structure of the exploration strategy to be used. The full potential very old crystalline basement that underlies most of the crustal anomaly measurements will not be continental areas. This structure will not necessarily realized for some time, but the near-term applica­ parallel known younger trends. tion of the initial results is obviously promising. Interpretation of the basic scalar anomaly map begins with the construction of a model of regional GEOMAGNETIC FIELD MODELING susceptibility contrasts. The inclusion of vector One of the principal contributions of satellite data will improve model accuracy. Vector data are magnetic field measurements to geomagnetism has then used to infer the magnetic-moment direction been to make available a rapidly obtained global for each anomaly, permitting the presence of large­ survey instead of conventional surface measure­ scale remanent magnetism to be distinguished from ments that present the problem of the long-term, or susceptibility contrasts. Correlative data, such as secular, variation in the main geomagnetic field gravity anomalies and known geology, are used in (which can amount to as much as 1% per year in interpretation so that the resulting geological and some localities). In order to represent the global geophysical models closely resemble reality. These geomagnetic field accurately at any given instant, models will contribute substantially to our knowl­ measurements must be made worldwide at nearly edge of the unexposed fundamental geology of the the same time. This can be achieved only by satel­ continents. Models yield information for both the lite observations and, even then, only by spacecraft shallow crustal features that are important to re­ that are in polar orbits and that are equipped with source assessment and the deep features that relate on-board tape recorders. In addition, accurate to faulting and earthquake mechanisms. global respresentation of the secular variation re­ One application of Magsat anomaly maps will be quires periodic worldwide surveys. Magsat will fur­ the long-range planning of mineral and hydrocar­ nish one such survey and, together with existing bon exploration programs. Such planning is con­ surface data, will permit accurate global representa­ cerned with delineating entire regions (frequently of tion of secular variation for the period beginning subcontinental extent) that should be explored with OGO-2 to the demise of Magsat (October rather than with discovery of specific oil or mineral 1965 to June 1980). deposits. For example, in Australia serious explora­ Although Pogo data were global and taken over tion for oil began in areas that were known to have a short time, the limitation of measuring only the thick accumulations of sedimentary rock (i.e., sedi­ field magnitude resulted in some ambiguity in the mentary basins). Regional studies eventually nar­ field direction in spherical harmonic analyses. rowed the broad target areas to a few promising Magsat is eliminating this ambiguity by providing structures, which were then explored in detail. global vector data. Some idea of the relationship between the re­ gional geology and mineralogy can be seen by INNER-EARTH STUDIES noting that metallic mineral deposits are usually as­ The radius of the earth is about 6378 km. Direct sociated, either directly or indirectly, with igneous measurements in mines, drill holes, etc. are avail­ rocks. From a broader viewpoint, the distribution able only in the upper few kilometers of the crust. of such deposits is closely interrelated with regional In particular, for information regarding the core

Volume I, Number 3, 1980 173 and mantle, the only sources of data are extruded be used to investigate changes of the fluid motions geologic structures (upper mantle only), the gravity in the core that give rise to the field, properties of field of the earth, the rotation rate and polar mo­ the core-mantle boundary that greatly affect fluid tion, seismic measurements, and the magnetic field motions of the core, and the transmission of this of the earth. Therefore, we must depend on only a temporarily varying field through the lower mantle. few indirect measurements for information on When magnetospheric fields are time varying, more than 96070 of the earth's volume. they result in induced fields within the earth To understand the tectonic processes that con­ because of the finite conductivity of the earth. The tribute to the dynamics of the earth's crust (i.e., characteristics of these induced fields are deter­ plate motions, earthquakes, volcanism, and mineral mined by the composition and temperature of the formation), it is necessary to investigate the mantle materials in the earth's mantle. At present the and core in which these processes originate. limiting factor in determining a precise conductivity Characteristics of magnetic fields at and near the profile within the earth with adequate spatial reso­ earth's surface (e.g., secular change, field reversals, lution is the accuracy possible in determining the and power spectra of magnetic-field models) have external and induced fields. Although the ultimate heretofore been used to infer properties internal to usefulness of satellite vector measurements in these the earth. Combining Magsat data with earlier studies is not completely clear, preliminary results satellite surveys and with surface data from in­ are encouraging, suggesting that Magsat vector tervening times will permit more accurate deter­ measurements will enable a much more accurate mination of secular variation. This information can analysis to be made.

174 Johns Hopkins A PL Technical Digest FREDERICK F. MOBLEY

MAGSAT PERFORMANCE HIGHLIGHTS

This article describes the commands given to the spacecraft during the first week after launch to prepare for collection of scientific data. It also discusses the performance of systems and in­ struments during the data-gathering phase.

POST-LAUNCH ACTIVITY 80°,-----,-----,----,,----,-----,-----, OCTOBER 30 - NOVEMBER 3,1979 ~....--v~St a rt

Attitude Control It was critically important for Magsat to obtain the proper attitude relative to the sun very soon after launch. Most satellites are designed to gener­ ate adequate electrical power regardless of their solar orientation; in such cases the post-launch at­ titude control adjustments can proceed gradually. However, Magsat had many electrical load require­ ments that could not be dispensed with, and the O~--~~------~ solar array could not meet those loads except when I the B axis of the satellite was within 60° of the sun Sun line

(see Glossary, Fig. A). At the moment of satellite _20°'----____'---- ____'- _____"'- _____"~ _____" ______' release in orbit, this angle (() ) would be near 90°, _20 0 o and the solar array output would be zero. There­ L;. right ascension (deg) fore, an immediate attitude control maneuver was Fig. 1-Predicted track of + B·axls maneuver In Initial or· required to move the B axis toward the sun before bits of Magsat. Initial spin rate: 0.67 rpm. the battery became dangerously discharged. Figure 1 shows the attitude control maneuvers that were programmed to occur in the first two or­ to carry out the following commands in the first bits of Magsat. These maneuvers were accom­ orbit: plished by commanding a B-axis coil to be ener­ 1. Turn off a tape recorder a few minutes after gized with a current of 0.9 A, producing a magnet­ injection into orbit to save power; ic dipole parallel to the satellite B axis. This dipole 2. Fire pyrotechnics to release two of the five interacted with the earth's magnetic field and pro­ mounting points for the optical bench and to duced a torque on the satellite. Since the satellite release the mechanical retention of the boom had substantial angular momentum (due to an in­ links (but not the magnetometer platform - ternal wheel spinning at 1500 rpm as well as to the that would come later); rotation of the satellite itself) and the torque is ap­ 3. Turn the Doppler transmitter on for the plied perpendicular to the momentum vector, the benefit of a receiving station in Winkfield, effect is to shift slowly the direction of the momen­ England; tum vector in space. 4. Turn the B coil on and off at the right times To move the vector in a desired direction re­ and with the correct polarity in order to quires knowledge of the magnetic field strength and maneuver the B axis toward the sun; and direction at the satellite position and the proper 5. Extend the aerotrim boom to balance choice of command timing to take advantage of the aerodynamic yaw torques. field direction. This led to a particular command Figure 1 shows the predicted track of the + B sequence and timing; the result is the predicted axis with an assumed satellite spin rate of 0.67 track of the B axis shown in Fig. 1. The commands rpm. Figure 2 compares the predicted variation of for these maneuvers consisted of a series of B-coil the angle () versus time with the actual result in or­ commands, e.g., + SENSE, COIL ON, OFF, - SENSE, bit. Since the actual spin rate was 1.22 rpm, the ON, OFF, etc., each at a particular time. angular momentum was greater than expected and The Magsat command system could accommo­ the B axis failed to reach the sun line. Nevertheless, date 82 stored commands in each of its two redun­ the final angle of 20° was more than adequate to dant systems. Almost all of this capability was used provide the power requirements of the satellite.

Volume 1, Number 3,1980 175 0 system for active roll-and yaw-angle control was activated. Full control of pitch, roll, and yaw angles was maintained thereafter. 20 ...... c; o ...... Boom Extension and A TS Capture Q) "., :.':'. 40 "., ""- "., Late on November 1, 1979, the magnetometer ~. boom was deployed by command. Figure 3 shows Cl ".,if' C 60 ",-C1" the observed extension process versus time. The en­ '"c :J (f) tire extension took 20 minutes, and was followed ."c1 ./ ~ctu a l (spin rate = 1.22 rpm I immediately by a further extension of the aerotrim ./ 80 ,.. ",­ boom to 6.99 m. ./ At this point the "moment of truth" came for V

100L-~~ __L-~~ __L--L~ __L--L~ __L--L~ one of the most challenging engineering tasks of 10 30 50 70 90 110 150 the Magsat program. The pitch/ yaw head of the at­ Time after lift-off (mini titude transfer system (ATS) sent a narrow beam of Fig. 2-Sun angle versus time for initial attitude collimated light from the satellite to a small mirror maneuver of Magsat. A 8-axis coil is turned on to interact (9 by 9 cm) mounted at the end of the boom, 6 m with the earth's magnetic field to precess the satellite away. The mirror reflected the light back into the spin axis toward the sun in a programmed maneuver. The predicted motion was not fully achieved because the lens of the sending unit in order to measure the satellite spin rate was higher than expected. The solar ar­ mirror angles of pitch and yaw. The ATS measures ray generated 135 W at the final orientation. the mirror angles accurately over a range of ± 3 arc-min (0.01667") and is able to detect a signal out to ±6 arc-min (but cannot measure the angle ac­ Two additional manually controlled magnetic ma­ curately)_ Beyond that angle there is no signal. The neuvers were carried out in the next 48 hours to critical question was, would the boom position and bring the B axis within 50 of the sun line. angle be correct for an ATS signal to be received? The magnetic spin/ des pin system was used to re­ We had simulated the zero-gravity condition of duce the spin rate from 1.22 to 0.05 rpm in the space in a flow-tank test setup, but the pitch and first day. This system used X- and Y-axis coils that yaw angles had to be simulated separately. Were were energized proportionally to Y and X magne­ our ground calibrations correct? Had some last tometers, respectively, making the satellite behave minute changes disturbed our final alignments? like the armature of a DC electric motor and chang­ Had the vibration of launch changed some critical ing its spin rate slowly. angle? We were relieved and happy to discover af­ On October 31, 1979, the scalar and precision ter the boom extension that ATS signals in pitch, vector magnetometers were turned on to help ab­ sorb some of the excess solar-array power (even though the magnetometer boom was not yet ex­ 6 tended and the data would be useless for scientific purposes). Very little drift in B-axis attitude was observed during this time period. The aero trim 5 boom had been extended to 4.63 m as part of the initial stored command sequence. Even though this was shorter than the planned length of 5.30 m, the 4 yaw aerodynamic torques were well trimmed. E .J:: Until this time the attitude of the satellite was 1J, c 3 not under any form of continuous active control. ~ E The momentum wheel was running at a constant 0 0 speed of 1500 rpm, and the associated angular co • Telemetered data points momentum provided a form of "gyro" stability. Now the orientation of the satellite allowed the IR scanner to see the earth, and closed-loop pitch con­ trol was possible. At 1644 UT (Universal Time) on October 31, the attitude control was changed by command from the constant wheel-speed mode into 4 6 8 10 12 22 the active pitch-control mode. In the latter mode Time (min) the pitch angle of the satellite is detected by the IR scanner and if the pitch angle differs from the Fig. 3-Magnetometer boom extension on November 1, desired value, the wheel speed is increased or de­ 1979. The boom is extended by a screw drive at the base of the boom. Static friction is overcome and the boom ex­ creased to produce a reaction torque on the satellite tends rapidly, overshoots, and partially retracts - then in pitch, driving the pitch angle to the desired the process repeats. This accounts for the irregular ex­ value. At 1730 UT on November 1, the automatic tension process.

176 l ohll5 Hopkins APL Technical Digest yaw, and roll were immediately received! On No­ cumulated at "" 2000 bitsls during the previous 6 to vember 2 the three-axis gimbals at the base of the 7 hours. Before a playback was initiated the second boom were commanded to adjust the boom angles tape recorder was started so that no scientific data to bring all three ATS outputs into the linear range were missed during the playback itself. Doppler of the system. Subsequent boom deflections due to transmitters were on continuously and Doppler temperature changes are discussed on p. 203. It is data at 162 and 324 MHz were received by the important to note that the ATS signals were in their worldwide Tranet stations of the Defense Mapping nominal range continuously, a credit to the design Agency. These data were forwarded to APL where of the boom. the definitive satellite position versus time was pro­ duced by B. Holland. Position accuracy of better Star Camera Operation than 70 m root mean square was achieved in spite With attitude control fully established and the of the high drag condition of the Magsat orbit. magnetometer boom extended, the star cameras Observation of attitude drift indicated that we were turned on during the morning of November 2 had overcompensated for yaw aerodynamic torque and functioned normally. Figure 4 shows typical with an aerotrim boom length of 6.99 m. A series outputs of a star camera. The position of a star in of retractions brought the boom back to 4.48 m the field of view of the camera is recorded in terms where a very good trim condition was established. of its X I and Y I coordinates. While tracking a Figure 5 shows the drift track of the satellite B axis star, the Y I voltage stays relatively constant and from November 22 to December 3. The B axis drifts in a small clockwise circle about 1.5 ° in the X I voltage increases steadily, which is expected because the satellite rotates slowly about its B axis. At a rate of "" 4 ° /min, a star would seem to move across the field of the star camera in "" 2 min Note the curvature of these tracks, resulting from nutation because the star camera field of view is 8 by 8 ° . However, the star camera was programmed to

Volume I, Number 3,1980 177 diameter in each orbit. The circle gradually drifted 600 about in a large counterclockwise elliptical path whose center was displaced about 4 0 above the or­ 500 bit normal. We believe this apparent tendency to circle about an orientation offset from the orbit E 400 ~ normal was due to the rotation of the earth's at­ Q) "0 mosphere - a predicted effect. However, subse­ ..,~ quent drift tracks have not been as simple and the « 300 cause is uncertain. It is important to note that hardly any magnetic torquing was necessary to 200 maintain the satellite attitude except when the satellite came into lower altitudes and encountered 100 higher aerodynamic forces. 265 315 0 50 100 150 200 (1979) (1980) Orbit Decay Day number

Figure 6 shows the actual decay of the apogee Fig. 6-Magsat orbit decay. The decay was slower than and perigee of the Magsat orbit versus time as predicted because the density of the atmosphere was compared with an initial prediction made shortly less than expected. The satellite reentered on June 11, after launch. Magsat remained in orbit longer than 1980. predicted, allowing more data collection when the magnetic field was quiet. However, there was a analyzed it became apparent that the battery disadvantage because the low altitude data were capacity had dropped to about 12% of its nominal delayed, to the extent that eclipsing of the satellite capacity of 8 .ampere-hours. This loss in capacity by the earth in April 1980 had deleterious effects has been ascribed to the "memory" effect asso­ on power, so that duty cycling of subsystems was ciated with shallow discharging of nickel-cadmium necessary. batteries (which occurred prior to the eclipsing period) and also to simultaneous exposure to ele­ Vector and Scalar Magnetometers vated temperatures. The reduced battery capacity caused operational The scalar magnetometer data were noisy from problems. During each orbit various subsystems the start, probably because of lamp instability were commanded OFF prior to eclipse and ON after problems similar to those encountered before eclipse. In spite of this effort, a low battery voltage launch. However, the 20 to 40070 of the data points condition occurred on April 17. The battery voltage that were valid have been used to make final ad­ dipped below 13.2 V and the satellite automatically justments in the calibration of the vector magne­ went into a self-protective mode, which included tometer to achieve a correlation of 1.2 nT rms in turning off the gyro. The attitude control system the total field as measured by the two instruments. responded by going automatically into another (More detail on these devices may be found in the pitch control mode that does not require gyro in­ Farthing and Acuna articles in this issue.) put. Several hours later, the satellite was restored Star Camera Operation to normal operation by commands from the ground. A similar event occurred in May. Loss of star camera data for time periods of 30 In early June, the satellite altitude decayed to to 40 minutes was observed beginning in early "'" 240 km and eclipse times of 35 minutes were ex­ November. It occurred only in the southern hemi­ perienced. It was necessary to turn off the scalar sphere and is believed to be caused by sunlight fall­ magnetometer, the star cameras, and the ATS to ing directly on the sides of the sunshades and pene­ save power. The vector magnetometer data were trating the black plastic skin of the sunshade. As acquired until a few hours before reentry. Since the expected, this phenomenon shifted from the south­ star cameras were off, the vector data cannot be ern to the northern hemisphere in March and April used to determine the field direction. However, the as the sun moved north. three components will be combined to form the magnitude of the field, and those data will be ana­ FINAL EVENTS lyzed for evidence of geomagnetic anomalies. On April 12, 1980, eclipsing of Magsat by the Reentry of the satellite occurred at 0720 UT on earth began, as predicted. The loss of solar array June 11, 1980, in the Atlantic Ocean. The satellite power during the eclipse meant that the satellite probably vaporized from the heat of aerodynamic systems were entirely dependent on the stored friction. However, the momentum wheel (made of energy in the nickel-cadmium battery. The battery tungsten) may have survived. Some mariners of voltage dropped much more rapidly than we ex­ future civilizations may be puzzled at the strange pected during the eclipse. After the data were " anchors" we used.

178 Johns Hopkins APL Technical Digesl WALTER E. ALLEN

THE MAGSAT POWER SYSTEM

The Magsat power system was required to generate, store, and condition energy for use by all spacecraft systems. It consisted of a solar cell array, a rechargeable battery, redundant battery charge regulators, and several converters, inverters, and regulators to condition power for use by Magsat loads.

SOLAR CELL ARRAY and the sun-earth line. Figure 1 illustrates the pre­ An array of silicon solar cells mounted on four launch predicted power available at the battery as a deployable panels generated electrical power for the function of this angle. Magsat spacecraft. Each panel consisted of two in­ BATTERY terhinged, curved segments that were designed to be folded inside the rocket heat shield and re­ The spacecraft battery consisted of 12 series­ strained against the side of the base module during connected 8 A-h (ampere-hours) nickel-cadmium launch by a thin-wire despin cable. After the spin­ cells operating at a nominal voltage of 16.7 V. The stabilized last rocket stage was fired, a special pur­ battery supplied energy to meet brief demands in pose timer activated pyrotechnic devices that re­ excess of the generating capability of the solar ar­ leased the despin weights and cables. This action ray and operated the spacecraft during transient caused the spacecraft to despin while simultaneous­ phases of orbit acquisition and attitude adjustment. ly allowing deployment of the spring-loaded panels. It also sustained operation of the spacecraft during The deployed panels formed a planar cruciform ar­ periods of solar eclipse that occurred late in the ray, with the long axis of each panel perpendicular mission lifetime. to the spacecraft B axis. Each of the eight array segments was a curved, 180,-----,-----,-----,-----,----.-----. lightweight, aluminum-honeycomb structure ap­ proximately 64 cm long by 36 cm wide. Two of the eight segments flown were from another spacecraft 160 program and had 2 by 2 cm silicon solar cells on both sides . The remaining six segments were of more recent manufacture and had 2 by 4 cm, high­ 140 efficiency silicon solar cells on one side only. All segments were configured with two circuits of 57 ~ series-connected cells in order to permit effective ~ 120 o recharge of the spacecraft battery. c. Opposing panel pairs in the array could be in­ >- ~ dependently rotated about their common axis, if ro ro 100 desired, by ground command. Rotation to any o (f) angle from 0° (the normal value at deployment) to 90° could be effected. A separate synchronous 80 motor drive was employed aboard the spacecraft to perform this function for each axis. The feature could have been used to optimize power generation 60 by the array if the spacecraft had assumed some anomalous attitude. The Magsat attitude control system was designed to maintain the B axis of the spacecraft nearly perpendicular to the sun-synchronous orbit plane. Angle '" (deg) This resulted in relatively close alignment of the Fig. 1-Power output of the Magsat solar array as a sun-earth line and the B axis of the spacecraft, and function of angle if; between the + B spacecraft axis and full orbit sunlight illumination throughout most of the sun-earth line. Variations in instantaneous array the mission lifetime. Therefore, the power gener­ power output illustrated here result from array shadowing by both the base and instrument modules. These losses ated by the solar array was largely a function of were evaluated with the assistance of a 3/B·scale model the angle if; between the + B axis of the spacecraft of the spacecraft.

Volume 1, Number 3,1980 179 BATTERY CHARGE CONTROL A block diagram of the subsystem is shown in Fig. Since the Magsat battery was to be exposed to 3, and its salient characteristics are provided in extended periods of continuous overcharge, con­ Table 1. siderable design effort was expended to provide a Table 1 charge control system that would permit rapid recharge of a depleted battery and would limit POWER SYSTEM CHARACTERISTICS overcharge to levels that were electrically and ther­ mally acceptable. The power system therefore in­ Power available 120to 163 W, average corporated a lightweight shunt regulator of special Main regulator capacity 4.0 A, maximum design to dissipate excessive solar array power as Unregulated bus voltage 18.6 V, maximum heat (up to a maximum of approximately 120 W) 16.7 V, nominal on three externally mounted radiator panels. 15.0 V, minimum expected The dissipative shunt regulator and its control (circuit breaker actuates at element were fully redundant. The inputs to the 13 .2 V) control element were bus voltage, battery tempera­ Regulated bus voltage 13.5 V ± 20/0 ture, and a voltage proportional to battery current. Charge control system Redundant shunt regulator Depending on the operating mode selected (either provides trickle charge andl fixed-rate trickle charge or voltage-limited charge), or temperature compensated the controller exercised active elements in the voltage limit. Shunt capacity: regulator to shunt unwanted battery current. The 120W electronic elements (shunt drivers) were mounted on an external panel on the + B axis of the space­ Battery 12 series-connected 8 A-h craft. The resistive elements were distributed on ex­ cells. Battery cell manufac­ ternal radiators mounted on the sides of the base turer: SAFT America, Inc. module. The selectable voltage limit charge control Solar cell array type Four panels in rotatable co­ characteristics are shown in Fig. 2. planar pairs. Each panel We anticipated that ground controllers would consists of two inter hinged primarily employ shunt regulator No. 1 in the volt­ segments. age-limited mode to permit rapid recharge of a de­ Number of cells 1824, 2 x 2 cm pleted battery while limiting maximum battery volt­ 1368, 2 x 4 cm age to a conservative value. The trickle charge Solar cell type Centralab 10 n-cm, Si, NIP mode, a viable one for noneclipsed orbits, was con­ cells,2 x 2 cm, and sidered a secondary operating mode. The trickle Spectrolab 10 n-cm, Si, NIP charge limit for both shunt regulators was 250 mAo cells,2 x 4 cm Number of cells in series 57 17.2 r---,---,------,------,------,---, Coverglass type 6 mil microsheet with anti­ reflective filter applied by Optical Coating Labora­ tories

SYSTEM OPERATION UNDER ABNORMAL CONDITIONS

~ The Magsat power system was designed to ac­ :§ 16.9 commodate malfunctions within the spacecraft OJ automatically. A low-voltage sensing system, acting L'l'" o as a circuit breaker, controlled all commandable > og 16.8 relays that supplied power to noncritical loads. If ..0 C the main bus voltage fell below 13.2 V (which in­ ·ro ~ dicates a depleted battery), the system automatical­ 16.7 ly took remedial action as indicated in the box on the next page. Ground controllers were required to reset the desired spacecraft power system configu­ 16.6 ration and to reenable the system to function by command, upon recovery from an undervoltage fault. 16.5 '------'----'------'------'------'-----' 22 24 26 28 30 32 34 Battery temperature eCI ORBITAL PERFORMANCE Fig. 2-Voltage limit of the Magsat main bus as a func· The Magsat power system performed satisfacto­ tion of battery temperature. rily. The output power of the solar array exceeded

180 Johns Hopkins APL Technical Diges{ Auxiliary bus I LVISS I J Instrument Main bus J +X r ~module J J interface I I ~..J o -x

~ ~ + y ~-+-.....

- y

TLM TLM

\l - Relay controlled by ground command L_ TLM - To telemetry system

Fig. 3-Block diagram of the Magsat power subsystem. The loads are divided into critical and noncritical categories. Critical loads are defined as those that are essential to mission success and that must be powered continuously. The critical power system loads are the command system, selected attitude control system elements, and instrument module thermal control. A" loads derive power from the main power bus. The solar array is equally divided between the main bus and an auxiliary bus that is normally connected to the main bus when the noncritical loads are powered. the predicted value by approximately 10070. This panel illumination enhancement from earth albedo, was attributable to the seasonal increase in solar il­ and sunlight reflections from the highly reflective lumination intensity at the time of observation, thermal control surfaces of the base and instrument modules onto the solar panels. We believe that re­ flections are the principal contributor to array Actuation of the Low-Voltage Sensing System power enhancement. Main bus voltage drops below 13.2 V: The orbital operating temperatures of elements in Cause I-Insufficient solar array generating capability (most probably attitude related). the Magsat base module were somewhat higher Cause 2-System electrical overload. than anticipated. In an attempt to minimize tem­ Cause 3-Shunt regulator overload. peratures in the base module, the battery had been Cause4-Battery failure. primarily operated in the OR mode with the battery OR System Response: diode not bypassed. mode operation, as the term suggests, permitted either the solar array or Action I-Noncritical loads are turned off to conserve power. the battery (or both) to supply power to the main Action 2-The auxiliary solar array is connected directly bus through their respective "one-way" blocking to the battery to effect a rapid battery diodes. System operation in the OR mode prevented recharge. The battery is simultaneously oRed to the battery from being charged but had the positive the main bus as a power source by the battery effect of reducing internal power dissipation in the diode. Action 3-The controller of the shunt regulator is base module by approximately 5 W, thereby lower­ automatically switched to the voltage limit ing temperatures. The battery was discharged brief­ mode. ly during recovery of tape-recorded data, and con­ Action 4-Shunt regulator 2 instead of I is switched on tinuously by internal self-discharge mechanisms. As (Fig. 3) . This also permits the main bus to achieve the slightly higher voltage limit. a result the battery required periodic recharge in order to maintain the necessary reserve capacity to If the low voltage condition results from cause I or 2, ac­ tion I will be helpful. If it results from cause 3, action 4 is meet anticipated storage demands and to cover helpful. If it results from cause 4, action 2 will be helpful. contingencies. In the case of cause I, actions 2 and 4 will help restore the In order to recharge the battery in orbit while battery to a charged condition and permit the determination operating in the OR mode, the battery diode was of the particular subsystem causing the overload. typically bypassed for one orbital revolution every

Volume 1, Number 3, 1980 181 fifth day. Figure 4 illustrates the telemetered bat­ ment module temperatures, The battery charging tery performance data obtained during recharge on current was tapered as the selected instrument tem­ Day 316, 1979. Small step jumps in the data are at­ perature was reached, The battery charge regulator tributable to the granularity of a digital telemetry was activated; it shunted excess current to maintain system. Relatively large perturbations in initial bat­ the appropriate bus voltage throughout the re­ tery charge currents were caused by load variations mainder of the charge period. that occurred as heaters were automatically Particularly noteworthy in Fig. 4 is the change in switched on and off to maintain selected instru- Magsat battery cell temperature during recharge. During the early part of the charge cycle, the tem­ peratures of cells 3 and 9 dropped because of the ~ 3 5r--.--.--.---'--'--'--'---'-~--'--' endothermic nature of the chemical reactions tak­ i'l '" Cell 9 ing place during recharge. During the last 40 min­ ~ 34 ·-I.. __ '\.. ___.1--_.1-- '" utes of the charge cycle, temperatures began to ~ u Ce ll 3 --- ;"--33 "- '\" _ _\. rise, indicating the onset of overcharge. ~ --- All telemetered performance parameters of the Magsat power system were nominal, including load currents and converter output voltages. The main bus voltage was closer to 16 .5 V than to the pre­ dicted 16.7 V. This was a direct result of the some­ what higher than nominal operating temperature of i~1 f~ "'," ,: ,:. ,.~.~ ,-- ,0 ' . -(' . ~r1 l the base module. 1 A significant loss in battery capacity was ob­ served late in the Magsat mi ssion lifetime. This loss is believed to be attributable to sustained high tem­ " I I6 ~V I J j perature operation, As a result of this capacity i4:/ loss, operation of the spacecraft proved somewhat 14 0020 0040 0100 01 20 0140 0200 problematic during periods of solar eclipse; selected T ime (UT, min) loads had to be switched off briefly during these Fig. 4-Magsat battery 100 minute charge profile, Day periods in order to reduce the electrical load im­ 316,1979. posed on the battery.

182 Johns Hopkins A PL Technical Digesl ARK L. LEW, BRUCE C. MOORE, JOHN R. DOZSA, AND RONALD K. BUREK

THE MAGSAT TELECOMMUNICATIONS SYSTEM

The Magsat telecommunications system, consisting of the command and telemetry subsystems, performed the command, control, data-gathering, and transmission functions between Magsat and the NASA ground stations.

INTRODU CTI ON checked for validity, and distributed to the users, The design of the Magsat telecommunications either as relay drive pulses from the pulse com­ system was based in part on using hardware from mand matrix or as relay contact closures from the the Small Astronomy Satellite (SAS-3) wherever power switching unit. Command sequences could possible. However, the telemetry modulation con­ also be loaded into the command processor for de­ trol and tape recorder interface electronics were layed command execution at times when the satel­ new designs. In addition, the need to accommodate lite was out of range of a ground station. The com­ the new NASA phased shift keying (PSK) uplink mand subsystem was completely redundant and was format, together with other considerations, led to a designed so that a single failure could not prevent new command processor design, supported by ex­ commands from being processed. isting SAS-3 power switching modules and com­ The telemetry subsystem received data from all mand DC/ DC converters. Figure 1 illustrates the ma­ of the spacecraft systems, formatted the informa­ jor telecommunications hardware functions and in­ tion into a single serial digital bit stream, and dicates the primary signal flow paths. routed it to the transmitter and to the two tape The NASA standard transponder and associated recorders. Redundant precision crystal oscillators diplexers, couplers, and antennas were shared by and digital clock divider logic in the telemetry sub­ the command and the telemetry subsystems. From system provided timing signals for other spacecraft each command receiver, the bit stream from the systems. The telemetry subsystem also had a data command decoding unit went to the command pro­ interface that allowed the command processors to cessor, where the commands were decoded, preempt the entire telemetry frame for verification

Subsystems Pulse NASA standard interface commands transponder Receiver Command coding unit Relay outputs Transmitter

Telemetry inputs Tape>recorder Telemetry transponder (SAS-3) interface Signal outputs

Transmitter Fig. 1-Magsat telecommuni· Command coding unitl-----I~ Relay outputs cations system. Receiver NASA standard transponder Subsystems Pu Ise interface commands Government furnished equipment New APL designs

Volume I, Number 3,1980 183 of the stored command sequences before execution SHORT DATA (24 data bits), or LONG DATA (memory was begun. load) commands. Delayed command sequences were extracted from the working random-access­ COMMAND SUBSYSTEM memory space and loaded into a designated de­ Since the introduction of the first integrated cir­ layed-command random-access-memory space. The cuit controllers in the early 1970's, microprocessors delayed-command storage area could accommodate have matured to the point where the design of up to five different sequences totaling 82 com­ complex spacecraft electronics must consider the mands; however, each delayed-command load was alternatives of microprocessor-based versus random limited to a maximum of 31 commands. For each logic design. Trade-off evaluations resulted in the load, an automatic telemetry readout was generated selection of a microprocessor-based design for the for ground verification before sequence execution Magsat command subsystem. I This new design rep­ was started with an epoch set command. The de­ resented the first in a new generation of command layed commands were handled by two flags (for subsystems, offering expanded capabilities and in­ preservation of delayed-timing integrity) within the creased flexibilities in real-time and delayed com­ polling routine. manding operations for APL satellites. A block diagram of the command subsystem is Subsystem Hardware shown as part of Fig. 1. Except for the S-band an­ The considerations that led to the decision in tenna, it incorporated full addressed redundancy. favor of independent, redundant command chan­ Two receivers detected the PSK RF uplink and nels were reliability, the reliance of other subsys­ delivered command data to each respective com­ tems on the command subsystem for configuration mand processor for execution of the following and control, and their use as a troubleshooting tool functions: RELAY, SHORT DATA, LONG DATA, and with the telemetry subsystem. Both channels had PULSE commands. All command functions were ex­ equivalent command capabilities so that a failure in ecutable in real time or on a delayed basis. Addi­ either channel would not impair any command tionally, each command processor was programmed function. Within each channel there existed a mea­ for limited semiautonomous operations with the sure of circuit redundancy to reduce the probability telemetry, power, and attitude subsystems. of executing erroneous commands. Reinforcing the Multiple command execution was an important hardware redundancy were numerous software feature of this new design. Figure 2 shows the up­ checks on the command message that had to be link command message depicting a real-time com­ successfully verified before command execution mand sequence that must be limited to a maximum could proceed. of 2048 bits (the equivalent of 32 real-time com­ Each" channel consisted of a receiver, a proces­ mands). Within the microstructure of each com­ sor, and the power switching electronics. The re­ mand, a truncated Hamming code byte was used ceivers are discussed in the transponder section. for error control. The command processor was implemented with Both command processors processed the uplink the RCA COP 1802 family of circuits consisting of command message; however, only the selected pro­ the COP 1802 microprocessor, COP 1852 interface cessor executed the desired command once all in­ circuit, COP 1822 lK x 1 random access memory, terlocks were enabled. Both processors would nor­ and HA 6611 256 x 4 programmable read-only mally be in the main general polling routine shown memories . Complementary metal-oxide semicon­ in Fig. 3. The routine sampled flags according to a ductor (CMOS) support circuits were utilized for prescribed priority to determine the desired com­ power considerations. The COP 1802 is the large­ mand operation: real-time commands, low battery­ scale-integration CMOS, 8-bit register-oriented cen­ voltage commands for load minimization, delayed tral processing unit with circuitry for fetching, in­ commands, spacecraft attitude maneuvers, and terpreting, and executing instructions in external automatic transmitter power turn-off. The routine memory and generating input-output control sig­ could be interrupted for the loading of a real-time nals. command message bit stream by the INTRCDLD sub­ Each command processor contained four elec­ routine. Upon sensing a real-time hardware inter­ tronics boards that were interconnected via flexible rupt from the command decoder unit, the com­ cabling, as is seen in Fig. 4. The four boards mand sequence was loaded into a working area in folded in accordion fashion and were housed in a the random access memory for processing by the magnesium casing. general polling routine. All real-time commands The centralized power switching modules were were processed similarly for either RELAY , PULSE, housed separately. In addition to relay contact out-

1---128 bits ·1· SYNC word .1. 2048 bits ·1· 16 bits ---1 1 0 1 0 1 0 -- - 1 0 1 1 1 1 0 0 1 0 F 1 F2 F3 FM 1 0 1 0 1 0 --- 1 0 Fig. 2- Command message stream.

FM = 64 bits command frame M ';; 32

184 J()hns H()pkins APL Technical Digesl Execute next real time command and reset real time flag at end

Execute low voltage sensing Reinitialize processor switch command

Execute No next delayed commands

No Execute next delayed commands

No Execute attitude no. 1 command

No Execute attitude no. 2 command

No Execute attitude no. 3 command

No Execute Fig. 3-Magsat command; transm itter off general polling scheme. command

puts, the two relay matrices also provided for low Subsystem Software voltage sensing switch operations. A low voltage sensing circuit external to the command system There are four routines for each command pro­ provided a pulse to selected relays to switch off all cessor, labeled as follows: INGENPOL, CMDEX, nonessential loads upon sensing low battery volt­ INTRCDLD, and STORED CMDS. INGENPOL performed ages. The same pulse was also routed to each com­ all initialization operations and continually exe­ mand processor to initiate the low-voltage stored cuted the general polling routine during the back­ commands. Functions not accommodated by the ground mode (Fig. 3). Within each loop the CMDEX SAS-3 power switching modules were assigned to routine was called to process command frames ,in­ the pulse command matrices (board 4 of Fig. 4). dividually after each real-time command load.

Volume I, Number 3,1980 185 Board 1 Board 2 Board 3 Board 4

Fig. 4-Command processor. Each of the four electronics boards is 5.9 inches square.

After servicing subsequent flags, INGENPOL again single-point failures, but there was much redundan­ invoked CMDEX to execute another real-time com­ cy in the hardware (such as two independent for­ mand until all real-time commands were executed. mat memories). In addition, the recorder-trans­ Real-time command sequences were loaded into ponder interface was completely redundant. The in­ each processor's random access memory by inter­ terface unit required two standard welded wire rupting INGENPOL with INTRCDLD. The STORED boards of circuitry that fitted into the space that CMDS portion of the firmware consisted of stored became available by eliminating the variable format commands that were executed via software for generator. Pertinent characteristics of the telemetry spacecraft operations. electronics are summarized in Table 1. The entire command firmware required 2.8 kilo­ bytes of programmable read-only memory. Soft­ Table 1 ware for this project was developed with the aid of the APL 1802 cross assembler. TELEMETRY CHARACTERISTICS TELEMETRY SUBSYSTEM Bit Rate (b/s) 1953 Split phase PCM on 640 kHz subcarrier For Magsat, the flight-qualified spare telemetry 312,000 Tape recorder playback on baseband "books" from the SAS-3 spacecraft were modified and requalified. Some of the flexibility designed in­ Frame to the SAS-3 telemetry subsystem 2 was not re­ Size quired; this allowed some hardware to be deleted. Minor frame: 960 bits The changes included deleting the variable format Major frame: 256 minor frames generator and its memory, the variable bit rate, Rate (s) and the transmitter and recorder interfaces, and Minor frame: 0049 changing the time code generator to a major frame Major frame: 125.8 counter. Also, the programmable read-only memo­ Subcommutators ries that contained the SAS-3 fixed format program Number: 4 analog, 4 digital were removed from the format generator board Length: shortest, 4 channels; and replaced with programmable read-only memo­ longest, 256 channels ries containing the Magsat format. The relative ease with which the Magsat frame format was ac­ Size (cm) commodated illustrates the flexibility inherent in a Main telemetry hardware 16.0 x 18.3 x 35 .6 programmable controller like the SAS-3 format Crystal oscillators lOA x 13.0 x 15.5 generator. Dual power converter 16.0 x 18.3 x 5.1 Since the interfaces to the transmitter, the NASA Weight (kg) standard tape recorders, and the SAS-3 spare te­ 7.7 (excluding NST* and tape recorders) lemetry hardware were all predetermined and were Power (W) not mutually compatible, a section of interface 9 (excluding NST* and tape recorders) electronics had to be designed to interconnect them. The recorder-transponder interface represents the only new hardware design that was required for 'NASA standard transponder the telemetry subsystem. It provided the signal cross-strapping and m~de selection that allowed data from either tape recorder to be played back S-BAND TRANSPONDER via either of the transmitters. The telemetry sub­ The S-band transponders used on Magsat were system as a whole was not completely immune to the NST - near earth type. They served as com-

186 Johns Hopkins APL Technical Digest mand receivers, phase-coherent ranging trans­ tical commutation. Housings were pressurized by ponders, and telemetry transmitters. Each trans­ nitrogen to protect the recorders from the vacuum ponder consisted of a double-conversion, phase­ of space. Table 2 summarizes other tape recorder coherent receiver and a transmitter that was in­ parameters. tegrally related in both frequency and phase with the receiver. Table 2 The transponder operated on a 2102.723 MHz uplink received signal and produced a 2283.5 MHz SALIENT CHARACTERISTICS OF THE MAGSAT TAPE downlink transmitted signal. The phase-locked RECORDERS downlink RF carrier signal was precisely 2401221 times the uplink received RF signal frequency when Record speed 0.55 cmls operated in the ranging mode. On ground com­ Playback speed 88.2 cmls mand, the transponder would provide a noncoher­ Tape 256 m x 0.635 cm x 25.4 /Lm ent downlink RF signal frequency. In this nonrang­ Power consumption 9 W record, 25 W playback ing mode of operation, the transponder operated Weight 7.85 kg strictly as a command receiver and a telemetry Size 35.4 x 22.9 x 15 .5 cm transmitter that were independent of each other. In the absence of an uplink RF carrier signal, the downlink RF carrier signal was derived from a self­ PERFORMANCE contained oscillator. Operation and performance of the telecom­ MAGSA T TAPE RECORDERS munication system were highly satisfactory both during prelaunch and in orbit. For the prelaunch Tape recording the real-time telemetry data en­ qualifications, the command processor, power sured that no information was lost during the time switching, and telemetry electronics were exten­ the spacecraft was out of contact with a ground re­ sively tested in automated test configurations con­ ceiving station. Two recorders, manufactured by trolled by an SEL 32 computer. The spacecraft op­ Odetics Corp., recorded up to 9 X 10 7 bits each, erators found the versatility, in orbit, of the com­ over a 12.5 hour maximum period. Upon com­ mand processor in handling delayed commands mand, the accumulation of data was played back particularly useful. Successful execution of stored to the ground at 312,000 bls, which is 160 times commands was crucial to spacecraft operations the rate at which it was recorded. This took less during the first orbit when the spacecraft was not than 4.7 minutes, well within the 6 to 10 minute in view of a ground station. duration of a satellite pass .over a ground station. The reels of tape were stacked one above the other and contrarotated to minimize disturbances to the REFERENCES spacecraft attitude by recorder speed changes. To reduce angular momentum transients further and to tA. L. Lew, The Microprocessor Based MAGSA T Command Syslem, JHUlAPLCP077 (1980) . eliminate rewind time, the tapes were played out 2M. R. Peterson, "The SAS-3 Programmable Telemetry System," APL backwards so that the last datum recorded was the Tech. Dig. 14, No.4, pp. 7 - 13 (1975). first recovered. Each recorder had three capstans ACKNOWLEDGMENT - The authors wish to recognize the contribu­ coupled by redundant Kapton belts and was driven tions of H . Goodnight, D. Guynn, J. Kroutil, M. Peterson, D. StOtl, and by a servo-controlled brushless DC motor with op- G. Weaver in the production of this system .

Volume I, Number 3, 1980 187 KEVIN J. HEFFERNAN, GLEN H. FOUNTAIN, BARRY E. TOSSMAN, AND FREDERICK F. MOBLEY

THE MAGSAT ATTITUDE CONTROL SYSTEM

An account is presented of techniques used for orienting Magsat in the desired three-axis con­ trolled attitude in space. The pitch angle was sensed by an infrared horizon scanner and con­ trolled by reaction torques from an internal reaction wheel. Roll angle was also sensed by the in­ frared scanner and controlled by torquing the satellite with a magnetic coil that interacted with the earth's magnetic field. A small on-board computer assessed the roll error and computed the proper strength of the magnetic coil in order to correct the error, accounting for the actual magnetic field strength and internal angular momentum. No direct measurement or control of yaw was necessary.

INTRODUCTION tion to keep the roll and yaw angles from changing rapidly. However, the wheel momentum did not The first two Small Astronomy Satellites, SAS-l force the roll and yaw angles to return to zero. The and SAS-2, are good examples of previous APL at­ IR earth horizon scanner provided a pitch error sig­ titude control systems. A constant speed momen­ nal, and closed-loop control in pitch was achieved tum wheel provided open-loop gyro stabilization of by modulation of the wheel speed. A pitch-axis the Z axis in space (see Glossary, Fig. A and Ref. gyro system provided a measure of pitch rate and 1). X- and Y-axis coils were used to interact with was used to enhance the damping of the pitch con­ the earth's magnetic field to control the satellite trol system. In time the accumulated effects of ex­ spin rate. A Z-axis coil was used to precess the Z ternal pitch disturbance torques caused the wheel axis to the desired orientation in space. Nutation speed to drift away from 1500 rpm. When the devi­ damping was accomplished using a mechanical de­ ation reached a specified threshold, a magnetic vice to dissipate nutation energy in the form of torquing system was turned on to restore the wheel heat. Only the Z-axis orientation was controlled, speed to 1500 rpm. and that only by command from the ground. The IR scanner also sensed the roll angle of the The third Small Astronomy Satellite, SAS-3, had satellite. This information was used at four specific a somewhat more sophisticated attitude control points in the satellite orbit to decide whether cor­ system (ACS). An internal gyro system was used to rective action was necessary. If the roll angle ex­ sense the satellite spin rate, and closed-loop control ceeded a specified threshold (which could be set by of spin rate was achieved by modulation of the command in the range from 0 to 16°), the B-axis wheel speed to provide the necessary torque. The magnetic coil was turned on with the correct polar­ wheel also incorporated an IR earth horizon scan­ ity and strength to precess the roll error to zero. ner for closed-loop pitch angle control of the This calculation was done by a new electronic sys­ satellite. While this was not the desired flight mode tem called the attitude signal processor (ASP). It for SAS-3, we did have the opportunity to demon­ was a small on-board computer system using the strate its proper operation several times during the RCA 1802 microprocessor. The ASP was the four years of SAS-3 life. This form of control be­ "brains" of the ACS - it sampled the roll angle came the primary operating mode for Magsat. and computed the necessary B-coil strength based In Magsat, the satellite was three-axis controlled on the sampled strength of the X and Y magnetic to follow the local vertical axis system as the satel­ field components and the internal angular momen­ lite moved in orbit. The axis system makes one tum as indicated by the wheel speed. In addition, it complete rotation about the pitch axis during each decided when to initiate momentum dumping, se­ orbit. The angles describing the satellite attitude are lected the proper polarity, and shut off momentum called pitch, roll, and yaw, where pitch refers to dumping when the wheel speed reached 1500 rpm. nose-up or -down (B axis), roll is rotation about No direct measurement or control of yaw was the flight path axis (A axis), and yaw is rotation made, nor were they necessary. By controlling roll about the local vertical axis (C axis). at four points 90° apart in the orbit, the yaw angle The momentum wheel had its axis of rotation was automatically controlled. We relied on the fact aligned with the satellite pitch axis. The angular that yaw would not change by more than 1 or 2 0 in momentum of the wheel, spinning at approximately the 23 minutes during which the satellite moved 1500 rpm, provided a form of weak gyro stabiliza- from one roll sample point to the next. This de-

188 Juhns Hupkins APL Technical Digesl pended on an adequate ratio of internal angular Pitch Control momentum to external torque. Being able to ignore Maintaining pitch control of the spacecraft is yaw permitted a very important simplification in vital to the success of the mission. Loss of control system design. Yaw is difficult to observe directly for more than one orbit could prove di sastrous; the in a satellite; an inertial reference platform with motion of the spacecraft could cause the B axis to three-axis gimbals and gyros would be required, move away from the sun line far enough to reduce with a substantial increase in complexity and cost, the power-generating capability below that required and a loss in reliability. by the critical loads. As a result, five pitch-control SYSTEM REQUIREMENTS modes were designed into the system to be used during different phases of the mission or in failure Four requirements played a major role in deter­ situations. mining the design of the attitude control system. The ASP monitored a number of points as a These were: check on the system's ability to make correct con­ 1. Stabilize pitch, roll, and yaw to ±5° to orient trol decisions and, upon detecting a malfunction, the star cameras, sun sensor, and solar panels switched to a safe mode. A microprocessor bypass properly; mode was provided in the event of a microproces­ 2. Minimize angular rates and short-term motion sor failure. Gitter) to avoid boom bending and to ease the Figure 2 is a block diagram of the Magsat pitch analysis of the star camera data; control system. There were five modes, four of 3. Minimize the demand on the ground station which were closed loop. Relay position numbers in operators for rapid attitude control; and the diagram correspond to the mode numbers. 4. Minimize the amount of magnetic torquing activity because the associated magnetic fields Mode I: IR Scanner Pitch-Bias Mode with Gyro would degrade the fields at the primary scien­ Damping - This was the primary mode of the mis­ tific data sensors. sion. The IR scanner provided pitch angle feedback, SYSTEM DESIGN and a gyro simultaneously provided pitch rate feed­ back for damping purposes. The error in pitch A block diagram of the ACS is shown in Fig. 1. angle was reduced to zero by accelerating or decel­ Some of these elements are in common with the erating the momentum wheel. The equilibrium SAS-3 design described in Ref. 2, namely, the IR pitch angle could be offset from zero by a com­ scanner wheel, the redundant gyro system, the mand from the ground. This "pitch bias" could be B-axis coil used for orientation of the B axis, the X commanded to any angle in the range of - 10 to and Y coils and X and Y magnetometer and elec­ + 10°. We correctly anticipated that this feature tronics that make up the magnetic spin/ despin would be useful in seeking a pitch angle that had system, and the passive nutation damper. The ASP minimum pitch disturbance torque. and the yaw aerodynamics trim boom are new fea­ Mode 1 was designed with high damping and tures for Magsat. minimum bandwidth to provide a slow but smooth The control of pitch and the control of roll and pitch response and thereby minimize the pitch jitter yaw use quite different and independent schemes. motion. The system bandwidth was approximately Both schemes are managed and operated by the 0.01 Hz. ASP. Great care was taken in the design to eliminate

r-;:--=-=- =-= ~-3·axis~ -3--;-axIs'-~ --: - - II Attitude signal processor (ASP) : Provides automatic. : ••• .••.••••• ••• ~": II magnetometer =- coils' signal processing and control capability for various : Nutation dampmg. I 'I attitude control systems via a 5 kilobyte micro- ; system : " 'computer and associated circuitry. • • I ,-____ --, Momentum wheel : Provides angular momentum for ~.INutation damper/:.: I ~ YawbaOeOromtrim I spin axis stabilization and a torquing capability . - used to control pitch rate and/or angle. • • , I Infrared scanner (within the reaction wheet): detects • ••••_ ••••• ~.... II ______' '------1 infrared radiation above the earth and converts r- ---++ n this information to signals proportional to space- I, I~ II I I craft pitch and roll angles. •. Gyro: Provides a signal proportional to pitch rate. Orthogonal magnetic coils (3) , 3-axis vector magneto- ~ I meter, magnetic control book: Provide magnetic I I torquing capability for aligning spacecraft spin axis, controll ing pitch rates and dumping reaction I Gyro processor : I wheel momentum. a Momentum I Nutation damper: Dissipates nutational energy via I L~ °.!'l!.n!.u~ ~u~p~g_Sy~t~ _ wheel J I mechanical damping of a torsion wire pendulum. I Yaw aerotrim boom: Reduces frequency of roll/yaw I R scanner control maneuvers by providing in-flight balancing I I I of yaw aerodynamic torques. LRoli/yaw control syst~ _ _ _ _ I _ _ -.-J ...!'itchI control system______..-J

Fig. 1-Block diagram of the Magsat attitude control system.

Volume I, Number 3,1980 189 1, 3, 5

5 2

+

L..----~-~-----~--__lIR scanner J..+-.-'------.J

Microprocessor Analog signal processor fR scanner/wheel

I...... ------Attitude signal processor ------l.~1 1. I R scanner pitch-bias mode with gyro damping 2, I R scanner mode with pitch-bias 3. Pitch rate control mode 4. Constant wh eel speed mode 5. Mic ro processor bypass mode

Fig. 2-Block diagram of the Magsat pitch control system. any internal noise effects on system operation. The been used only in the event of a microprocessor pitch voltage, which was noisy due to a differenti­ failure and, due to loop gains, would have resulted 0 ating network in the IR scanner, was digitally in a steady state pitch angle of - 0.7 • All micro­ filtered at 0.02 Hz in the microprocessor. Gyro processor control and signal processing functions noise was measured and found to be negligible. would have been disabled in this mode. This was This mode was initiated approximately 24 hours the only pitch control mode that was not used dur­ following pitch capture and was intended to con­ ing the lifetime of the mission. tinue for the lifetime of the mission. Momentum Dumping Mode 2: IR Scanner Mode with Pitch Bias - This was a pitch angle control mode, again with a Momentum dumping was required to keep the 0 wheel speed in the range where it was capable of commandable pitch bias capability to ± 10 • However, the gyro was not required for this mode; producing reaction torque (1100 to 1900 rpm). it was a backup mode in case of gyro failure. Wheel speed changes were the result of external disturbance torques acting on the satellite. There Mode 3: Pitch Rate Control Mode - In this were two techniques available to achieve momen­ mode the pitch rate was controlled, rather than the tum dumping, pitch angle bias and magnetic spin/ pitch angle. The gyro signal was used as a measure despin torquing. The pitch angle bias was used to of the actual pitch rate, which was compared with allow aerodynamic, gravity-gradient, and other the desired rate (as loaded by command). This disturbance torques to offset one another, the accu­ mode was intended as a backup mode in case of an mulated effect being to minimize the overall exter­ IR scanner failure or a roll angle so excessive that nal pitch torques. The bias angle was determined the IR scanner did not see the earth. The desired from analysis by the satellite operators, and a com­ rate was set at 1 revolution per orbit. mand was sent to the satellite to implement the bias. Mode 4: Constant Wheel Speed Mode - In this The magnetic spin/despin system was used auto­ mode the wheel speed was held fixed at 1500 rpm matically under the control of the attitude signal and no control of pitch angle or rate was at­ processor. If the wheel speed deviated from 1500 tempted. The system was operating in this mode rpm by more than a predetermined amount, say during launch and during the initial attitude ma­ 200 rpm, the ASP selected the proper polarity and neuvers before closed-loop control was established. turned on the magnetic spin/despin system. When the wheel speed reached 1500 rpm, the ASP turned Mode 5: Microprocessor Bypass Mode - As in off the magnetic torquing. the primary mode, both pitch angle and rate feed­ In order not to interfere with roll/yaw control, back information were provided. However, unlike this could only be done at specific times in the or­ Mode 1, this mode had no microprocessor interface bit. Figure 3 shows the orbit track about the earth and therefore no pitch bias capability or 0.02 Hz and the assigned points for various allowed activi­ filtering of the pitch signal. This loop would have ties by the ASP. Only at points Al and AS could

190 Johns Hopkins APL Technical Digesl _ - B coil declination correction angle provided a measure of the error in right - B coil right ascension correction ascension between the B axis and the orbit normal. _ - Spin/despin system may be on The earth's magnetic field is oriented north in this to dump momentum region. With knowledge of the roll error, magnetic A2 field strength, and wheel speed, the B coil could be energized at a specific strength and for a specific length of time to precess the B axis in right ascen­ sion to correct the error almost exactly. By energiz­ ing the B coil for an integral number of nutation periods (150 seconds each), residual nutational mo­ Al tion of the spacecraft was minimized. Two nutation AD periods were used. This action was initiated at points A 7 and AI4 (Fig. 3) if the roll angle ex­ A8 ceeded a specified threshold value. The torquing was terminated at A8 and A I, respectively. In a similar manner, spacecraft roll angle at the orbit poles is a measure of the declination error of the B axis from the orbit normal. Energizing the B coil when the earth's magnetic field is nearly paral­ Satellite orbit A9 (planar view) lel to the earth's equatorial plane would precess the Orbit B axis in declination and correct the observed er­ location Action ror. Figure 3 shows the orbit locations at which the AD Orbit ascending node roll angle was sampled (A2 and A9) and the B coil A 1, A8 B coil turned OFF after right was energized (A4 and All) if the roll angle ex­ ascension correction; wheel speed sampled, spin/despin system turned ceeded the threshold. ON if desired and necessary In the interest of minimizing B-coil activity, a A2, A9 Roll angle sample for declination correction threshold on roll error was chosen below which no A3, A6, A 10, A 13 Spin/despin system turned OFF to sample magnetometers B-coil activity would occur. This threshold was 0 A4, All B coil turned ON for declination correction normally set by command between 1 and 3 • if desired and necessary Computer analysis of the attitude dynamics A5, A 12 B coil turned OFF after declination correction showed that a significant yaw aerodynamic torque A7, A14 Roll angle sampled and B coil turned ON if desired and necessary for right ascension correction existed because the center of mass of the satellite was offset from the center of frontal area. This Fig. 3-0rblt locations of roll/yaw control and momentum yaw torque would have continually perturbed the dumping events. satellite, requiring frequent control activity. We therefore decided to balance the frontal area by in­ stalling a yaw aerodynamic trim boom. It was a momentum dumping be initiated. Momentum motorized boom made up of a silver plated berylli­ dumping had to be completed by A3 or AlO, um-copper tube 1.2 cm in diameter that could be respectively, or it would have been terminated in extended to any length up to 12 m. Our analysis preparation for possible roll control maneuvers. showed that about 5 m of extension would be re­ The magnetic spin/despin system worked in the quired to balance the yaw torques. This boom pro­ following manner: Voltages proportional to the X truded from the base of the satellite. and Y components of the earth's magnetic field were amplified and used to drive the Y and X coils, respectively. One of the signals was inverted. This Nutation Damping produced a net magnetic dipole that led or lagged The primary means of reducing satellite nuta­ 0 the earth's field by 90 • The result was a torque tional motion during the attitude acquisition phase about the satellite spin axis, the sense of the torque was the nutation damper. It consisted of a damped being determined by the ASP. pendulum pivoting on a torsion wire. The pendu­ lum's arm swung in a plane normal to the intended Roll/Yaw Control spin axis and oscillated when the spacecraft nu­ The roll/yaw control system was designed to tated. Damping in the pendulum movement dissi­ maintain nominal alignment of the B axis with the pated kinetic energy until all nutation ceased. orbit normal. The automatic roll/yaw control mode The predominant nutation damping mechanism took advantage of the system's ability to detect for the mission phase used product-of-inertia cou­ both declination and right ascension errors from pling between nutational motions and the pitch the roll output of the IR scanner and corrected control loop. Damping in the pitch loop simultane­ these errors by using an application of a B-axis ously damped nutations via the passive coupling. magnetic dipole at appropriate places in the orbit. Loop gains and phase shifts were analyzed to as­ For example, at equatorial crossings the roll sure stability. The nutation period was approxi-

Volume I, Number 3,1980 191 mately 150 seconds, and the damping time constant PERFORMANCE IN ORBIT was 40 minutes. Initial satellite activity following launch included the yo-yo despin, pitch-axis alignment with the or­ ATTITUDE SIGNAL PROCESSOR bit normal, satellite despin using the spin/ despin system, and acquisition of closed-loop pitch con­ The attitude signal processor (ASP) coordinated trol. Details of the initial attitude maneuvers are and controlled the activity in the attitude control given in the "Magsat Performance Highlights" ar­ system. It used the RCA 1802 microprocessor with 3 ticle in this issue. 4096 bytes of programmable read-only memory The satellite was commanded into the primary (PROM) and 1024 bytes of random-access memory pitch control mode on November I, 1979, and re­ (RAM). The 1802 microprocessor employed a four­ mained almost exclusively in that mode until June level individually maskable, priority-interrupt 10, 1980, the day before reentry. Attitude transfer sche:ne. The highest priority interrupt used a direct system outputs indicated a peak-to-peak pitch oscil­ memory access to enter commands to the ASP di­ lation of 1.5 arc-s at 0.6 Hz, well within the design rectly into the RAM. The computer was synchro­ specification. nized to the telemetry clock by an interrupt gener­ Momentum dumping operations occurred period­ ated every half second. ically during the first week of the mission. Pitch A command of 256 bits was required to initiate angle bias was used very successfully to minimize the ASP activity. This message contained such in­ magnetic momentum dumping for the majority of formation as the orbital period, the time when the the mission. However, toward the end of satellite satellite would cross the equator north bound, the life, magnetic momentum dumping became quite desired pitch mode, the pitch bias angle, the roll frequent as the aerodynamic disturbance torques threshold, the desired pitch rate, etc. These data increased. were stored in the RAM and used by the computer The yaw aerotrim boom was deployed to 4.63 m as it proceeded through its routine, which was before the initial satellite maneuvers and subse­ stored in the PROM. quently was extended to 6.99 m after the magne- One of the most important features of the micro­ computer was its diagnostic capability. The micro­ computer performed a "checksum" every half sec­ 8 r----.-----,----,-----~--_,----~ ond to verify that its embedded software had re­ mained static. Also it checked for certain "faults" in the IR scanner and the gyro system; if any were 7 detected, appropriate action was taken to change the pitch mode. Discrepancies in the command word were also detected and, depending upon the 6 error the command was either rejected or cor­ rected. Flags were generated for each fault both in a 128-byte telemetry output and in a special 4-bit 20 status word provided as a warning system for the ground operators. In the event of a momentary loss of the 5 V power supply, the destruction of each copy of ~he resident ASP command, or a program sequencmg 12 anomaly, the ASP would automatically restart, ini­ 3 tializing both the foreground and background por­ Target• tions of the firmware and placing the attitude con­ trol system in a safe mode. If the microprocessor had failed completely, a relay driven by the com­ 2 mand system would have placed the attitude con­ trol system in a microprocessor-bypass pitch con­ --Drift track trol mode and would have disabled all microcom­ --Magnetic cont rol puter control and signal processing capability. o /orbit normal The Magsat ACS was the first microcomputer­ based ACS built at APL and quite possibly the first o 6 satellite microcomputer that was programmed in a Right ascension offset (deg) high-level language (MICRO-FORTH). The 1802 mi­ croprocessor was chosen for the applicarion be­ Fig. 4-Magsat attitude activity from January 8 to 27, cause of its low power consumption, its compatibil­ 1980, showing drift of the + B axis in space. The roll ang Ie had to exceed a threshold of == 50 (with respect to ity with other systems on board, and its ~vailability target position) to initiate roll/yaw control activity. Lack in a high reliability version. The ASP weIghed 0.86 of activity was due to the aerodynamic balancing effect kg and consumed 0.95 W of power. of the yaw trim boom at the skewed target position.

192 johns Hopkins APL Technical Digest tometer boom extension. Careful observation of aerodynamic torque !1eeded to reduce magnetic tor­ the B-axis drift led to the decision to retract the quing operations. In addition, it provided the boom to 4.88 m, which resulted in a minimum of stable platform needed for arc-second-quality at­ B-coil magnetic torquing. Figure 4 shows the B-axis titude determination solutions and high-resolution drift over a 17 day period in January 1980, during magnetic field measurements. which only one roll/yaw control maneuver was re­ quired. REFERENCES and NOTES Nutational motion was reduced from 10 to 0.5 0 1 The B axis of Magsat is the same as the Z axis of SAS. The A and C axes are perpendicular to B. The X and Y axes are also perpendicular to in 12 hours by the mechanical damper prior to B but are rotated 45' from the A and C axes. magnetometer boom extension and was held to ap­ 2F. F. Mobley, P. Konigsberg, and G. H. Fountain, "The SAS-3 At­ 0 titudeControl System," APL Tecil. Dig. 14, No.3, pp. 16-23 (1975). proximately 0.07 peak-to-peak by inertial coupling ) A byte is an 8-bit word of data. damping. The entire ACS performed flawlessly in meeting ACKNOWLEDGMENT - The authors wish to recognize the special ef­ each of its requirements. It oriented the satellite for forts of T. Zaremba, D. Klein, and W. Schneider in designing the soft ­ optimum power generation and provided minimum ware for the ASP.

Volume 1, Number 3,1980 193 GLEN H. FOUNTAIN, FREDERICK W. SCHENKEL, THOMAS B. COUGHLIN, AND CLARENCE A. WINGATE

THE MAGSAT ATTITUDE DETERMINATION SYSTEM

The Magsat mission required knowledge of the magnetic field orientation with respect to a geocentric coordinate system with an accuracy of better than 20 arc-seconds. Attitude sensors were therefore incorporated into the spacecraft. The design, construction, and verification of these sensors as a system became a major task of the spacecraft development.

INTRODUCTION nT rss; for the maximum field strength anticipated (i.e., 64,000 nT) the angular error inferred was 14.5 The Magsat mission requirement for measure­ arc-so ments of the earth's magnetic field with an ac­ The design, fabrication, and verification of the 9 2 curacy of 6 nanoteslas (1 nT = 10. Weber/ m ) ADS was one of the major challenges of the Magsat root sum square (rss) per vector axis imposed a program. It is interesting, as a point of reference, stringent requirement on the spacecraft's attitude to note that an angular measurement of 1 arc-s is determination system (ADS) of 14.5 arc-s (0.00028 °) equivalent to measuring the eye of a needle at a rss. The major sources of error in making such distance of 164 meters! field measurements were the vector magnetometer, The Magsat ADS, shown schematically in Fig. 1, the satellite tracking system, and the ADS. The vec­ was a collection of sensors whose required accura­ tor magnetometer measured the magnetic field at a cies were a few arc-seconds. These accuracies had given position in a geocentric coordinate system as to be verified by ground tests where component determined by the satellite tracking network. The weight effects and thermal environments differed orientation of the field was determined by the slightly from the flight conditions. From these spacecraft attitude sensors. Initial estimates of the measurements the instrument alignment and accu­ errors associated with the vector magnetometer and racies in orbit were inferred. the spacecraft tracking system required the errors The ability to determine the attitude of satellites caused by attitude measurement to be less than 4.5 has been steadily improving. The Small Astronomy " "'khl;[/"

-A (roll, twist) ~-c - B Orientation axes

Fig. 1-The attitude determi· nation system consists of two star cameras mounted on an optical bench, which also con· tains an attitude transfer sys· Roll dihedral tem for relating the orienta· PlY ATS tion of the star cameras to the vector magnetometer. The sun sensor (and a rate gyro 10· Sun cated in the spacecraft) pro· sensor 6m vides additional information to allow interpolation between star sightings.

ma gnetometer

194 Johns Hopkins APL Technical Digest Satellites SAS-1 and -2 made measurements with If-o"'f------8.0 0 ------l,.~1 accuracies of a few arc-minutes. Large complex satellites such as the Orbiting Astronomical Ob­ servatory (OAO) made angular measurements of 1 arc-min. SAS-3 was the first small spacecraft to at­ s~rch deflection tempt to make measurements in the sub-arc-minute pattern range, but only after in-flight calibration. It was for that mission that a "strap down" star camera for high accuracy attitude determination was devel­ oped. Analysis of the in-orbit performance of SAS- X coordinate 3 indicated that the star cameras had the inherent --1 of star ~ accuracy to provide primary attitude data for Mag­ sat. 1 However, to meet the entire Magsat attitude l requirements, significant changes in the SAS-3 de­ Y coord mate sign were required, including better ground calibra­ of star tion, a more stable structure, and additional angu­ lar (or rate) sensing to interpolate between star cjJ 1 sightings. The three rotations chosen to describe the angu­ lar orientation of the vector magnetometer are roll, "-----,(f pitch, and yaw. As shown in Fig. 1, these rotations ~ are measured about a set of body-fixed axes that would nominally be aligned to the spacecraft velocity vector (roll), the orbit normal (pitch), and T0 the local vertical (yaw). For the sun-synchronous 0.3 orbit chosen for Magsat, the pitch (B) axis also pointed in the general direction of the sun. For I '- - 1 each of these axes, a detailed error budget was determined. The error budget established in No­ Track pattern vember 1977 for each axis had the form shown in Table 1. Fig. 2-When searching for a star, the camera field is searched in a left·to·right, right·to·left pattern. If an il· 1 Table luminated area is detected during the scan pattern, the camera will automatically initiate a small cruciform scan ERROR BUDGET FOR VECTOR MAGNETIC FIELD about the center of the illuminated area. If the il· MEASUREMENT luminated area moves in the field of view, the camera automatically recenters the cruciform scan and tracks it. arc-s nT The deflection coil currents required to keep the aperture Vector magnetometer 3.8 centered on the illuminated spot are direct measures of A ttitude determination the star coordinates. Star camera II Attitude transfer system 5 deflection current the entire area of the photocath­ Optical bench 4 ode was scanned for the presence of star images. Subtotal (rss) 13 4.1 Figure 2 shows the scanning pattern for the Magsat Satellite position error 1.7 cameras. Total error (rss) 5.8 The cameras were designed to detect stars as dim as 6.0 visual magnitude with a probability of 0.98. In fact the cameras were typically tracking stars as STAR CAMERAS dim as 7.2 visual magnitude. This guaranteed that The two star cameras that provide the primary several detectable stars were in the camera's field attitude determination data were built by the Ball of view at all times. Brothers Research Corp., based on the SAS-3 star The cameras were mounted on the spacecraft in camera design. Each camera consisted of a Super­ such a way that both swept the same circle on the Farron objective lens that focused an 8 by 8 0 star celestial sphere while the spacecraft rotated at one field onto the photocathode of an image-dissector revolution per orbit. This circle was 32 0 off a great tube. The image dissector consisted of a photomul­ circle to minimize the effect of both sun and earth tiplier with a small aperture in front of the first albedo interference. As each camera swept out its dynode and a gradient-free drift space between the circle, the stars appearing in its field of view were aperture and the photocathode on which the optical tracked for a specific time. The tracking time was image was focused. Deflection coils steered elec­ commandable between 4 and 128 s; for Magsat a trons from a given portion of the photocathode time of 30 s was selected. After the 30 s period (or surface through the aperture. By programming the loss of track caused by the star failing outside the

Volume I, Number 3, 1980 195 Compensation Compensation camera's field of view) , the camera was com­ detector reticle manded to break lock and search for another star. Main In this manner a "snapshot" of stars was located beamsplitter in the camera field of view . Light from objective lens Like most real devices the camera outputs were not truly linear or independent. As a result there were variations from an idealized linear response of Yaw detector several arc-minutes. The camera's ultimate ac­ Sp lit detectors curacy was achieved by calibrating these effects Beam image& during ground tests and removing them during in focal plane ~ post-flight data processing. Each observed position, once these effects were removed, was accurate to 10 arc-s or better with respect to the camera optical Pitch detector axis. ~Beam image ~ in focal plane

ATTITUDE TRANSFER SYSTEM Fig. 3-The light-emitting diode (LED) of the attitude The transfer of each observed star position to the transfer system generates an infrared signal that is vector magnetometer depended on the accuracy of formed into a square image by the LED mask at the focal plane of the objective lens. The source beamsplitter the attitude transfer system (ATS). The system, built diverts a small amount of the transmitted and received by the Barnes Engineering Corp., measured the energy to the compensation and automatic gain control relative orientation of the vector magnetometer detectors, which stabilize the sensor performance. The with respect to the star cameras via two optical majority of the returned signal is diverted to the pitch and systems that measured the rotational angles of yaw detectors by the detector beamsplitters. There the image formed by the LED mask is focused on the two pitch, yaw, and roll. Both optical systems used a silicon cells in each detector. Yaw motion causes the 125 Hz, 50070 duty cycle, modulated infrared beam beam image to move along the horizontal axis of each de­ at 930 nanometers to perform the optical measure­ tector, producing a change in the differential output of ment. This allowed both optical filtering and a syn­ the yaw detector while leaving the output of the pitch de­ chronous detection scheme to eliminate effects of tector unchanged. Pitch motion causes the image to move along the vertical axis of the detector, producing interfering light sources such as sun or earth change in the output of the pitch detector. albedo. The simplest system was one that measured the pitch and yaw angles of the vector magnetometer axis collimated beam that produced a pseudo-yaw with respect to the optical bench. The pitch/ yaw signal as a function of roll rotations. The roll opti­ transceiver generated at the transceiver exit pupil a cal system is illustrated in Fig. 4. A transceiver was 1.8 cm square beam that was reflected by the flat located on the optical bench but was displaced mirror mounted on the vector magnetometer plat­ from the roll axis. The collimated signal was trans­ form. The return beam formed a square image in mitted to a dihedral mirror on the magnetometer the focal plane of the transceiver 0.03 cm on a platform that reflected the beam to a second dihe­ side. When perfectly collimated, the image was dral mirror on the optical bench. From this base centered on two split detectors as is shown in Fig. dihedral mirror the beam was reflected back on it­ 3. Pitch or yaw rotations of the vector magnetom­ self to the transceiver. eter platform produced displacements of the image. In its simplest form the dihedral mirror on the The two detectors were oriented so that, to first magnetometer platform can be thought of as re­ order, the pitch sensor was insensitive to yaw mo­ flecting the transmitted image onto a circle whose tion and the yaw sensor was insensitive to pitch radius is defined by how far the transceiver is off motion. A 1 arc-s rotation in either pitch or yaw the roll axis. As the magnetometer dihedral mirror produced displacement of the image on the order rotates, the transmitted image moves around the of 1.3 x 10 - 4 cm, which generated an output in circle, producing a pseudo-yaw motion. In fact, the the appropriate split detector. The maximum two dihedral mirrors could be flat mirrors tilted at angular rotation, allowed to keep the pitch/ yaw the appropriate angles and produce the same result, system within its linear region, was ±3 arc-min. A but both pitch and yaw motion of the magnetom­ second constraint on the pitch/ yaw system was the eter would corrupt the roll measurement. The dihe­ allowable translation of the vector magnetometer. dral mirrors are insensitive to rotations about the If the flat mirror translated more than ± 1.9 cm, ridge line. Thus the dihedral mirror on the magne­ the collimated beam would be vignetted or possibly tometer platform effectively decoupled yaw from not returned to the pitch/yaw transceiver. the roll measurement and the decoupled pitch mo­ The roll system was much more complicated. It tion of the base dihedral mirror. was, in essence, trying to measure the rotation The transmitted roll image was focused onto (twist) about the line of sight of the optical system. another split detector in the transceiver that de­ The measurement was made by generating an off- tected the pseudo-yaw motion. In this case, how-

196 johns H opkins A PL Technical Digest Roll c

~~:;';:~4Sr---Dihedral ridge line I

c A periodic pattern reticle

B periodic pattern reticle

Light Imagey~h

A coarse reticle

sensor B coarse reticle B ./ "'Base dihedral mirror ------A

Fig. 4-The roll system transceiver generates a colli· Fig. 5-The precision sun sensor resolves the orientation mated beam that is reflected by the magnetometer dihe· of the sun line into two angles (ex and (3) in the sensor dral mirror. As the dihedral mirror rotates about the roll coordinate system by means of the A and B reticle axis, the reflected beam traces out a circle in the assemblies. The periodic patterns determine the ex and {:3 pitch/yaw plane. The base dihedral mirror causes the angles with a precision of 2 arc·s in a 2.2' repeating pat· reflected beam to be returned to the transceiver when the tern. The coarse reticles determine which one of the 2.2' magnetometer dihedral ridge line lies in the roll/yaw segments (in the 32' field of view) actually contains the plane. For small roll rotations, the returned image is sun line. translated parallel to the pitch axis (as a yaw rotation would produce in an autocollimator), which is measured by the split detector. ever, the pseudo-yaw motion for a 1 arc-s roll rota­ tion was only 1 X 10 - 5 cm. In addition, because the roll rotation appeared as a yaw motion, actual yaw rotations between the transceiver and the base dihedral mirror appeared as roll motion and, un­ fortunately, at an even more sensitive level than the true roll rotation. This put a very stringent require­ ment on the Magsat optical bench and required the rotation about the line connecting the transceiver and the base dihedral (yaw) in the plane of the bench to be less than 0.2 arc-s if such motion was to affect the roll measurement by less than 2.5 arc-so This effect was noted during ground calibra­ tion of the roll system. Measurements taken with the spacecraft in two configurations that reversed Fig. 6-The periodic pattern reticle assemblies consist of gravity-induced distortion in the optical bench pro­ two thin, fused, silica plates separated by a fused silica duced significant changes in the roll output. This spacer. Four photocells are located beneath the bottom 1 g bias was removed by the roll system calibra­ plate. A periodic grating deposited on the lower surface of the top plate has equal·width lines and spaces with a tion. grating period w. The bottom plate has four identical periodic patterns deposited on the upper sun·facing sur· PRECISION SUN SENSOR face, with the same grating period as the upper plate. However, each of these four lower patterns is displaced The precision sun sensor (see Figs. 5 and 6), by one-quarter of the grating period. When combined, manufactured by the Adcole Corp., was the only these four periodic patterns form a 4·phase filter for light primary attitude sensor that was sufficiently clean reaching the photocells beneath the four patterns.

Volume 1, Number 3, 1980 197 magnetically to be mounted near the vector magne­ OPTICAL BENCH REQUIREMENTS tometer. The sensor optical system resolved the angle between the sun line and a sensor coordinate For the ADS to function, the attitude sensors had system into two angles with a resolution of better to be connected by means of extremely stable struc­ than 2 arc-s and an accuracy of 12 arc-s root mean tures. The spacecraft optical bench (Fig. 7) and the square (rms) based on preflight calibration. The magnetometer platform were designed and built to sensor optics contained two reticle assemblies for hold the alignment of the system elements mounted each angle measurement. Each assembly consisted on them to deflections of 1 to 2 arc-s during orbital of one periodic pattern reticle and one coarse angle operations. In addition, they could suffer no align­ reticle. The periodic pattern reticle provided angu­ ment changes during the launch and prestabiliza­ lar information with 2 arc-s resolution over re­ tion phases of the mission. Severe weight con­ peated angular periods of 2.20. The coarse angle straints, in conjunction with the thermal, struc­ reticle determined which of the periodic patterns tural, and magnetic requirements, led to the choice the sun line was in over a range of ± 32 0 • of graphite-fiber reinforced epoxy (GFRE) for the The periodic pattern reticle acted as a 4-phase construction of both structures. Active temperature filter for the incident sunlight with a spatial distri­ control was necessary to meet thermal deflection bution of intensity. The position of the intensity objectives. The optical bench was attached to the distribution, and consequently the amount of light spacecraft by means of a kinematic mounting ar­ transmitted by the 4-phase grating to the photo­ rangement to prevent significant deflection caused cells, was a function of the sun angle. As the sun by spacecraft bending. angle moved in the plane perpendicular to the grat­ The selected material was superior to the other ing lines, the output of each of the 4 photocells materials (such as aluminum or beryllium) in varied sinusoidally with a period of 2.20 and with strength and stiffness ratios and in coefficient of each cell in phase quadrature. The sensor electron­ thermal expansion, and it is not magnetic. It has ics converted the photocell outputs to digital data. some unique properties that require special care but By combining the periodic pattern reticle outputs are tractable. GFRE consists of graphite fibers lying with the coarse pattern reticle outputs, which un­ in a single plane, held together by resin. The desir­ ambiguously determine the angles ex and (3 with a able properties of GFRE exist only in the fiber 0 resolution of 1 , the sun angle could be determined plane. Normal to the plane, the thermal properties with a resolution of 2 arc-so of GFRE degrade, approaching those of epoxy. By Like most other highly accurate sensors, small using an egg crate construction technique degrada­ imperfections in the system created nonlinearities in tion of the anisotropic properties was reduced. the actual sensor output. For the sun sensor, these Another factor considered was hygroscopicity. nonlinearities were caused by a number of factors Strains caused by water absorption by GFRE during such as the flatness of the grating substrates, the satellite construction and testing can approach 100 photocell spectral response, the skew (misalign­ {(m/m and can produce deformations far exceeding ment) between the input and output gratings, and those caused by thermal sources. Critical align­ the thickness of the grating patterns. Analysis by ments of bench-mounted components, performed the Adcole Corp. resulted in a transfer function de­ when the bench had a high moisture content, were pendent on higher order harmonics of the fine reti­ likely to change in orbit as the moisture was re­ cle period that reduced the errors to the level of 12 leased in vacuum. This undesirable property was arc-s rms. reduced to tolerable levels by the application of a moisture barrier to the external surfaces of the op­ tical bench. The moisture barrier consisted of 0.017 mm aluminum foil bonded to the GFRE. A comparison of the critical alignment goals and the expected thermal deflections of the satellite structure when in orbit indicated that the bench should be attached to the structure in a manner that would not introduce bench bending. This anal­ ysis led to a kinematic arrangement for attaching the bench to the spacecraft. Of the three attach­ ments, one restricted all three translational degrees of freedom and the other two restricted the three rotational degrees of freedom without introducing additional translational constraints. Fig. 7-The Magsat optical bench shown here was fabri· Even with the use of GFRE, the requirement for cated by the Convair Division of General Dynamics. The temperature control was reasonably stringent. For inclined planes on the top surface hold the star cameras. the optical bench, which was 3.8 cm thick and had The ATS components are mounted on the underneath side as illustrated in Fig. 1. The silver-like surface of the struc­ a maximum separation between any two compo­ ture is the aluminum moisture barrier. nents of 66 cm, a temperature gradient through the

198 fohns Hopkins APL Technical Digesl bench of 0.5°C would produce a deflection of 1 stitute of Technology's Center for Space Research, arc-so In addition to controlling the bench tempera­ based on the calibration data furnished by the Ball ture, -it was necessary to maintain each of the Brothers Research Corp., were verified. The cali­ bench-mounted components at a constant tempera­ bration of the ATS system related the sensor out­ ture. The basic approach taken was to prevent puts to the angular relationships between the space­ large temperature gradients by minimizing heat craft optical bench reference cube and a reference flow within the bench. The bench was first en­ cube on the vector magnetometer (see Fig. 1). The closed in its own multilayer thermal blanket within alignments between the precision sun sensor and the spacecraft to reduce heat transfer by radiation the vector magnetometer were also measured at this to its surroundings. Second, the five components time, but no verification of the Adcole calibration and three mounting points that must penetrate the of the sun sensor was performed. blanket were heated to 25.0 ± 0.2°C. Third, ther­ The flexure induced in the bench by the weight mal connections between the bench and the heater­ of the components was measured by making all controlled surfaces had relatively high thermal re­ alignment measurements with the spacecraft B axis sistance. both up and down (i.e., ± 1 g). The measurements The three spacecraft attachment fittings were de­ indicated that star camera No. 1 moved 9 arc-s and signed to minimize heat flow between the bench star camera No. 2 moved 30 arc-s because of and the spacecraft structure by controlling the tem­ weight reversal. The ATS pitch/yaw system moved perature to 25 0 C through the use of thermostati­ less than 3 arc-s, but the roll system changed by cally controlled heaters. Thermal resistance in­ 70 arc-s because of the sensitivity of the roll output troduced between the bench and the spacecraft re­ to yaw rotation between the roll transceiver and the duced the required heater power substantially. base dihedral mirror. Changes of 10 arc-s in the Also, in those few cases when the temperature of sun sensor alignment were measured when the vec­ the structure was greater than 25°C, the heat flow tor magnetometer plate assembly was flipped to re­ into the bench was limited by the resistance. verse the weight loading. The final calibration of Because the optical components (i.e., the two the system used these measured changes to deter­ ATS optical heads, the base dihedral mirror, and mine the relative alignments when in space. The the two star cameras) have direct heat leaks to weight loading effect was removed by averaging the space, they had to be directly controlled to the de­ difference in alignment in the ± 1 g cases. sired temperature. In addition, they had to be A second set of alignment measurements was mounted so that precise alignment was maintained made after the spacecraft had been exposed to en­ through all mission phases. Thus titanium, a mate­ vironmental testing. Residual changes caused by en­ rial with low thermal conductivity, was used for the vironmental stress were less than 3 arc-s for the joints between the bench and each component. A ATS and star camera No.2. Changes in star camera heater placed directly on the bottom of each com­ No.1 were slightly larger, reaching 10 arc-so ponent served the dual function of preventing heat The data from the various attitude sensors were flow from the bench and controlling the compo­ used as inputs to a computer program developed by nent's temperature. This design resulted in a bench the Computer Sciences Corp., that generated three­ with minimal thermal gradients through and across axis attitude information at 0.25 s intervals. 2 The the bench. Post-launch data show that gradients program computed the attitude for each 0.25 s in­ were less than 0.1 ° C. terval based on the sensor data available. If both cameras were tracking identified stars, the magne­ tometer orientation was computed using the two SYSTEM VERIFICATION AND star sightings and ATS data. If only one star camera PERFORMANCE was tracking an identified star, the second vector required for an attitude solution was derived from The Magsat attitude determination system was a the sun sensor data. If neither star camera was composite of individual units that had to be assem­ tracking an identified star, a motion model was bled into a calibrated system. Each of the sensors used to interpolate between valid star tracks. This was tested and calibrated by its manufacturer to motion model determined the right ascension and various levels depending on the manufacturer's fa­ declination of the B axis from sun sensor data and cilities and the ability of that particular component the rotation (pitch) about the B axis by integrating to operate without assembly into the spacecraft. the output of the pitch axis gyro. Verification of the primary attitude system was per­ The attitude determination program was capable formed at the Calibration, Integration, and Align­ of making certain self-consistency checks to ascer­ ment Facility at the NASA/Goddard Space Flight tain the validity of the data and the stability of the Center. Tests were performed to determine the system throughout the mission. One major check alignments of the star cameras and the ATS com­ was the determination of biases in the system from ponents to an optical reference mounted on the data sets in which all three sensors generated valid spacecraft optical bench. The star camera calibra­ data simultaneously. These bias determinations, tion algorithms generated by the Massachusetts In- computed from data taken early in the mission, in-

Volume 1, Number 3,1980 199 dicated that the two star cameras were within 6 arc­ between a reference vector (determined by star s of the prelaunch alignment measurements. Differ­ camera No.2, the ATS, and the sun sensor) and the ences between the attitude as computed by the star same vector determined solely by star camera No. 1 cameras, the A TS, and the sun sensor were larger was computed to be 5 arc-s root mean square than expected (based on ground testing). Since the (rms).3 This variation was a measure of the system star camera alignment did not appear to have noise and the ability of all sensor transfer functions changed, the discrepancy appeared to lie either in to remove the system nonlinearities. the A TS or the sun sensor. Further analysis, using a Computations of system biases were performed least squares fit of the observed magnetic field several times during the mission life. Variations in data, determined the distribution of the error. The system alignments were small, but with some drift best fit of the data to a model of the earth's field that was correlated with temperature changes. A indicated that the major discrepancy of 200 arc-s in failure of the heater on one of the star cameras in roll was due to the ATS (i.e., the precision sun sen­ December 1979 caused a shift of 6 arc-s o The data sor measures the roll angle correctly). Likewise, a set of attitude measurements for the Magsat mis­ smaller yaw discrepancy of 55 arc-s was associated sion has good internal consistency and, with resolu­ with the sun sensor. The sources of these errors in tion of the observed bias, the system has an ab­ the individual instruments have not been deter­ solute accuracy of 20 arc-s rms. mined. In time, the input of each sensor varied over a REFERENCES JR . L. C1eavinger and W. F. Mayer, "Attitude Determination Sensor for large portion of its range; i.e., the sun moved in a Explorer 53." AIAA Paper 76-114, Proc. 14th Aerospace Science circle about the B axis during each orbit of the Meeting (1976). spacecraft, the ATS angles varied, and many stars 20. M. Gottlieb et al., MAGSA T Fine Aspect Baseline System Overview and Analysis, Computer Science Corp. Document CSC SO - 78 - 6067 were tracked over the entire field of view. During (1978). such a period of time, variation in the difference 3F. Van Landingham, personal communications (March 1980).

200 Johns Hopkins APL Technical Digest JAMES F. SMOLA

THE MAGSAT MAGNETOMETER BOOM SYSTEM

The boom connecting the sensor platform with the Magsat spacecraft provided position and tilt adjusting capabilities for the remote mirrors referenced to the vector magnetometer. Once set in orbit, the mirror positions were to be maintained passively. Only vanishingly small angular deviations caused by thermally and dynamically induced structural distortions were permissible, which made the design and test problems extremely difficult.

INTRODUCTION precautions were taken to control thermal distor­ The need for a simple, extendable, lightweight, tions, mechanical misalignments, and dimensional precisely alignable, virtually distortion-free struc­ instabilities in the design of the structural system ture for Magsat, capable of overcoming the stiff­ that supported the sensors and separated them ness of sizable multiconductor electrical cabling, from the spacecraft instrument section. presented a most difficult design requirement. The critical requirement was the need to position with precision magnetically sensitive instruments at a SYSTEM DESCRIPTION considerable distance from sources of magnetic The magnetometer boom system consisted of a contamination in the spacecraft. The problem was 14-link scissors boom, a 3-axis gimbal, and the sen­ particularly acute because the magnetometer sen­ sor platform (see Fig. 1). The platform was con­ sors were designed to measure a 60,000 nanotesla nected to the instrument module electrically with (1 nT = 10-9 weber per square meter) magnetic multiconductor cabling that was routed through the field with a precision of ± 1 nT. interior of each boom link. Two independent, py­ Very' precise knowledge of the vector magnetom­ rotechnically actuated caging systems were used eter sensor's angular orientation was important. during launch to contain and protect the boom and Because of the extremely narrow bandwidth of the platform in their stowed configurations. The 3-axis attitude transfer system (ATS) , which measured the gimbal located at the boom base provided the angular orientation of the magnetometer, special boom with pitch, yaw, and roll capabilities of ±2,

Magnesium end fitting Graphite epoxy link Thermistors Length-indicating potentiometer

Boom caging T -rod

Alignment guide

SP caging rod Pitch gimbal actuator

Fig. 1-Magnetometer boom system (partially extended).

Volume I. Number 3.1980 201 ±2, and ±5°, respectively, for acqulSltlOn of the Table 1 ATS. The drivers for each gimbal actuator, con­ sisting of a 491-Hz square-wave inverter powering WEIGHTS OF BOOM SYSTEM COMPONENTS (kg) two phase-synchronous hysteresis gear motors cou­ pled to gearboxes, provided average pitch, yaw, Link structure 2.72 and roll scan rates of 30 arc-sis. A pair of tension Drive assembly 2.93 springs attached to the drive base and the instru­ Gimbal actuators (3) 3.09 ment module structure eliminated all unrestrained Caging subsystems 1.13 play in the gimbal actuator adjusting screws. Rota­ 9.87 ry potentiometers geared to the output shafts of Inverters (2) 0.62 each of these gearboxes were calibrated to give the Electrical subsystem 0.76 angular orientation of the drive base. 1.38 The boom drive consisted of a right- and a left­ Sensor platform assembly 7.03 handed ball screw that was rotated by a fourth, Boom cable 2.72 inverter-driven, synchronous hysteresis gear motor. 9.75 A rotary potentiometer geared to the output shaft of the gearbox was calibrated to indicate boom Total weight 21.00 length during deployment. Confirmation of total extension was given by a second potentiometer made of nonmagnetic materials and pinned to one OPERATIONAL REQUIREMENTS of the boom hinges closest to the platform. Total deployment time was about 20 minutes. Magnetic contamination of the magnetometer Weight and thermal distortion dictated the use of sensors was avoided by locating the star cameras graphite-epoxy material for the boom links. The (which were magnetic) on an optical bench in the basic link was 94 cm long and 1.07 X 5.08 cm in spacecraft instrument module. (Since the potential cross section. Magnesium fittings were fastened to for magnetic contamination of the sensors was so the ends and the center of each link with semirigid great, the urge to locate the cameras on the sensor epoxy to prevent interface cracking caused by dif­ platform was dispelled well in advance of any ferential expansion. Each link was a hollow box serious design effort.) On command, the boom was with a 0.076 cm thick wall and was covered inter­ to uncage and displace the sensor platform 6 m nally and externally with an aluminum foil mois­ away from the instrument module. To satisfy re­ ture barrier to control moisture absorption and quirements imposed by the ATS, which measured desorption - sources of dimensional instability - the vector magnetometer tilt relative to the star in the inherently hygroscopic graphite epoxy. A cameras, the boom was to maintain the platform final wrapping of aluminized Kapton coated with position so that the center of a plane mirror, pre­ aluminum oxide was added for temperature con­ cisely attached to the backside of the vector magne­ trol. The links were hinged to each other with pins tometer sensor, remained within a ± 1.91 cm that were forced through compliant undersized square target zone. This zone was centered on the bushings. This permitted rotation but eliminated all axis defined by an infrared light beam emanating unrestrained mechanical side play. from an A TS optical head located in the instrument The aluminum-foiled, graphite-epoxy platform module. The plane mirror had to remain orthogo­ was attached to the tip of the boom through a nal to the optical axis within 3 arc-min; this turned hollow graphite-epoxy box-like spacer and a out to be the greatest challenge. "figure-eight" mechanism that enabled the plat­ form to translate while maintaining its attitude nor­ AREAS OF MAJOR CONCERN mal to the boom axis as the boom extended. At­ Although the precision scissors boom was a tached to the platform with a kinematic suspension flight-proven concept, the remote, precisely aligned was the temperature-controlled vector magnetome­ platform was not. Concern over magnetic interfer­ ter base, which in turn had attached to it the vector ence disallowed electrically powered adjusting magnetometer sensor, the remote plane and dihe­ mechanisms at the platform end. Consequently, tilt dral ATS mirrors, and a precision sun sensor. The adjustments could only be achieved by gimbaUing kinematic suspension provided a compliant mount the boom at its base, which would cause the plat­ for the magnetometer base and isolated it from form to translate. This side effect, coupled with the thermally induced structural distortions that would narrow field of view of the ATS, severely limited be detrimental to the alignment of the vector mag­ the tilt adjusting capability. For this reason con­ netometer and the remote mirrors. The scalar mag­ siderable engineering analysis and testing were netometer was attached to a 1.27 cm thick epoxy­ necessary to demonstrate that the Scout launch en­ glass thermal isolator that was fastened to the side vironment, uncaging, deployment, thermal distor­ of the graphite-epoxy spacer. tions, and attitude control disturbances would not The weight breakdown for the boom system is compromise the ability to achieve the precise align­ shown in Table 1. ments that were required.

202 Johns Hopkins APL Technical Digest The following three major areas of concern re­ were to provide 16 W of power to warm the boom ceived considerable attention. cable and drive just prior to platform separation and boom extension. Torsion springs located at Control of Thermal Distortion each boom-link pivot were designed to overcome The requirement was to limit initial boom ther­ cold-cable stiffness as well as hinge frictional mal distortions caused by broadside solar illumina­ resistance. The existence of many single-point failure mechanisms in this area of concern resulted tion sufficiently to permit acquisition of the ATS remote mirrors by gimballing the boom at its base. in a considerable amount of cold temperature testing to demonstrate and guarantee system in­ Although the gimbals were adjustable ±2° in pitch tegrity. and yaw and ± 5 ° in roll, any tilting of the boom at its base would translate as well as rotate the Pre-Launch Mechanical Alignments remote mirrors. Since the target zone was ± 1.91 cm square, any mirror tilting that was necessary for To bring the remote mirrors mounted on the sen­ acquistion had to be achieved within 0.33 ° once the sor platform into the fields of view of the ATS mirrors entered the target zone. This made the ini­ pitch/yaw and roll optical heads and then into tial tilt angle of the mirrors and direction of tilt ex­ linear range by fine adjustment, a terrestrial test tremely important. If, for instance, the mirrors that negated the 1 g bias had to be contrived . The were perfectly centered within the target zone ini­ best proposal utilized a pair of 6.1 m long water tially, acquisition would have to be achieved within troughs (Fig. 2) . Specially designed floats with gim­ 0.16° or 9.6 arc-min of angular displacement. balled pulleys were attached to each of the boom­ The ±3 arc-min requirement on mirror angle link pivots. The idea was to simulate a zero-g con­ limited the permissible transverse offset of boom dition in a plane parallel to the plane of the water. tip position to ±0.51 cm for simple mechanical Remote mirror transverse or angular offsets could misalignment and ±0.25 cm for misalignments then be corrected by gimballing the boom at its caused by thermal distortion. It was important for base or introducing shims at the platform interface. the system to be free of play and for the boom ele­ The boom system was installed in a special fixture ments to be made of some material that is virtually that was attached to a rotary table, the purpose of immune to thermal distortion. (Active temperature control was ruled out because of the complexity re­ quired for implementation and lack of sufficient electrical power.) Graphite fiber-reinforced plastic (graphite epoxy) with a coefficient of thermal ex­ pansion of very nearly zero (+0.34 x 1O - 6 rq was chosen to cope with the thermal distortion pro­ blem. To control boom-link temperatures and tempera­ ture gradients in the presence of the solar and earth infrared environments, each link was taped with aluminized Kapton coated with aluminum oxide. This thermal covering was selected to give link equilibrium temperatures close to 25 ° C where all mechanical alignments would be checked. Thermal distortion tests conducted in the God­ dard Space Flight Center simulator on a four-link version of the flight boom yielded favorable results and indicated that the transverse and angular dis­ placements of the platform caused by boom ther­ mal distortion would be well within the allowable range.

Fig. 2-The Magsat magnetometer boom float system. U ncaging, Separation, and Extension The magnetometer boom is suspended from floats in Two independent caging systems were provided 6.1 m long flotation tanks that provided zero-g simUlation to contain the boom during launch and to prevent in the horizontal plane. The boom was rotated about its longitudinal axis into four positions 90 ° apart. the 7 kg sensor platform from transmitting damag­ Measurements of inclinations of sensor platform­ ing loads into the relatively delicate boom struc­ mounted mirrors provided data that were used to align ture. Both were actuated by pyrotechnic piston-type the mirrors by operating the gimbal at the base of the actuators. Because the boom system was stowed in boom or by shimming the vector magnetometer base at an unheated external compartment, boom-link and the sensor platform end. Final measurements, adjust­ ments, and checks were made by the attitude transfer cable temperatures as low as -70°C were expected. system with the instrument module mounted on the ro­ Resistive heaters attached to the boom drive base tary fixture.

Volume I, N umber 3,1980 203 which was to rotate the boom into its pitch and Acquisition ranges: Pitch/yaw ±1S0 arc·s; Roll ±300 arc-s Nominal: 0 arc-s yaw planes for orthogonal plane, zero-g measure­ 20,--.---,--,---,--,---,--,---,---,--, '" ., ments, and adjustments. With the aid of an autore­ ~ o ------., flecting telescope, a theodolite, and numerous mir­ ~ -20 ----- rors to measure the transverse and angular offsets ~ -40 R2!L - __ of the plane remote mirror, sufficient data were ",1ij -60 -- "- ".­ ,/ obtained to guarantee remote mirror acquisition in ~ e -SO Pitch .~ -100 orbit. -120 OPERATIONAL PERFORMANCE u ~----Orbital period: 92.S min------;~~I -"'~ 44 On November 1, 1979, following a successful ~ ~ 40I--S_u_n_sid_e_Design temperature bandwidth: 25°C ±10°C launch a few days earlier, the spacecraft attitude Eo ~ 36 ...... --- o " Darkside was stabilized. At 6:27:53 PM EDT, after successful en E32 uncaging, the magnetometer boom was extended. e 0~~---L--~--~~--~--~--S~0---L~100 Telemetry indicated that the extension was proper Time (min) and complete (see Fig. 3 of the Mobley article in this issue). Surprisingly, telemetry indicated that Fig. 3-Magsat ATS remote mirror angles and magnetom­ eter boom-link temperatures, 029:22:40:15 (January 29, the remote mirrors were within the field of view of 1980, orbit 1410). the ATS optical heads. This obviated the need for extensive gimbal searching for ATS acquisition. On the following day slight gimbal adjustments were this same cyclical characteristic. This coincidence made to bring the remote mirrors into the ATS lin­ leads to the inference that the boom is being ther­ ear range. Subsequent gimbal adjustments were un­ mally excited at the orbital frequency. The contin­ necessary. ually varying temperatures are caused by the once­ Figure 3 - data from one orbit 92 days into per-orbit coning of the solar vector. Their magni­ mission life - shows roll, pitch, and yaw angles tudes are functions of the boom-link angle relative relative to orbital time and boom-link temperatures to the sun, which is set by the link angle relative to as measured by thermistors attached directly oppo­ the boom axis and the seasonal variation in the site one another on a boom link. Oscillations are solar vector. evident, with periods approximately equal to the The landmark performance of the boom system orbital period and amplitUdes well within the ± 180 in orbit is an endorsement of the concept and of arc-s pitch and yaw and ±300 arc-s roll limitations the special precautions that were exercised to of the ATS. The boom-link temperatures exhibit guarantee its successful use.

204 Johns Hopkins APL Technical Digest WINFIELD H. FARTHING

THE MAGSAT SCALAR MAGNETOMETER

The Magsat scalar magnetometer is derived from optical pumping magnetometers flown on the Orbiting Geophysical Observatories. The basic sensor, a cross-coupled arrangement of absorption cells, photodiodes, and amplifiers, oscillates at the Larmor frequency of atomic moments pre­ cessing about the ambient field direction. The Larmor frequency output is accumulated digitally and stored for transfer to the spacecraft telemetry stream. In orbit the instrument has met its principal objective of calibrating the vector magnetometer and providing scalar field data.

INTRODUCTION (nT), was therefore used even though, from the The cesium vapor optical pumping magnetometer standpoint of resonance line width, it is the poorest used on Magsat is derived from the rubidium mag­ of the three. In the interim between OGO and netometers flown on the Orbiting Geophysical Ob­ Magsat, cesium-133 came to be the most commonly servatories (OGO) between 1964 and 1971. The used isotope in alkali vapor magnetometry and was Polar Orbiting Geophysical Observatories (Pogo) thus the natural selection for Magsat. used rubidium-85 optical pumping magnetometers, I while those on the Eccentric Orbiting Geophysical PRINCIPLE OF SENSOR OPERATION Observatories (Eogo) used the slightly higher gyro­ The alkali vapor magnetometer is based on the magnetic ratio of rubidium-87. phenomenon of optical pumping reported by Data generated by the Pogo instruments pro­ Dehmelt4 in 1957. The development of practical vided the principal data base for the U.S. input to magnetometers followed rapidly, evolving in one the World Magnetic Survey, 2 an international co­ form finally to the dual-cell, self-oscillating magne­ operative effort to survey and model the geomag­ tometer5 shown in Fig. 1. Two of these twin-cell netic field . It is partly this body of data that under­ magnetometers are combined in the Magsat magne­ lies the most accurate analytical models of the main tometer. In each twin-cell sensor an electrodeless geomagnetic field in present use. 3 Further studies discharge lamp driven by the RF exciter produces of the Pogo data base and other sources led to the resonance radiation that is collimated and passed recognition of the need for a dedicated mission, through an interference filter, which selects the DI such as Magsat, to increase the resolution for spectral line (894.5 nm for cesium-133) and rejects crustal studies, to incorporate vector measure­ the D2 spectral line (852.1 nm for cesium-133). The ments, and to correct the models for secular varia­ light is then circularly polarized before it is allowed tion. to optically excite cesium vapor contained in the When considering various options for implement­ absorption cells. ing a scalar magnetometer for use in space, optical Two identical sets of optics are employed, with pumping instruments are clearly superior in view of the direction of light propagation anti parallel, thus requirements for accuracy, dynamic range, infor­ defining the "optical axis" of the system. The mation bandwidth, ease of data collection and stor­ transmission of each absorption cell is monitored age, power and weight requirements, and reliabil­ by a photodetector, amplified, and cross-fed back ity. For the self-oscillating type used in OGO and to solenoidal coils wound about the absorption Magsat, cesium-133 is superior to other alkali va­ cells coaxially with the optical axis. A closed loop pors because of its narrower resonance line width, is thus formed that oscillates if the loop gain is followed by rubidium-87 and rubidium-85. When greater than unity at the frequency where the loop instrumentation for Pogo was selected, cesium had phase shift is zero. The frequency of oscillation is a not been used extensively in magnetometry and direct measure of the strength of the ambient mag­ thus was not considered sufficiently developed to netic field, i.e., the Larmor frequency of atomic use in spaceflight. Rubidium-87, with its higher magnetic moments precessing about the ambient gyromagnetic ratio, would have resulted in a wide field direction. range of frequencies over which it would have been The modulation of light transmission obtained difficult to control the electronic phase shifts that through application of a Larmor frequency signal are so important to accuracy. Rubidium-85, with to the solenoidal coil, termed the "HI" coil, results its gyromagnetic ratio of 4.66737 Hz per nanotesla from transitions between the discrete hyper fine

Volume 1, Number 3,1980 205 Boom Main body

L------+--fAA;m~pmlif~ie~rl-----~ Amplifier Output signal

Fig. 1-Dual·cell cesium magnetometer. The loop, composed of absorption cells, photocells, and amplifiers oscillates at a frequency proportional to the ambient field magnitude.

energy states that exist in the cesium atom when it fn = A Ho + Bn Ho2 is subjected to a magnetic field. The hyperfine splitting of cesium-I33 is shown in Fig. 2. Prior to A = 3.49847 B1 = -Ba = -93.5 the application of pumping light, the atoms can be B2 = -B7 = -66.7 considered to have a Boltzmann distribution among B3 = -86 = -40.0 the energy states. Optical pumping polarizes the t B4 = -Bs = -13.4 sample so that a nonequilibrium concentration of D2 852.1 nm atoms in either the m (the Larmor frequency for F=3-+------transition) = + 4 or m = - 4 sublevel of the 62P1 /2 ground state is obtained, depending on both the F=2·~~------sense of circular polarization and the direction of the optical axis component of the magnetic field . Application of a (Larmor frequency) signal to the HI coil at the transition frequency for m m = + 4, + 3 (or m = - 4, - 3) causes the selective D1 894.5 nm +41 depumping of a group of atoms having a certain +3 I f1 (4 -+ 3) phase with the driving signal. Since the pumping t21(f2 (3 -+ 2) probability depends on the angle to the light wave +1 If f3 (2 -+ 1) r------_~::::::::::::::=-- 0 f4 (1 -+ 0) normal as the group of depumped atoms precesses, 2l F = 3 -1 f fs (0 -+ -1) c the light transmission is modulated at the Larmor W'" -2 ff6 (-1 -+ -2) frequency. -3 ff f7 (-2 -+ -3) A single-cell absorption cell process is subject to - 4 fa (-3 -+ -4) "heading error" in that the Larmor frequency for transition m = + 4, + 3 differs from that for tran­ -3 -2 sition m = - 4, - 3. The dual-gas-cell configura­ tion has a strong advantage over single-cell magne­ F = 2 '----_~§:==== -6 +1 tometers in that the sense of circular polarization is +2 such that when one cell is pumped to the m = + 4 +3 state, the other is pumped to the m = - 4 state, Fig. 2-Energy diagram for cesium·133. Transition frequencies f1 to fa are in hertz for the applied field Ho in resulting in a reverse skew for the two absorption 5 cells. Since the dual-cell loop transfer function in­ gauss (1 gauss = 10 nT). cludes the product of the individual absorption lines, a symmetrical two-cell composite line is ob­ absorption frequencies of the Zeeman spectrum for tained. Thus the frequency of operation should n()t F = 3. This yields for the theoretical oscillation change under reversal of the ambient field with frequency respect to the sensor. The frequency of oscillation of the loop is deter­ f = 3.49847 Ho , mined by the open loop phase shift of all the com­ ponents. If the net phase shift from all sources where f is given in hertz if the ambient field Ho is other than the absorption cells is zero at any fre­ expressed in nanoteslas. quency, the closed loop oscillates at the frequency The various processes of modulating and moni­ where the phase shifts of the two absorption cells toring light in the twin-cell sensor are such that the are equal and opposite. By the symmetry of the loop gain depends on the product sin2 e cos e, system, this frequency is the average of the eight where e is the angle between the optical axis and

206 Johns Hopkins APL Technical Digest the applied field. Thus null zones are to be ex­ pected in a twin-cell sensor when the ambient field is either along or perpendicular to the optical axis. These zones are termed "polar" and "equatorial" null zones, respectively. The Magsat magnetometer incorporates two such sensors, with their optical axes crossed at 55 0. The polar null zones are thereby eliminated because they have no intersec­ tion, and the only remaining composite null zone is I the intersection of the two equatorial null zones, J-I _ _ _ +8 orbit shown in Fig. 3. In the Magsat magnetometer, the norma l composite null zone is directed normal to the orbit plane, such that in normal operation in near-polar orbit the ambient field will never lie within the composite null zone. It is desired, of course, to minimize the width of the null zones for each individual sensor. It is thus necessary to use high gain amplifiers in the system, the limitations being the noise level of the system and the requirement to maintain near-zero phase _ Null zo ne for sensor A shift over the operating frequency range. The width Null zone f or se nsor 8 _ Composite zo ne f or t wo du al-ce ll sensors of the null zones is also quite dependent on the operating temperature of the absorption cells. For Fig. 3-Spatial coverage of the scalar magnetometer. The composite null zone, or intersection of the two individual cesium-133, the optimum cell temperature is 52°e. sensor null zones, is oriented normal to the spacecraft or· The four absorption cells in the Magsat magnetom­ bit plane. It should never contain the magnetic field vec­ eter are maintained at the desired temperature by tor. individual thermostatically controlled heaters. The heaters are made from bifilar-wound resistance wire to avoid magnetic signature, and each heater dissipates 0.5 W at 30 V when the thermistor-oper­ ated control circuitry closes the circuit. The overall thermal environment is achieved by controlling sep­ arately the beryllia yoke (into which the sensors are mounted) at 41°C, and enclosing the entire sensor in a passive thermal enclosure with a radiator whose area is selected to radiate the required amount of energy. Figure 4 shows the Magsat sca­ lar magnetometer sensor configuration without its thermal enclosure or radiator.

DIGITAL DATA GENERATION Figure 5 is a diagram of the electronics necessary to operate the sensors and to interface the space­ craft telemetry system. In order to prevent inter­ ference between the two sensors, they must be op­ Fig. 4-Magsat scalar magnetometer sensor. Each erated in frequency lock. This is accomplished by cylinder contains optics and hybrid preamplifiers for one dual cell magnetometer. The central yoke is made of using a combining amplifier on one side of the sen­ beryllia to minimize the temperature gradient. A flat sors. The combining amplifier is repeated in order beryllia radiator (not shown) is attached to the top of the to implement fully the design objective of main­ yoke. The entire structure is housed in a fiberglass and taining the maximum amount of redundancy for multilayer thermal blanket enclosure. single sensor operation. The output signals from the two combining amplifiers are processed through mandable aperture type, a design feature that was completely independent signal processors consisting incorporated to allow the sampled data to vary of phase-locked loop tracking filters, accumulators, from the normal situation of a 30 ms aperture time and readout gates. In each signal processor chain, (in which the information bandwidth is primarily the bandwidth of the tracking filter is set at 25 Hz that of the tracking filter) to a maximum aperture to ensure that the loop will track the varying input case (where the frequency characteristic of the sam­ frequency adequately and to make negligible any pling function is primarily determined by the aper­ errors associated with time lag in the closed-loop ture time and the resulting output data are substan­ transfer function. The accumulators are of a com- tially protected from frequency aliasing effects). In

Volume I, N umber 3,1980 207 .---__ Aperture status A (8 bit)

r---'-----.,.....i-Aperture time A Sample A Bit clock A \.-___j

Output data A

Frame sync Housekeeping data Word gate

Output data B

.----'L--~Word gate B Bit clock B Sample B Exciter A I....-_.,...-_...... ,...Aperture B

Power bus A---"--~ Converter A '---_Aperture status B (8 bit) Exciter B \------1 Filter Regulated power r------, board Power bus B---~ Converter B Survival heater power

Fig. 5-Block diagram of scalar magnetometer electronics.

order to make this design objective compatible with both of the lamps must be operating within a pre­ resolution requirements, the tracking filters were determined brightness range throughout the ac­ designed to multiply the Larmor frequency by a cumulation interval. This form of the logic func­ factor of 16. tion was selected in order to be compatible with the The output data word assigned to each scalar operation of both processing chains with a single magnetometer data point. is 20 bits long. The last sensor and yet to be an efficient rejector of a bit is made a parity bit, and the next to last bit was number of possible malfunctions within the instru­ desired to be a data quality flag associated with ment. that particular sample. This leaves 18 bits for the The sampling of each scalar is done at 4 samples primary data sample, which was not compatible per second, and the samples of scalar A and scalar with the full range of commandable aperture times. B are interleaved to obtain a total of 8 equidistant Thus a pre scalar is included in each data processing samples per second. chain. The prescalar is under the control of the commanded aperture time, which is delivered to the OPERATION IN ORBIT experiment in the form of a binary number, N, The Magsat scalar magnetometer met its prin­ that specifies the aperture time as (2N + I)Tb, cipal in-orbit objective of calibrating the vector where Tb is the telemetry bit time of 0.512 ms. For magnetometer and returned scalar field data, al­ N :;; 63, the prescalar factor, P, is 1. For 64 :;; N though intermittent noise in the lamp excitation cir­ :;; 127, the prescalar factor is 2. For N ~ 128, the cuitry caused loss of lock in the tracking filters prescalar factor divides by 4. and, therefore, a reduction in the expected data The logic equation for the data quality flag, Q, rate. associated with each data point is, for instance, for Engineering data returned from the instrument scalar A, verified that the thermal control circuitry per­ formed as expected in controlling lamp and absorp­ QA = TFA . (SIGA + SIGB) . (BRA + BRB) , tion cell temperatures. Monitors in the lamp RF control loops were only reliable when the intermit­ where each term in the logic function has to be true tent noise was absent but at such times showed that over the accumulation time in which that particular lamp A operated at control parameters identical to data sample was collected. TFA = 1 means that the those observed during ground thermal-vacuum test­ tracking filter associated with that data point was ing. Lamp B, which ran extremely efficiently dur­ in lock throughout the accumulation time. SIGA + ing ground testing, required slightly more RF power SIGB = 1 is a condition that the signal amplitude in orbit. Sensors A and B were operated indepen­ in either sensor A or sensor B is above a predeter­ dently at times to serve as a cross check on ab­ mined threshold throughout the accumulation inter­ solute accuracy and to verify the integrity of the val. BRA + BRB = 1 is a condition that one or Larmor sensors.

208 Johns Hopkins APL Technical Digest The lamp noise problem manifested itself dif­ to fill in the A sensor null zones and to provide a ferently in the two lamps. It first appeared in lamp more complete data set for calibration of the vector A about 12 hours after the instrument was turned magnetometer. on and, except for a few periods after the lamp Incorporation of the data quality flag in the out­ had been off for a day or more, was consistently put word proved indispensable to the rejection of present in the lamp excitation until the spacecraft data when the tracking filter was out of lock and entered the eclipse season, when a dramatic im­ enabled the combination of vector and scalar mag­ provement occurred. The problem was first ob­ netometers to meet substantially all the objectives, served in lamp B two weeks after turn-on and was even in the presence of the lamp excitation noise. present intermittently over a period of several hours in a way that has not been explained. Transi­ tions from one state to another often occurred REFERENCES IW. H. Fanhing and W. C. Folz, " Rubidium Vapor Magnetometer for when one of the tape recorders was played back, Near Eanh Orbiting Spacecraft," Rev. Sci. 1nstrum. 38, pp. but the problem was not associated with a par­ 1023 - 1030 (1967). ticular tape recorder. The lamp problem was most 2A. J. Zmuda (ed.), The World Magnetic Survey, IAGA Bulletin No. 28 (1971). likely caused by a feedback mechanism to the 3Ibid., p. 148 . spacecraft power that was not present to the same 4H. G. Dehmelt, Phys. Rev. 105 , p. 1487 (1957). degree on the ground. 5A. L. Bloom, " Principles of Operation of the Rubidium Vapor Magnetometer," Appl. Opt. 1, pp. 61 - 68 (1962) . The effect of the interference on tracking filter lock was much more serious when the noise was ACKNOWLEDGMENT - The Magsat scalar magnetometer was present in lamp B. Thus the operational mode was built for the NASA/ Goddard Space Flight Center by Bell Aerospace Systems Division, Western Laboratories, and Varian Associates, Canada, to operate lamp A continuously and to operate Ltd. Mr. David Synder, BASD, was program manager for the project. lamp B approximately one day out of five in order The RF exciters were provided by Mr. L. J . Rogers of GSFC.

Volume 1, Number 3,1980 209 MARIO H. ACUNA

THE MAGSAT PRECISION VECTOR MAGNETOMETER

The Magsat preClSlon vector magnetometer was a state-of-the-art instrument that covered the range of ±64,OOO nanoteslas (nT) using a ±2000 nT basic magnetometer and digitally controlled current sources to increase its dynamic range. Ultraprecision components and extremely efficient designs minimized power consumption.

INTRODUCTION stable and linear triaxial fluxgate magnetometer with a dynamic range of ±2000 nT (1 nT = 10,9 The instrumentation aboard the Magsat space­ weber per square meter). The principles of opera­ craft consisted of an alkali-vapor scalar magnetom­ tion of fluxgate magnetometers are well known and eter and a precision vector magnetometer. In addi­ will not be repeated here. The reader is directed to tion, information concerning the absolute orienta­ Refs. 1, 2, and 3 for further information concern­ tion of the spacecraft in inertial space was provided ing the detailed design of these instruments. by two star cameras, a precision sun sensor, and a To extend the range of the basic magnetometer system to determine the orientation of the sensor to the 64,000 nT required for Magsat, three digi­ platform, located at the tip of a 6 m boom, with tally controlled current sources with 7 bit resolution respect to a reference coordinate system on the and 17 bit accuracy were used to add or subtract spacecraft. automatically up to 128 bias steps of 1000 nT each. The two types of instruments flown aboard the The X, Y, and Z outputs of the magnetometer spacecraft provided complementary information were digitized by a 12 bit analog-to-digital con­ about the measured field. The scalar magnetometer verter that, in conjunction with the 7 bits asso­ measured the magnitude of the field independent of ciated with the digital current sources, yielded an its orientation with respect to the sensor, with an overall instrument resolution of ±0.5 nT. absolute accuracy that ' is determined by atomic The ambient magnetic field was sampled 16 times constants and thus is not subject to change as a per second along each of the three orthogonal di­ function of time. On the other hand, the precision rections, and the digital current sources were up­ vector magnetometer measured the projections of dated at the same rate. The digital data corre­ the ambient field in three orthogonal directions sponding to the "fine" (12 bit) and "coarse" (7 with an absolute accuracy determined by calibra­ bit) information were fed directly to the spacecraft tions with respect to a standard; thus they were through a serial interface. The fine data were sam­ subject to error and drift. Accuracy goals for the pled at 16 samples per second; the coarse data were mission required a vector magnetometer capable of sampled at only 4 samples per second. This sam­ measuring the ambient field with a maximum error pling scheme took advantage of the fact that the of ± 1 part in 64,000 in magnitude and 5 arc-s in ambient magnetic field changed slowly over the orientation (1 arc-s = 0.00028°). The development Magsat orbit; hence, coarse updates were not of such an instrument within the constraints im­ needed as frequently as fine updates. The response posed by the spacecraft represents a major techno­ of the instrument to increasing and decreasing am­ logical achievement that would have been impossi­ bient fields is shown in Fig. 2. The current steps, ble without parallel developments in the areas of which are added or subtracted depending on the ultraprecise linear integrated circuits and miniature magnitude and direction of the external field, resistors. The design implemented for Magsat rep­ maintained the effective magnetic field seen by the resented an optimum compromise among many fluxgate sensor within its operating range of 2000 conflicting requirements and limitations imposed by nT. The stepping threshold was actually 1000 nT to available resources, reliability considerations, and allow the magnetometer to follow rapid changes in state-of-the-art electronics. This paper presents a the field between two adjacent updates of the bias brief description of this unique instrument. steps without loss of data. As can be seen from Fig. 1, the magnetometer INSTRUMENT DESCRIPTION electronics, analog-to-digital converters, and digi­ Figure 1 is a block diagram of the vector magne­ tally controlled current sources were implemented tometer. The heart of the instrument is a highly with redundant designs. This was also true for the

210 Johns Hopkins APL Technical Digest -250 KHz clock A Sample A

Coarse word gate A Attitude transfer Coarse data A system f----Shift clock A t t Fine data A r------t I I I Triaxial I I fluxgate I sensor assembly ~*-'l---l...,.. ___ ..,. ______':-:"":'~':"-,..I----I I I L __ - __ oJ Fine word gate B Fine data B Thermalr control +----Shift cl ock B Coarse data B Sun sensor Coarse word gate B assembly Calibration ~ Timing and 'Sampl e B }A power in ~ control ~- 250 KHz clock B Data processor B _ Housekeeping }B power in -l-=-=-=-::: an alog signals ---+-

Fig. 1-Block diagram of the precIsIon vector magnetometer. All subsystems are implemented with redundant designs to avoid loss of data in case of failures. digital processors and power converters. The block­ dition, its uniform expansion characteristics were redundancy approach provided a fundamental mea­ essential for achieving the angular stability re­ sure of reliability to this important instrument and quired. Figure 3 is a schematic diagram that shows eliminated from consideration single-point failures the construction of each sensor. Crucial elements in in the electronics. Selection of the particular system the Magsat fiuxgate sensor design were the feed­ used (A or B in Fig. 1) for both the analog and back coil that nulls the external field , and the sen­ digital portions of the instrument was executed by sor core itself; they constitute the most important ground command. sources of error in terms of alignment stability as well as variation of scale factor with temperature. Any distortion or motion of the sensor core within TRIAXIAL FLUXGATE SENSOR the feedback coil represents an effective alignment The fiuxgate sensor used in the Magsat vector shift. Structural deformations larger than 50 J.tm magnetometer was a unique design that took ad­ were sufficient to exceed the alignment stability vantage of the ring core geometry. This particular tolerance. geometry exhibits superior performance characteris­ The fundamental strategy followed for the Mag­ tics in terms of noise and zero-level stability. In ad- sat vector magnetometer was to match the expan-

EOU! [V ] 4095 't / "E 3071 J / c: :J ~ :; 2047 -5~A,I".; /1 / B- I, ,,-', .t, .c' ":0 ,,-' ,,-" ,' , , 0 ,,-' .: 0 .co I [nT x 1000] :Jo rl'I'I':l:,:,.:1':f:,:,: ,:r Q) c: / :/ :/ :/ :1 ~ / :/ :,' 1/ :/ :/ : / :/ u. 1023 ~ " v V v _41... ~ ~ ~ f I I ( Fig. 2-lnstrument response for increasing (solid curve) and decreasing (dashed curve) / 1 ----~c.B~-- ambient field. Note the large / - 8 o safety margin in the switching o 57 58 59 60 61 62 63 64 65 66 67 68 125126 127 n I I I I I I I 11 I I threshold (half scale). o 1 2 58 59 60 61 62 63 64 65 66 67 68 69 126 127 I- --+ - -t - --u- - t - -t - -t - -t - --j - - + - -t - -t - + - -I- - +- - ~I- - -I- - --j Field offset generator step (coarse output)

Volume I, Number 3,1980 211 quired of the analog signal processing circuits dic­ Feedback tated the use of ultraprecise components, some of coil which had to be specifically developed for Magsat. The maximum allowable error voltage for the digi­ tally controlled current sources was only 125 J.N so considerable attention had to be paid to the prob­ lem of thermally generated electromotive forces that are produced across dissimilar metal junctions. Circuit layouts with a minimal number of solder joints, the proximity of essential conductors in order to minimize thermal gradients, and the use of Fig. 3-lndividual fluxgate magnetometer sensor construction. All parts have identical expansion coeffi­ cadmium-tin alloys for soldering and assembly re­ cients. duced the instrument errors to an absolute mini­ mum. The resistors used in the digital current sources as well as the magnetometer feedback cir­ sion coefficients of all the materials used in the cuit were ultraprecise, hermetically sealed units construction of the sensor assembly (core, feedback with an absolute temperature coefficient of less coil, and support structure) so that differential than 0.5 ppm/oC and a long term stability of 20 stresses induced by temperature variations were ppm/year. The calibration of the analog circuits re­ minimized. Since the sensor core expansion coeffi­ quired special test instrumentation with accuracies cient is approximately 10 ppmrC, platinum wire established by the National Bureau of Standards was used to wind the feedback coil, and the sup­ and resolution of about 1 ppm (a type of design port structures were machined from solid blocks of usually restricted to laboratory-grade instruments). Macor, a machinable glass-ceramic. The triaxial Extensive reliability and performance trade-off sensor assembly was mounted on a temperature-sta­ analyses were carried out to ensure mission success. bilized baseplate whose temperature was actively The digital processors were implemented with maintained at 25 ± 1°C. The attitude transfer CMOS digital integrated circuits to reduce power system mirrors were mounted under this baseplate. consumption to a minimum. Only one processor Their expansion coefficient was nearly zero; hence, was powered at any given time by its respective active thermal control of this crucial interface was power converter; the same was true for the analog needed. electronics subsystem. As is illustrated in Fig. 1, Since the overall expansion coefficient of the sen­ there were then four possible arrangements for sor feedback coil was nonzero, the magnitude of these subsystems, depending on how they were in­ the feedback field was a strong function of temper­ terconnected. To simplify testing, the "A-A" and ature. To minimize this effect, the magnetometer "B-A" combinations were calibrated by an indirect electronics incorporated a correction circuit that analysis of the "A-A" and "B-B" configurations sensed the coil temperature by changes in its resis­ test data. A summary of performance parameters tance and adjusted the feedback current to make and technical characteristics of the magnetometer the generated field independent of temperature. and the electronics system is given in Table 1. The external magnetic field generated by the tri­ axial fluxgate sensor assembly was less than ± 1 nT at the location of the scalar magnetometer. This CALI BRA nON AND ALIGNMENT low field value minimized the interference with the The problem of determining the orientation of a accuracy of the mea3urements obtained by the sca­ given magnetic field vector traditionally has been lar instrument. solved by assuming that the field orientation can be To determine the absolute orientation of the sen­ established accurately by the geometry of a calibra­ sors with respect to a reference coordinate system, tion coil. This method is generally sufficient to two optical cubes were bonded to the sensor determine sensor orientation within a few arc­ mount. They allowed the sensor assembly to be ro­ minutes from its true direction, but it is certainly tated exactly 90° during calibrations to establish not accurate enough for the Magsat case where ac­ the absolute direction of the magnetic axes and curacies of the order of 2 arc-s were required. The later to reference the scalar instrument measure­ method to determine the sensor alignment assumed ments to the principal spacecraft coordinate that the deviations from orthogonality of the sen­ system. sor assembly and triaxial test coil system were small. In this case, a very accurate alignment of the MAGNETOMETER ELECTRONICS sensor and test coil system can be obtained simulta­ The electronics design for the vector magnetom­ neously by rotating the sensor twice through exact­ eter incorporated a number of advanced develop­ ly 90° in two planes. This method has been de­ ments derived from the Voyager 1 and 2 magnetic scribed in Refs. 4 and 5. An accurate optical refer­ field experiments 1.2 that were directly applicable to ence coordinate system was first established at the this instrl)ment. However, the extreme accuracy re- Goddard Space Flight Center (GSFC) Magnetics

212 Johns Hopkins APL Technical Diges/ Table 1 of the magnetometer and coil system, excellent SUMMARY OF TECHNICAL CHARACTERISTICS OF THE results were obtained, as is evident by the outstand­ MAGSA T PRECISION VECTOR MAGNETOMETER ing in-orbit performance of the vector magnetom­ eter. Basic Fluxgate Magnetometer The individual axis scale factors were calibrated Dynamic range: ±2000 nT using a proton precession magnetometer as a refer­ Resolution: 12 bit A I D converter (±O.S nT) ence standard for fields greater than 20,000 nT. Noise: O.OOS nT rms (S Hz bandwidth) For small fields, the calibration constants were syn­ Zero level stability: thesized from selected incremental measurements Sensor ( - 60 to + 60°C): ±0.2 nT above 20,000 nT to determine the exact "weights" Electronics ( - 20 to + SO°C): ±0.2 nT of each of the 7 bits in the digitally controlled cur­ Drive frequency: 12.S kHz rent sources. Final overall calibration included co­ Linearity error (compensated): less than I part in 10 5 ordinated tests with the scalar magnetometer to Angular stability better than ± 3 arc-s over a tempera- determine mutual interference and relative accu­ ture range of 10 to 40°C racy. Significant problems were encountered in this respect with the relatively large RF power required Offset Digitaliy Controlled Current Sources for operation of the scalar magnetometer. Prior to Dynamic range: ±64,000 nT the installation of RF shielding on the spacecraft, Quantization step: 1000 nT this interference was rather serious, and consider­ Temperature coefficient: less than O.S ppmrC able effort was dedicated to its elimination. Long-term stability: within 2 parts inlO 5 per year

Sensor Assembly REFERENCES Mass: 0.6 kg Dimensions: 11.4 x 5.72 x S.Scm t M. H. Acuna, "Fluxgate Magnetometers for Outer Planets Explora· tion," IEEE Trans. Magn. MAG - 10, No.3 , p. 519 (1974). 2K. W. Behannon, M. H. Acuna, L. F. Buriaga, R. P. Lepping, N. F. Electronics (redundant) Ness, and F. M. Neubauer, "Magnetic Field Experiment for Voyagers 1 Mass: 2.6 kg and 2," Space Sci. Rev. 21, pp. 235 - 257 (1977). Dimensions (approximate): 22.23 x 17.S x 11.4 cm 3M. H. Acuna, C. S. Scearce, 1. B. Seek, and J. Scheifele, Tile MAGSA T VeClOr MagnelOmeler-A Precision Fluxgale Magnelollleler Total power consumption: for lile Measuremenl of lile Geomagnelie Field, NASA Technical I.S W at 2SoC Memorandum 79656 (1978). 2.0 W at O°C 4R. L. McPherron and R. C. Snare, "A Procedure for the Accurate Calibration of the Orientation of the Three Sensors in a Vector Magnetometer," IEEE Trans. Geosci. Eleclron. GE·16, No.2, pp. 134 - 13 7 (1978). 5M. H. Acuna, MAGSA T - Veelor MagnelOmeler Absolule Sensor Test Facility by means of a pair of first-order Alignmenl Delerminalion, NASA Technical Memorandum 79648 theodolites referenced to a 400 meter baseline. (1978). Numerous magnetic contamination problems asso­ ACKNOWLEDGMENTS - The author would like to acknowledge ciated with the optical system had to be solved to the dedication and support provided during the development, test, and realize the intrinsic accuracy obtainable by this integration of this instrument by J. Seek, C. Scearce, J. Scheifele, and E. Worley of GSFC; C. Fewell of Northrop Services; R. Lundsten, and J. method. The short- and long-term mechanical sta­ Scarzello of NSWC; and F. Mobley, J. Roberts, E. Marshall, and J. bility of the triaxial coil system presented very dif­ Smola of APL. The outstanding optical support provided by S. Hinkal ficult problems at times in terms of the consistency was essential to the success of the calibration activity. The GSFC Mag­ netics Test Facility personnel, C. Harris, R. Bender, and R. Ricucci , of test results. Nevertheless, after many hours of spent many hours waiting for us to understand our spacecraft; their cOn­ dedicated testing to determine the true performance tribution is deeply appreciated.

Volume I, Number 3,1980 213 ROBERT A. LANGEL

MAGSAT SCIENTIFIC INVESTIGATIONS

The nature of the geomagnetic field and of presatellite and early satellite measurements is de­ scribed. The objectives of each investigation in the areas of geomagnetic modeling, crustal mag­ netic anomaly studies, investigations of the inner earth, and studies of external current systems are presented, along with some early results from Magsat.

INTRODUCTION ment of magnetic fields. Such surveys usually, The near-earth magnetic field, as measured by although not always, measured only the scalar the magnetometers on Magsat, contains contribu­ magnitude of the field. Many countries have been tions from three sources: the earth's core, the surveyed in their entirety, some more than once. earth's crust, and external current systems in the Unfortunately, the United States has not yet been earth's ionosphere and beyond. By far the largest totally surveyed. In addition to their obvious value in magnitude is the field from the core, or the for modeling the earth's main field, such surveys "main" field. The main field is nearly dipolar in are particularly useful for mapping the anomaly nature. Its strength is more than 50,000 nT (nano­ field at low altitude and, as a result, have been teslas) at the poles and near 30,000 nT at the conducted by the oil and mineral exploration in­ equator. Its variation with time (" secular" varia­ dustries on a local scale. tion) is slow, with a maximum of about 1070 per Satellite measurements of the geomagnetic field year. External current systems are time varying on began with the launch of Sputnik 3 in May 1958 a scale of seconds to days and can vary in magni­ and have continued sporadically in the intervening tude from a fraction to thousands of nanoteslas. years. Table 1 is a list of spacecraft that have made The current systems are located in a cavity-like significant contributions to our understanding of region surrounding the earth, known as the "mag­ the near-earth geomagnetic field. Each had its own netosphere." Although these currents are always limitations, ranging from a lack of global coverage present, their strength and location vary consider­ caused by the absence of on-board tape recorders ably between periods of magnetic quiet and periods to limited accuracy due either to instrumental of magnetic disturbance. shortcomings or to ambient spacecraft fields. Prior Magnetic fields from the earth's crust are by far to Magsat, only the Polar Orbiting Geophysical the smallest in amplitude of the three sources at Observatory (Pogo) satellites (OGO -2, -4, and -6) satellite altitudes. Their strength at Magsat altitudes had provided an accurate, global geomagnetic sur­ is between 0 and 50 nT. Also called "anomaly vey. Their alkali vapor magnetometers provided fields," the sources are associated with variations measurements of the field magnitude about every in the geologic or geophysical properties in the half second over an altitude range of about 400 to 1 earth's crust; accordingly, their temporal variations 1500 km. ,2 are on a geologic time scale. A new era in near-earth magnetic field measure­ Prior to the satellite era the earth's magnetic ments began with NASA's launch of Magsat in Oc­ field was (and still is) monitored by means of per­ tober 1979. Magsat has provided the first truly manent magnetic observatories that measure the global geomagnetic survey since the Pogo satellites ambient field continuously and by periodically and the very first global survey of vector compon­ repeating measurements at selected sites. Field ents of the geomagnetic field. Designed with two surveys are necessary to fill in the spatial gaps be­ major measurement tasks in view, Magsat provided tween observatories and repeat sites. Such surveys a global vector survey of the main geopotential were first conducted by early mariners and land field and a lower altitude measurement of crustal surveyers. Edmund Halley made a sea voyage dur­ anomalies. These tasks stemmed directly from the ing 1689-1700 expressly to survey the magnetic field Magsat mission objectives outlined by Ousley in over the oceans. In 1701 he published the first this issue. Data from Magsat are being analyzed by chart of the magnetic declination in the region of many investigators, some of whom are working the Atlantic Ocean; in the following year he ex­ cooperatively. tended his chart to the Indian Ocean and to the sea near China. OVERVIEW OF INVESTIGATIONS In addition to land and sea surveys, some air­ Investigations are being carried out by scientists craft have been especially adapted for the measure- at the Goddard Space Flight Center (GSFC) and

214 Johns Hopkins APL Technical Digest Table I

SATELLITES THAT HAVE MEASURED THE NEAR-EARTH GEOMAGNETIC FI ELD

Altitude Approximate Satellite Inclination Range Dates Instrument Accuracy Coverage (degs) (km) (nT) Sputnik 3 65 440-600 5/ 58 - 6/ 58 Fluxgates 100 USSR Vanguard 3 33 510-3750 9/ 59 - 12/59 Proton 10 Near ground station· 1963-38C Polar 1100 9/ 63 - 1174 Fluxgate 30 - 35 Near ground (I-axis) station Cosmos 26 49 270 - 403 3/ 64 Proton Unknown Whole orbit Cosmos 49 50 261 - 488 10/ 64 - 11 / 64 Proton 22 Whole orbit 1964-83C 90 1040 - 1089 12/64- 6/ 65 Rubidium 22 Near ground station OGO-2 87 413 -1510 10/ 65 - 9/ 67 Rubidium 6 Whole orbit OGO-4 86 412 - 908 7/ 67 - 1/69 Rubidium 6 Whole orbit OGO-6 82 397 - 1098 6/ 69 - 717 1 Rubidium 6 Whole orbit Cosmos 321 72 270 -403 1170- 317 0 Cesium Unknown Whole orbit A zur 103 384 - 3145 11 / 69 - 6/ 70 Fluxgate Unknown Near ground (2-axis) station Triad Polar 750- 832 9/ 72 - present Fluxgate Unknown Near ground station . " Near ground station" indicates no on-board recorder. Data were acquired only when the spacecraft was in sight of a station equipped to receive telemetry. the U.S. Geological Survey (USGS), and by scien­ main field of the earth in a least-squares sense. tists selected in response to a NASA Announce­ Theoretically, such a potential function could be ment of Opportunity (AO). The work at GSFC in­ made to represent both the core and crustal fields cludes derivation of the first geomagnetic field exactly. Practically, a restricted model must be models from Magsat, derivation of the global chosen because of the finite limitation on computer anomaly map from Pogo, and some interpretations size and speed. Most researchers attempt only to of that map in terms of geologic/geophysical represent the core field with a potential function models of the earth's crust. The USGS effort, and use alternate methods of describing the crustal under the direction of Frank Frischknecht, includes fields. both field modeling and magnetic charting as car­ One of the principal contributions of satellite ried out by Joseph Cain and Eugene Fabiano, and magnetic field measurements to geomagnetism has crustal modeling by Jeff Phillips and others. A been to make available a truly global distribution total of 19 domestic and 13 foreign investigators of data. Surface measurements are notably sparse, were selected from responses to the AO. Table 2 particularly in oceanic and remote regions. The lists these investigators, their institutions, and a problem is compounded by the secular variation in brief description of their proposed investigations. the main geomagnetic field that, as was stated More detailed information regarding the investiga­ earlier, can amount to as much as 1070 per year in tions will be given in the remainder of this paper, some localities. This means that to represent the which is divided into four sections corresponding to global geomagnetic field accurately at any given four relatively distinct areas of scientific investiga­ time (epoch), worldwide measurements must be tion, namely: made at times near that epoch, a feat only achieved I. Geomagnetic field modeling; by satellite observations and even then only by the 2. Crustal magnetic anomaly studies, i.e., postu­ Pogo and Magsat satellites with their on-board tape lating the crustal structure and composition recorders and near-polar orbits. Accurate global that cause the magnetic anomalies; representation of the secular variation itself would 3. Investigations of the inner earth (the core, require periodic worldwide surveys, something mantle, and core/ mantle interface area); and often spoken of but yet to become a reality. The 4. Studies of external current systems. Pogo satellites accomplished one such survey and Magsat provided another. These, together with ex­ GEOMAGNETIC FIELD MODELING isting surface data, permit accurate global represen­ Geomagnetic field modeling derives the spherical tation of secular variation for the period beginning harmonic potential function that best represents the with OGO-2 until the demise of Magsat. Future

Volume I, Number 3,1980 215 Table 2

LIST OF MAGSAT INVESTIGATORS

Investigator Institution/Country Title Objectives

David R. Barraclough Institute of Geological Spherical Harmonic Represen- Produce an accurate model of the main geomagnetic field, to­ Sciences, United King­ tation of the Main Geomag- gether with reliable estimates of the accuracy of coefficients dom netic Field for World Charting and Investigation of Some Fundamental Problems of Physics and Geophysics

Charles R. Bentley University of Wisconsin Investigation of Antarctic Using Magsat data, devise a general framework for the struc­ Crust and Upper Mantle Using ture of Antarctica into which more specific and local measure­ Magsat and Other Geophysical ments can be integrated Data

Edward R. Benton University of Colorado Geomagnetic Field Forecasting Adjust the gauss coefficients of the main field model of the and Fluid Dynamics of the Magsat data set to satisfy dynamic constraints; use Magsat Core data to test the ability to forecast the structure of the internal geomagnetic field

B. N. Bhargava Indian Institute for Geo- Magsat for Geomagnetic Stud- Prepare a regional geomagnetic reference field and magnetic magnetism ies in the Indian Region anomaly maps over Indian and neighboring regions: (a) to gain a clearer understanding of secondary effect features and the variability of the dawn/dusk field, and (b) to study in detail the equatorial electrojet and transient variations

Robert F. Brammer The Analytic Sciences Satellite Magnetic and Gravity Produce magnetic anomaly maps of the Indian Ocean; quantify Corporation (TASC) Investigation of the Eastern the comparison between Magsat data and Geos-3 gravity data; Indian Ocean interpret the magnetic data using ancillary data

J. Ronald Burrows National Research Coun- Studies of High-Latitude Cur- Understand the physical processes that control high-latitude cil of Canada rent Systems Using Magsat current systems; improve the confidence level in studies of in- Vector Data ternal field sources

Robert S. Carmichael University of Iowa Use of Magsat Anomaly Data Analyze Magsat anomaly data to synthesize a total geologic for Crustal Structure and Min­ model and interpret crustal geology in the midcontinent region; eral Resources in the U. S. contribute to the interpretation and calculation of the depth of Midcontinent the Curie isotherm

Richard L. Coles Energy, Mines and Re- The Reduction, Verification Select quiet-time data; correct Magsat data for disturbance sources, Canada and Interpretation of Magsat fields and apply the routines; compare Magsat and vector air­ Magnetic Data over Canada borne data; combine Magsat and aircraft data of magnetic anomalies; produce regional interpretations relating to earth structure

James C. Dooley Bureau of Mineral Magsat Data, the Regional Incorporate Magsat data into regional magnetic field charts to Resources, Australia Magnetic Field, and the improve their accuracy; determine if differences exist in Crustal Structure of Australia temperature-depth curves for different tectonic areas; study the and Antarctica boundaries between major tectonic blocks and between con­ tinental and oceanic crust; determine Curie point depth and crustal magnetization for Antarctica

Naoshi Fukushima Geophysics Research Proposal from Japanese Analyze the regional geomagnetic field around Japan and Laboratory, Japan National Team for Magsat Japanese Antarctica; study the contributions to magnetic varia­ Project tions by electric currents and hydromagnetic waves in and above the ionosphere

Paolo Gasparini Osservatorio Vesuviano, Crustal Structures under the Calculate the depth of the Curie temperature for the Italy Active Volcanic Areas of Cen- Mediterranean area and relate to areas of volcanic activity; in­ tral and Eastern Mediterra- vestigate the Italian and Tyrrhenian anomaly nean

Bruce P. Gibbs Business and Technolog- Geomagnetic Field Modeling Produce a state vector to predict field values for several years ical Systems, Incorpo- by Optimal Recursive Filtering beyond the Magsat model; obtain optimal estimates of field rated values throughout the 1900-1980 period

M. R. Godivier Office de la Recherche Magnetic Anomaly of Bangui Improve the explanation of the cause of the Bangui anomaly, Scientifique et Tech- using Magsat data, other magnetic data, and gravity, seismic, nique, Outremer and heat flow data (ORSTOM), France

Stephen E. Haggerty University of Massachu- The Mineralogy of Global Interpret Magsat data to locate mafic and ultramafic source setts Magnetic Anomalies rocks and lineament expressions of anomalies that can be cor­ related with crustal of upper mantle depths; determine mineral stabilities pertinent to magnetic anomalies to ascertain the magnetic properties of metamorphic rocks

216 Johns Hopkins APL Technical Digesl Table 2 (continued)

LIST OF MAGSA T INVESTIGATORS

Investigator Institution/Country Title Objectives

D. H. Hall University of Manitoba, Identification of the Magnetic Confirm and extend the model for crust mantle magnetization Canada Signatures of Lithostrato­ graphic and Structural Elements in the Canadian Shield Using Magnetic Anomalies and Data from In­ dividual Tracks from Magsat

Christopher G. A. University of Miami Investigations of Medium Determine the relationship of magnetic anomalies with surface Harrison Wavelength Magnetic Anomal- geological features ies in the Eastern Pacific Us- ing Magsat Data

David A. Hastings u.S. Geological Survey, An Investigation of Magsat Determine the Magsat magnetic signatures of various tectonic EROS Data Center and Complementary Data Em- provinces; determine the geological associations of these phasizing Precambrian Shields signatures; synthesize Magsat and other data with mineral and Adjacent Areas of West resources data globally Africa and South America

JohnF. Hermance Brown University Electromagnetic Deep-Probing Evaluate the applicability of electromagnetic deep-sounding ex­ (100-1000 km) of the Earth's perimems using natural sources in the magnetosphere Interior from Artificial Satel- lites; Constraints on the Re- gional Emplacement of Crus- tal Resources

William J. Hinze Purdue University Application of Magsat to Use magnetic anomalies to develop lithospheric models to Lithospheric Modeling in determine the properties of principal tectonic features; correlate South America: Part 1- magnetic anomalies of South America with those of adjacent Processing and Interpretation continental areas to attempt to reconstruct Gondwanaland (see of Magnetic and Gravity Keller below) Anomaly Data

B. David Johnson Macquarie University, An Investigation of the Produce a map of surface magnetization to understand the Australia Crustal Properties of Australia evolution of the crust and to aid in mineral exploration and Surrounding Regions Derived from Interpretation of Magsat Anomaly Field Data

G. R. Keller The University of Texas Application of Magsat to Provide models of the seismic velocity structure of the at EI Paso Lithospheric Modeling in lithosphere (see Hinze above) South America: Part 11- Syn- thesis of Geologic and Seismic Data for Development of In- tegrated Crustal Models

David M. Klumpar The University of Texas Investigation of the Effects of Apply a modeling procedure to the vector Magsat data in order at Dallas External Current Systems on to separate the terrestrial component from that due to extrater­ the Magsat Data Utilizing restrial sources Grid Cell Modeling Tech­ niques

John L. LaBrecque Lamont-Doherty Analysis of Intermediate­ Determine the distribution of intermediate wavelength magnetic Geological Observatory Wavelength Magnetic Anom­ anomalies of lithospheric origin in the oceans, the extent to alies over the Oceans in which Magsat describes the distribution, and the cause of these Magsat and Sea Surface Data anomalies

J ean-Louis LeMouel · Institut de Physique du Magsat Investigations Consor- Reduce Magsat vector data for a global analytic field model Globe de Paris, France tium and constant altitude field maps; compare Magsat data to regional studies; study features of the core field; correlate globally and regionally Magsat and gravimetric data

Michael A. Mayhew Business and Technolog- Magsat Anomaly Field Inver- Construct a regional crustal temperature/heat flow model ical Systems, Incor- sion and Interpretation for the based on a developed magnetization model, heat flow/ produc- porated U.S. tion data, and spectral estimates of the Curie depth

Michael A. Mayhew Business and Technolog- Equivalent Source Modeling Model the core field; compute equivalent spherical harmonic ical Systems, Incor- of the Main Field Using coefficients for comparison with other field models; examine porated Magsat Data the spectral content of the core field

Igor I. Gil Pacca Instituto Astronomico e Structure, Composition, and Construct preliminary crustal models in the Brazilian territory; Geofisico-UPS, Brazil Thermal State of the Crust in point out possible variations in crustal structure among dif- Brazil ferent geological provinces

Volume I. Number 3.1980 217 Table 2 (continued)

LIST OF MAGSA T INVESTIGATORS

Investigator Institution/Country Title Objectives

Thomas A. POlemra The Johns Hopkins A Proposal for the I nvestiga­ Identify and evaluate high-latitude external fields from the University Applied tion of Magsat and Triad comparison of data acquired by the Magsat and Triad Physics Laboratory Magnetometer Data to Pro­ spacecraft that can be used to improve geomagnetic field vide Corrective Information models on High-Latitude External Fields

Robert D. Regan Phoenix Corporation Impro ved Defini tion of Develop an improved method for the identification of magnetic Crustal Magnetic Anomalies anomalies of crustal origin in satellite data by defining better in Magsat Data and removing the most persistent external field effects

Dav id P. Stern NASA/ Goddard Space Study of Enhanced Errors and Estimate the secular variation over the period 1965-80 by Flight Center of the Secular Magnetic Varia- removing mathematical instability based upon scalar field in­ ti on Using Magsat Models and tensity alone Those Derived in P ogo Surveys

David W. Strangway University of Toronto, Proposal to Analyze the Examine the expected difference between the Grenville and Canada Magnetic Anomaly Maps from Superior provinces Magsat over Portions of the Canadian and Other Shields lhn Jae Won North Carolina State Compatibility Study of the Evaluate the compatibility between the Magsat and University Magsat Data and Aeromagnet- aeromagnetic data in the eastern North Carolina Piedmont ic Data in the Eastern Pied- mont of the U.S. satellite surveys will be needed for accurate global first geomagnetic field model from Magsat, desig­ representations beyond the lifetime of Magsat. nated MGST (6/80).7 The spherical harmonic coef­ Although Pogo data were global and taken over ficients are given in Table 3. Vector data above 50° a short time span, the limitation of measuring only latitude were not included because of non-curl-free the field magnitude resulted in some ambiguity in fields from field-aligned currents. Such fields are the field direction in spherical harmonic analyses transverse to the main field and, as shown by 3 6 based on Pogo data alone. - This ambiguity is be­ Langel,2 have little or no effect on the field ing removed by the acquisition of global vector magnitude. However, 3354 values of B" 3345 of data with Magsat. Bo, and 3361 of B were included for latitudes The USGS is currently in the process of updating below 50°. (B" Be, and B are the radial, north­ world and national magnetic charts and field south, and east-west components - in a spherical models. Three sets of charts are being prepared: a coordinate system - of the geomagnetic field.) U.S. chart of declination, a U.S. chart of total This model fits the selected data with the mean field intensity, and a set of world charts for all and standard deviations as follows: components. The declination chart was finalized in Mean Standard early 1980, the total intensity charts will be by the Component Deviation Deviation end of 1980, and the world charts by August 1981. (nT) (nT) Magsat data will contribute to all of these charts. To provide timely input for all applications, a Scalar mag- series of field models will be derived from data nitude 0.1 8.2 during magnetically quiet times over the period of B, 2.5 6.5 data accumulation. One model has already been Be -0.1 7.6 generated from two full days of data. 7 At ap­ B 0.5 7.4 propriate intervals it will be updated with addi­ tional data, culminating in a final model incor­ The deviations are mainly due to a combination of porating all data with fine attitude coordinate ac­ fields from unmodeled external and crustal sources. curacy, to be available about October 1981. Magsat As part of our preparation for Magsat, Pogo data will also be combined with data from the data were combined with surface data from obser­ Pogo satellites and other sources to derive predic­ vatories between 1960 and 1977, with selected tive models with temporal terms. repeat data, and with selected shipborne data to Of the Magsat data currently available for analy­ derive what we consider to be the best possible pre­ sis, November 5 and 6, 1979, showed the lowest Magsat model. 8 Constant and first-, second-, and value of magnetic activity indices. Plots of the data third-derivative time terms were included to degree indicated that those days were indeed very quiet and order 13, 13, 6, and 4, respectively. This magnetically. Accordingly, a selection of 15,206 model, designated PMAG (7/80), used a new tech­ data points from those days was used to derive the nique for dealing with the observatory data. As at

218 Johns Hopkins APL Technical Digest Table 3

SPHERICAL HARMONIC COEFFICIENTS FOR MAGSAT GEOMAGNETIC FIELD MODEL MGST (6/80)

Internal Coefficients (nn

m n m gn m h n m n m gflm h n 0 - 29989.6 9 8 1.4 -5.9 1 1 - 1958 .6 5608.1 9 9 -5.1 2.1 2 0 - 1994.8 10 0 - 3.3 2 1 3027.2 -2127.3 10 1 -3.5 1.4 2 2 1661.6 - 196 .1 10 2 2.5 0.4 3 0 1279.9 10 3 -5.3 2.6 3 I - 2179.8 - 334.4 10 4 -2.1 5.6 3 2 1251.4 270.7 10 5 4.6 - 4.2 3 3 833 .0 -251 .1 10 6 3.1 - 0.4 4 0 939.3 10 7 0.6 - 1.3 4 I 782 .5 211.6 10 8 1.8 3.5 4 2 398.4 -256.7 10 9 2.8 -0.5 4 3 -419.2 52.0 10 10 - 0.5 -6.2 4 4 199 .3 - 297.6 II 0 2.4 5 0 - 217.4 II I - 1.3 0.7 5 1 357.6 45.2 II 2 -1.9 1.7 5 2 261.0 149.4 11 3 2.2 -1.1 5 3 -73.9 - 150.3 II 4 0.1 - 2.7 5 4 - 162.0 -78. 1 II 5 -0.4 0.6 5 5 -48.3 91.8 II 6 - 0.3 - 0.1 6 0 48 .3 II 7 1.7 -2.4 6 I 65 .2 -14.5 II 8 1.8 - 0.3 6 2 41.4 93.4 11 9 - 0.6 -1.4 6 3 - 192 .2 70.6 II 10 2.1 -1.6 6 4 3.5 - 42.9 II II 3.5 0.6 6 5 13 .7 -2.4 12 0 - 1.6 6 6 - 107 .6 16 .9 12 1 0.4 0.6 7 0 71.7 12 2 - 0.1 0.6 7 I - 59.0 -82.4 12 3 - 0.1 2.3 7 2 1.6 - 27.5 12 4 0.6 -1.5 7 3 20.5 -4.9 12 5 0.5 0.5 7 4 - 12 .6 16.1 12 6 -0.6 0.2 7 5 0.6 18. 1 12 7 - 0.4 -0.4 7 6 10.6 -22.9 12 8 0.1 0.0 7 7 - 2.0 - 9.9 12 9 -0.4 0.0 8 0 18.4 12 10 -0.2 - 1.5 8 I 6. 8 6.9 12 II 0.7 0.3 8 2 -0.1 - 17 .9 12 12 0.0 0.7 8 3 -10.8 4.0 13 0 0.0 8 4 - 7.0 -22.3 13 I -0.5 - 0.4 8 5 4.3 9.2 13 2 0.3 0.4 8 6 2.7 16 .1 13 3 - 0.7 1.6 8 7 6.3 - 13.1 13 4 0.0 0.0 8 8 - 1.2 -14.8 13 5 1.2 - 0.6 9 0 5.6 13 6 - 0.4 -0. 1 9 I 10.4 - 21.1 13 7 0.4 0.8 9 2 1.1 15.2 13 8 -0.6 0.2 9 3 - 12.6 8.9 13 9 0.2 0.8 9 4 9.5 - 4.8 13 10 0.1 0.5 9 5 -3.3 -6.5 13 11 0.4 - 0.1 9 6 - 1.3 9.0 13 12 -0.4 0.0 9 7 6.8 9.5 13 13 0.0 - 0.1

External Coefficients (nn

n m gnl1l /1,,11/

0 20.4 -0.6 - 0.4

The mean radius of the earth is 6371.2 km The mean epoch is 1979.85

Volume I, Number 3,1980 219 the satellite, surface fields contain contributions investigators under the direction of LeMouel and from the three sources. For field modeling the an­ the British investigators under the direction of Bar­ nual mean value for each observatory is usually raclough. The USGS investigators, in addition to used so that the external contribution is some publishing charts of the field, will pay particular averaged value (we hope, small). However, crustal attention to the problem of separating the core, anomaly fields are typically quite large. For past crustal, and external contributions to the field, and models the rms residual of surface data to the of developing an adequate mathematical represen­ model has been typically between 100 and 200 nT. 9 tation of each. The French and British will attempt We have incorporated a procedure for solving in­ a very accurate model of secular variation for pre­ dependently for the "bias" or anomaly field in dicting the field in the future and for studying pro­ each component at each observatory. This pro­ perties of core fluid motions and interactions at the cedure was used for the first time in the derivation core/mantle boundaries. In addition to the global of the PMAG (7 / 80) model. After solving for the models, several investigators will attempt more ac­ coefficients of the potential function and the obser­ curate regional models for particular areas in order vatory biases, the rms residual of the observatory to isolate better the crustal anomaly fields in those data was reduced to below 28 nT for all com­ areas. ponents. For repeat data this procedure was not Magsat will provide an accurate model of the feasible because of a lack of extensive temporal main geomagnetic field, but its lifetime was too coverage. These data were incorporated by in­ short for determining the secular variation of the cluding only locations where three or more field. The secular variation can be determined measurements, made at different times, were through comparison with earlier global surveys of available. Each component at each location was the geomagnetic field from space, but such surveys then represented by a linear fit; only the rate of are known to suffer from enhanced errors in cer­ change was used in deriving PMAG (7 / 80). The tain sequences of harmonic terms. Stern (GSFC) residual to the fit was 10 nT per year for all com­ will use the Magsat model to evaluate the extent ~f ponents. For shipborne data, 39 long tracks in such enhanced errors, correct them, and then use regions devoid of other surface measurements were the corrected models for deriving the mean secular selected. These were low-pass filtered with a cutoff variation over the 1965-1980 period. of 500 km to eliminate the crustal anomaly field. Gibbs (Business and Technological Systems) will The rms residual to the fit was 38 nT. Extension of attack the secular variation problems from a sta­ this model by the addition of Magsat data does not tistical point of view by using recursive estimation change the above quoted residuals a great deal, and theory to combine conventional models into op­ the resulting model, GSFC (9/ 80), is in good agree­ timal estimates of the field parameters for any ment with MGST (6/ 80) at the Magsat epoch. given time. The statistical information so derived Using quick-look data from the first few days of should enable a more accurate prediction capability Magsat operation, various pre-Magsat models were as well as more accurate a posteriori models. evaluated. These results are summarized in Table 4. An alternative to the classical spherical harmonic Global geomagnetic models will be undertaken representation will be attempted by Mayhew by the USGS (Cain, Fabiano) and by the French (Business and Technological Systems). He will

Table 4

RESIDUALS OF SELECTED MAGSA T DATA TO SOME PUBLISHED FIELD MODELS (nT)

11 14 POGO (2 /72)10 IGRF 1975 A WC/75 12 IGS/75 13 WC80 PMAG(7/ 80)H Scalar: mean deviation 9 -91 60 21 -21 -2 standard deviation 105 124 125 119 117 79 rms deviation 105 154 139 121 119 82

Br : mean deviation 6 12 25 20 31 13 standard deviation 208 192 152 134 121 93 rms deviation 208 192 154 135 125 94

Be: mean deviation 18 51 -4 15 22 33 standard deviation 114 108 93 84 89 59 rms deviation 115 120 94 86 92 67

B .. : mean deviation 0 2 0 I standard deviation 165 125 89 89 61 61 rms deviation 165 125 89 89 61 61

220 Johns Hopkins APL Technical Digest adopt methods of anomaly modeling by equivalent become invalid, thus necessitating development of sources, representing the main field with an array new analysis techniques. of dipoles at a fixed radius within the core. If such Originally, it was thought impossible to detect a method proves feasible, it will potentially use less fields of crustal origin in satellite data. However, computer time than the usual methods. while analyzing data from the Pogo satellites, Regan et al. 20 discovered that the lower altitude CRUSTAL MAGNETIC ANOMALY data contained separable fields caused by crustal anomalies, thus opening the door to a new class of INVESTIGA TIONS investigations. None of the satellites shown in A crustal magnetic anomaly is the residual field Table 1 was designed for solid earth studies, yet after estimates of the core and external fields have results from the Pogo satellites have demonstrated been subtracted from the measured field. An the capability of mapping broad-scale anomalies. anomaly map is a contour map of the measured Although the map of Regan et al. was partially average anomaly field at the altitude of the data. contaminated by "noise" from magnetospheric and Anomaly maps have been derived from aeromag­ ionospheric fields, the reality and crustal origin of netic and shipborne data for many years and used several of the anomalies defined by the map were in the construction of geologic/ geophysical models clearly demonstrated. More recently Langel et al. 10 of the crust. Investigations with aeromagnetic and have compared a Pogo-derived anomaly map with shipborne magnetic data have mainly concentrated upward-continued aeromagnetic data from western on the very localized anomalies associated with Canada. Figure 1 shows the results of that com­ small-scale geologic features and localized minerali­ parison. The two maps are in substantial agree­ zation. However, in the past few years there has ment, demonstrating further both the reality and been an increased interest in studies of the broad­ crustal origin of the anomalies. scale anomalies that appear in regional compila­ The techniques for preparing such a map include 15 19 tions of aeromagnetic and shipborne data. - selecting suitable quiet-time data, removing the best Satellite anomaly maps are of recent origin and estimate of the fields not originating in the earth's describe only the very broadest scale anomalies. crust, and averaging data at the appropriate resolu­ Aeromagnetic and ship borne anomaly maps have tion. It is believed that these techniques can be usually been interpreted assuming a flat earth and a readily adapted to Magsat data, both scalar and constant ambient field over the region of interest. vector. It will be some months before an anomaly Because of the extremely large scale of satellite­ map is available from Magsat data. Individual pass derived anomalies, both of these assumptions residuals, however, clearly show the presence of

Fig. 1-Comparison of anomaly maps from upward continued aeromagnetic data and from Pogo data. Units are nanoteslas (nT).

Volume 1, Number 3,1980 221 these crustal fields. Figure 2, for example, shows ic crust and derives the magnetization in such a the Bangui or Central African anomaly first dis­ crust that would cause the anomalies seen at the covered by Regan et al. 20 spacecraft. All of the anomalous field is assumed The basic anomaly maps are only a starting point to be induced; i.e., remanent magnetization is for interpretation. To maximize their usefulness assumed to be zero. The results for the United they must be transformed to a common altitude States are shown in Fig. 4. In many regions, known and to the anomalies that would be present if the geologic features are clearly outlined (e.g., the earth's field had the same inclination everywhere Basin and Range Province, Colorado Plateau, Rio (reduction to a common inclination). Preliminary Grande Rift, Michigan Basin, and Mississippi Em­ techniques for reduction to common altitude now bayment) whereas some features are notable by the exist and have been applied to Pogo data between absence of magnetic features (e.g., the mid­ ± 50° latitude. The resulting map is shown in continent gravity high). It will be some years before Fig. 3. these maps are fully understood and interpreted, For geologic studies such anomaly maps must be but they promise to shed new light on the geology inverted to a description of the magnetic properties of the deep crust. of the crustal rocks. The inversions are not unique; Anomaly maps, or even magnetization maps, are constraints from other data sources will be required not an end in themselves. The object of these ef­ in their interpretation. As a first step in such forts is to derive models of the crust and upper modeling: traditional equivalent source methods, mantle for large-scale regions of the globe. There adapted for the case of a spherical earth with are many kinds of models; their common purpose changing field inclination, have been applied to the is to generalize observations and prediction. United States 21 and Australia.22 This technique Through synthesis of particular models, complex assumes a constant 40 km thickness of the magnet- models of crustal geologic systems are built up in terms of structural and compositional variations and the movements of material and energy. Con­ 25,------,------, clusions can then be drawn about the evolution of regions that lead to inferences about the distribu­ tion of natural resources. Figure 5 is an outline of an example of how one might think of the process of synthesizing models, aJ

-25~------~~------~ Curie isotherm can be developed from satellite 30° 0° Latitude data. This serves to quantify the anomaly map and can be of great utility in more detailed analysis of Fig. 2-The Bangui or Central African anomaly as seen in the data. Correlatively, from gravity measurements, Magsat data. The equatorial longitude is 14.6'; the altitude is 425 km . .lB is the residual anomalous field variations in mean density to some fixed depth can after subtracting a model of the core field from the be inferred, assuming homogeneity below that measured field. depth. These models can be combined with velocity

50

•

., 120 160 200 280 40 320 -'- - - Na noteslas 360 -8 _to - 10 -6_ to -8 -4_ to -6 - 2CJ to - 4 2C:= to 4 4 to 6 6 to 8 8 to 10 C] Fig. 3-Scalar magnetic anomaly map from the Pogo satellites reduced- to- 500 km altitude. 222 Volume I, Number 3,1980 Fig. 4-Equivalent bulk magnetization derived from Pogo satellite data assuming a constant·thickness magnetic crust of 40 km. Units are EMU/cm3 x 104 • models based on seismic data and with composi­ The Pogo satellites from which these maps were tional models based on laboratory measurements of derived had two severe limitations for the studies: rock properties to give large-scale models of crustal the altitude range was high (most data were from structure and composition. Similarly, models of rel­ above 500 km), and the data were of field magni­ ative movements and of temperature distribution tude only. Magsat addresses both shortcomings. Its can be built for very large regions. These large­ lower altitude increases the resolution by roughly a scale models can then be used to make more de­ factor of 2 and the field strength of the anomalies tailed models for smaller regions using a variety of by a factor of between 2 and 5, depending on the data, some of which are also listed in Fig. 5. There geometry of the source region. This is dramatically are no hard and fast rules about the combination illustrated in Fig. 6, which shows a comparison be­ sequence, which depends on the region and the tween the lowest altitude Magsat data and low­ data available. Further, the process is an iterative altitude Pogo data. The Magsat data are at 187-191 one in which new data are sought based on predic­ km, while the Pogo data are at 414-420 km. The tions from interim models. tracks are nearly coincident geographically.

Data (examples) Lithospheric Models

Satellite Overall regional model Magnetics Relative magnetization/curie isotherm depth Gravity Density Imagery (regional) Stru ctu ref to pograp hy / mega linea me nts

Correlative Seismic Regional velocity structure Heat flow Regional heat flux Field geology Geologic/tectonic maps Comparative planetology Evol utionary models

Satellite Imagery (visual/ I R/therm all Altimetry Radar Passive microwave Refined composite model for selected Fig. 5-Synthesis of geolog· Correlative subregions ic/geophysical models (ideal­ Aeromagnetic ~ ized outline). Surface gravity Detailed seismology Magnetote II uric Petrologic/ radiometric Tectonic history Tilt, vertical movement Resource assessment, Mineralization distributio n Hazard assessment

Volume I, Number 3,1980 223 Johnson and Anomaly map at 500 km DD •• Dooley Australia and Antarctica derived from Pogo data -4 to - 6 - 2 to -4 2 to 4 4 to 6 (nT) LeMouel Europe and Central Africa This type of modeling will be used by the in­ vestigators just listed for the continent-size regions indicated. When the larger region has been model­ ed, features of particular interest will become the subject of more intensive investigations. In some cases existing tectonic features have already been singled out for more localized modeling, such as the Narmada-son lineament in central India (Bhargava), the Superior province of Canada (Strangway), or the Japan Trench (Fukushima). Continental-scale studies will utilize maps made at GSFC, including those of Pacca (Brazil), Bentley (Antarctica), and Hastings (Africa and South America). Mayhew, Hinze, Coles, and Pacca will pay par­ ticular attention to mapping the Curie isotherm. Hinze, Keller, and Hastings are interested in the implications of the Magsat data for the plate­ tectonic reconstruction of Africa and South Ameri­ 140 160 180 ca, a topic now under study by Langel, Frey, and Longitude (deg) Mead at GSFC using the Pogo data. 2.0 Magsat data; June 10, 1980 In addition to the continental-scale studies, I titude: 187-191 km several investigators will study more limited areas ~ - 14.0 or particular tectonic features. Godivier '0 r''''"'---- _ a; _30.0L----"'--_~'-----"_ ___1_ ___1-=.___1_ ___1_--'_ ___' (ORSTOM) will extend the work of Regan and :;: Marsh24 in the region around the Central African Pogo-4 data; December 17, 1967 ~ 9.0 Republic where ORSTOM has a large amount of .'0;;; Altitude: 414-420 km ~ -3.0 correlative data. Gasparini (Osservatorio Vesu­ viano) will investigate the Curie depth and -15,0 '-----"~'--'--___1 _ ___1_ ___1_ ___1_ ___1_--'_ ___' volcanism in the Mediterranean area. Won (North 34.9 32.1 29.3 26.5 23.7 20.9 18.1 15.3 12,5 9.7 Latitude (deg) Carolina State University) will study a combination of Magsat, aeromagnetic, and regional gravity data Fig. 6-Comparison between Pogo (OGO·4) data and the in the eastern Piedmont of the United States. Car­ lowest altitude data from Magsat. michael and associates (University of Iowa) will study the central midcontinent of the United States with particular attention to the known midconti­ Magsat's lower altitude greatly enhances the resolu­ nent geophysical anomaly. tion and signal strength of the anomalies. With In contrast to the large number of investigators field magnitude data, only indirect estimates of studying continental-type regions, only three in­ remanent magnetization are possible. 23 The vector vestigators are giving concentrated attention to data from Magsat measure anomaly directions oceanic regions. There are several reasons for this. other than along the earth's main field. First, the satellite anomaly maps derived from GSFC will derive a basic global magnetic anoma­ Pogo data show very few anomalies in oceanic ly map from the Magsat data. Many investigators regions compared to continental regions. Second, will use that map or the associated magnetization theoretically, the thinner oceanic crust should not map directly in their investigations. Other investiga­ contain anomalous features of comparable size to tors will reexamine the basic derivation of the continental crust. Harrison (University of Miami) anomaly map and seek to extend or modify these notes that a minimum in the power spectrum techniques. Among the latter are: should occur between the contributions from core and crustal sources but that such a minimum is not present in spectra from shipborne data over oceanic Investigator Region Investigated basins. He will use Magsat data to study the inter­ Bhargava India mediate wavelength anomalies. Brammer (T ASC) Fukushima Japan and vicinity will concentrate his efforts on a study of Magsat Coles and Hall Canada and Geos-3 altimeter gravity data in the eastern In­ Hinze and Keller South and Central America dian Ocean. Mayhew United States LaBreque (Lamont-Doherty) will organize the ex-

224 johns Hopkins APL Technical Digesl isting shipborne data in an effort to help describe fields result in induced fields within the earth the secular variation over oceanic areas and to pro­ because of the finite conductivity of the earth. The vide surface anomaly maps suitable for upward characteristics of these induced fields are deter­ continuation and comparison with Magsat data. mined by the properties of the materials in the Underlying all crustal models derived from earth's mantle (i.e., composition, temperature). At Magsat and correlative data is a need for present the limiting factor in determining a precise understanding the basic magnetic properties of conductivity profile within the earth, with adequate crustal rocks. Such understanding depends on spatial resolution, is the accuracy possible in deter­ careful laboratory measurements, some at the mining the external and induced fields. Hermance higher temperatures and pressures of the lower (Brown University) will use Magsat vector measure­ crust. Preliminary studies25 have already claimed to ments together with surface data for a more accu­ show the extremely significant result that the Moho rate analysis than was previously possible. is a magnetic boundary even when the Curie iso­ therm lies in the mantle. Wasilewski and the other STUDIES OF EXTERNAL GSFC investigators are continuing these efforts. CURRENT SYSTEMS Particular attention to petrologic constraints, the Early observers of the earth's magnetic field effects of oxygen fugacity, and other properties will discovered that it continually undergoes transient be given by Haggerty (University of Massachu­ changes. These changes include systematic varia­ setts). A substantial refinement of the petrology of tions that occur with daily regularity and irregular source rocks responsible for deep crustal anomalies variations, both of amplitude and of a totally dif­ is expected. ferent kind, superimposed on the regular varia­ INVESTIGATIONS OF THE tions. Periods of time when the changes are mostly regular are called magnetically quiet; periods when INNER EARTH the magnetic disturbances become irregular are Man has directly penetrated only a few called magnetically disturbed. When the irregular kilometers of the 6371 km distance to the earth's fields are large but mainly confined to high lati­ center. Information about the inner earth must be tude, the condition is called a magnetic substorm; obtained by indirect methods such as seismology when the irregular fields are large and worldwide, and measurements of the gravitational and mag­ it is called a magnetic storm. To understand such netic fields. changes in field, one must realize that the space Combining Magsat data with Pogo and near­ surrounding the earth contains several "species" of surface surveys will permit more accurate deter­ electric current. The energy for these currents mination of the secular variation of the core field. comes ultimately from the sun. Figure 7 is an ar­ This variation will be used by Benton (University tist's conception (based on Fig. 1 of Heikkila/6 of Colorado) to study properties of the fluid mo­ wherein there is a more detailed explanation) of the tions in that core; in turn, appropriate magnetohy­ magnetic environment of the earth. A stream of drodynamic constraints will be investigated to charged particles from the sun, called the solar determine if they can aid in better modeling the wind, confines the earth's magnetic field to a cavity secular variation. known as the magnetosphere. This cavity is com­ When they are time-varying, magnetospheric pressed on the front, or sunward, side and drawn

Fig. 7 -Artist's conception of the configuration of the mag­ netosphere, adapted from Heikkila.26 The magneto­ sphere is the region into which the earth's magnetic field is confined by the solar wind.

Volume I, Number 3, 1980 225 out in a "tail" to the antisunward side. Currents flow, as shown, on the boundaries of this cavity and across the tail. Also, trapped particles within the cavity flow in a "ring current" in a westward direction around the equatorial plane. Most of these currents are relatively distant from the earth and cause only small fields at Magsat locations. However, the ring current intensifies considerably during periods of magnetic disturbance and causes substantial fields at Magsat altitude. In addition to the currents shown in Fig. 7, a 18 f----+--!.J.JIl-1Ll--=-i I--L-':'+--~----l----I 6 variety of currents flow in the conducting layer of the atmosphere known as the ionosphere. The regu­ lar daily variations of the field observed at the sur­ face are from such a current system, known as Sq (S for solar daily variation and q for quiet times). Sq is mainly a low- and mid-latitude phenomenon. At high latitudes very intense currents flow, often associated with auroral phenomena. These currents, illustrated schematically in Fig. 8,27 are coupled to the ring current and to currents in the tail of the o magnetosphere. Fig. a-Conceptual drawing of Ionospheric current flow Because the Magsat orbit was near sun­ at high altitudes. Arrows indicate direction of current synchronous, it sampled mainly twilight local flow. Closure is not shown because some circuits close times, a distinct disadvantage for synoptic studies in the outer magnetosphere via field·aligned current. of the magnetospheric fields, which are relatively fixed in local time. However, because it is the first near-earth satellite to obtain global vector measure­ Klumpar (University of Texas) will concentrate his ments and because its measurement accuracy far effort on extending existing models for high­ exceeds that of spacecraft that obtained some near­ latitude ionospheric currents and the field-aligned earth vector measurements, Magsat will be very currents coupling the ionospheric currents to the useful in extending previous research regarding cur­ magnetosphere. Burrows (NRC, Canada) and rent systems. Potemra (The Johns Hopkins University Applied One reason for investigating these external cur­ Physics Laboratory) will investigate the field­ rent systems with Magsat is to aid in isolating their aligned currents, also using correlative data from fields from the core and crustal fields. Several in­ satellite photographs of auroral phenomena (Bur­ vestigators will contribute to this effort. Regan rows) and with simultaneous data from the Triad (Phoenix Corp.) in particular will work toward re­ satellite (Potemra). moving external field effects from anomaly data. Figure 9 shows the effects of field-aligned cur-

Fig. 9-Magnetic field variation caused by field­ al igned current. The scale, in nT, is relative only.

Time (min) 1190.0 1190.2 1190.5 1190.71 1190.9 1191.1 1191.3 1191 .6 1191.8 1192.0 Latitude (deg) -57.1 -57.9 -58.7 -59.6 -59.6 -60.4 -61 .2 -62.0 -62.8 - 63.6 Longitude (deg) 142.4 142.1 141 .7 141 .2 140.8 140.4 139.9 139.4 138.9 138.3 Altitude (km) 524.6 526.0 527.3 528.6 529.9 531 .2 532.5 533.7 535.0 536.2

226 Johns H opkins APL Technical Digesl rents in Magsat data. b.X, b. Y, b.Z are residuals 9J. C. Cain, S. J . H endricks, R. A. Langel, and W. V. Hudson, "A Proposed Model for the International Geomagnetic Reference Fields - from a field model in a north-, east-, and down­ 1965 ," 1. Geomagll. GeoeleClr. 19, pp. 335 - 355 (1967). coordinated system. Large-amplitude, highly st ruc­ tOR. A. Langel, R. L. Coles, and M. A . Mayhew, "Comparisons of tured variations occur in the north and east direc­ Magnet ic Anomali es o f Lit hospheric Origin as Measured by Satellite and by Airborne Magnetometers over Western Canada," Call. 1. tions but not in the vertical direction, as expected Earlh Sci. X VII, pp. 876 - 887 (1980). from field-aligned currents. tt lAGA Divis ion I Study Group, " International Geomagnetic Reference Field 1975," EOS, 57, pp. 120 - 121 (1976). t2D. R. Barraclough, J. M. Harwood, B. R. Leaton, and S. R. C. CONCLUSION Malin, " A Model of the Geomagnetic Field at Epoch 1975" Geophys. Satellite-based magnetic field measurements 1. R. ASlroll . Soc. , 43, pp. 645 - 659 (1975) . l3N. W. Peddie and E. B. Fabiano, "A Model of the Geomagnetic Field make global surveys practical for both field model­ for 1975 ," 1. Geophys. Res. 81 , pp. 1539 - 2542(1976). ing and for the mapping of large-scale crustal 14F. S. Baker and D. R. Barraclough, " World Magnetic Chart Model for 1980," EOS 61, p. 453 (1980). anomalies. They are the only practical method of 15L. C. Pakiser and l. Zietz, "Transcontinental Crustal and Upper Man­ accurately modeling the global secular variation. tl e Structure," Rev. Geophys. 3, pp. 505 - 520 (J965). Magsat is providing a significant contribution, both t6l. Zietz, E. R. King. W. Geddes, and E. G. Lidiak, "Crustal Stud y of a Continental Strip from the Atlantic Ocean to the Rocky Moun­ because of the timeliness of the survey and because ta ins," Geolog. Soc. Am. BII/I. 77, pp. 1427 - 1448 ( 1966) . its vector measurement capability represents an ad­ 17R. T. Shuey, D. R. Schellinger, E. H. Johnson, and L. G. Alley, vance in the technology of such measurements. "Aeromagnetics and the Transition between the Colorado Plateau and the Basin Range Province," Geology 1, pp. 107 - 110 (1973). Data from Magsat are available for any inter­ 18D. H. Hall, "Long-Wavelengt h Aeromagneti c Anomalies and Deep ested user through the National Space Sciences Crustal Magnetization in Manitoba and North Western O ntario, Canada," 1. Geophys. 40, pp. 403 - 430 (1974). Data Center at GSFC. t9A. A. Kruithovskaya and l. K. Paskevich, " Magnetic Model for the With the success of Magsat, future mi ssions Earth's Crust under the Ukrainian Shield," Call . 1. Earlh Sci. 14 , pp. should take two courses. Field modeling requires 27 18 - 2728 (1977). 2o R. D. Regan, J . C. Cain, and W. M. Davis, " A Global Magnetic periodic surveys, but not low-altitude measure­ Anomaly Map," 1. Geophys. Res. 80, pp. 794 - 802 (J975 ). ments as required for crustal studies. On the other 21M. A. Mayhew, "Satellit e Derived Equivalent Magnetization of the hand, further advances in satellite crustal studies United States" (in preparation, 1979). 22M. A. Mayhew, B. D. Johnson, and R. A. Langel, "Magnetic will rest on NASA's ability to orbit magnetometers Anomalies a t Satelli te Elevation over Aust ralia" (submitted to Earlh at still lower altitudes; such concepts are still in the Plallel. Sci. Lell. , 1980) . 23 B. K. Bhattacharyya, " Reduction and Treatment of Magnetic stage of discussions as to their feasibility. Anomalies of Crustal Origin in Satellite Data," 1. Geophys. Res. 82 , pp. 3379 - 3390 (1977). 24R. D. Regan and B. D. Marsh, "The Bangui Anomaly: Its Geological REFERENCES Origi n," 1. Geophys. Res. (in press). 25p. J. Wasilewski, H . H. Thomas, and M . A. Mayhew, " The Moho as I J . C. Cain and R. A. La ngel, " Geomagnetic Survey by the Polar- Or­ a Magnetic Boundary," Geophys. Res. Lell. 6, pp. 54 1 - 544 (1979). biting Geophysical Obse rvatories," World Magnelic Survey 195 7- 26 W. J. Heikkila, " Penetration of Particles into the Polar Cap Regions 1969 (A. J. Zmuda, ed.) IAGA Bulleti n No. 28 (197 1) . of the Magnetosphere," Crilical Problems of MagnelUspheric Physics 2R. A. Langel, "Near Earth Magnetic Disturbance in Total Field at (E. R. Dyer, ed.), IUCSTP Secretariat (1972) . High Latitude, I. Sum mary of Data from OGO 2, 4, and 6," 1. 27R. A. Langel, "Near Earth Magnetic Disturbance in Total Field at Geophys. Res. 79, pp. 2363 - 237 1 (1974) . High Latitude, Interpretation of Data from OGO and 3G. E. Backus, "Nonuniqueness of the External Geomagnetic Field 2. 2. 4, 6," 1. Geophys. Res. 79, pp. 2373 - 2392 (1974). Determined by Surface Intensity Measurements," 1. Geophys. Res. 75 , pp. 6339 - 634 1 (1970). 4L. Hurwitz and D. G. Knapp, " Inherent Vector Discrepancies in ACKNOWLEDGMENTS - I wi sh to thank all members of the Magsat Geomagnetic Main Field Models Based on Scalar F," 1. Geophys. team for making possible the acquisition and analysis of these data. Res. 74, pp. 3009-3013 (1974). Although it is not feasible to name all those included, my special thanks 5D. P. Stern and J. H. Bredekamp, "Error Enhancement in go to Gil Ousley of GSFC and L. D. Eckard of APL for directing the ef­ Geomagnetic Models Derived from Scalar Data," 1. Geophys. Res. 80, fort that culminated in the success ful construction, launch, and operation pp. 1776 - 1782 (1975). of Magsat; to Mario Acuna and W. H. Farthing for the design and over­ 6F. J. Lowes, "Vector Errors in Spherical Harmonic Analys is o f Scalar sight of the magnetometers; to John Berbert and Earl Beard of GSFC Data," Geophys. 1. Roy ASlron. Soc. 42, pp. 637 - 651 (1975). and Don Berman of Computer Science Corporation and their colleagues 7R. A. Langel, R. H . Estes, G. D. Mead, E. B. Fabiano, and E. R. for their efforts at data preparation; and to Locke Stuart and hi s team at Lancaster, " Initial Geomagnetic Field Model from Magsal Vector GSFC for their successful organization of and interface with the Magsat Data," Geophys. Res. Lell. (in press). investigators. Finally, I particularly appreciate the encouragemenl, ad­ SR. A. Langel, R. H. Estes, G. D. Mead, and E. R. Lancaster, "A vice, and general support of Gil Mead, Lou Walter, and Barbara Model of the Earth's Magnetic Field, 1960 - 1980" (in preparation). Lueders of the GSFC Geophysics Branch.

Volume I, Number 3,1980 227 THOMAS A. POTEMRA

STUDIES OF AURORAL FIELD-ALIGNED CURRENTS WITH MAGSAT

Electric currents that flow between the outermost boundaries of the magnetosphere and the auroral zones, sometimes deliver more power to the earth than is generated in the entire United States at any given time. These field-aligned "Birkeland" currents are detected on a regular basis by Magsat.

INTRODUCTION modification to the attitude magnetometer system for the APLlU.S. Navy Triad satellite. The Triad Following earlier suggestions by Edmund Halley satellite was launched into a polar orbit at an and Anders Celsius that the auroral phenomena altitude of 800 km on September 2, 1972, and, un­ behave magnetically, Kristian Birkeland discovered til the launch of Magsat on October 30, 1979, was in his polar expeditions of 1902-03 that large-scale the only satellite making continuous and high­ currents were associated with the aurora. He was resolution magnetic observations in the polar also the first to suggest that these currents regions. The Triad magnetometer provides 2.25 originated far from the earth and that they flowed vector samples per second with a resolution of 12 into and away from the polar atmosphere along the nanoteslas (1 nT = 10,9 weber per square meter), geomagnetic field lines. The existence of such field­ made possible by the 13-bit analog-to-digital con­ aligned currents was widely disputed because it was verter incorporated into the system following the not possible from a study of only surface magnetic suggestion of Armstrong and Zmuda. The Triad field measurements to distinguish unambiguously magnetic field observations were used to determine between current systems that are field aligned) and for the first time the flow directions, spatial those that are completely ionospheric. 2 Further­ distribution, and intensities of Birkeland currents 3 more, theoretical calculations ,4 predicted that the and their relationships to a variety of geophysical effects of field-aligned currents may be impossible phenomena such as optical emissions, current­ to detect with ground-based magnetometers for cer­ driven plasma instabilities, ionospheric currents, tain models of these current systems. geomagnetic storms, and interplanetary phenome­ The first experimental confirmation of the pres­ na. During its brief lifetime, the improved Magsat 'ence of field-aligned ("Birkeland") currents was magnetic field observations were correlated with 5 provided by A. J. Zmuda and his colleagues ,6 who the Triad observations to compile more informa­ studied magnetic disturbances in the polar regions tion about the important Birkeland currents and to of the earth, observed with the single-axis magne­ calibrate the Triad magnetometer. Fortunately for tometer used in the attitude determination system these studies, Magsat was launched near the time on the APLlU.S. Navy 5E-I (1963-38C) satellite. of the peak solar cycle (the largest in many II-year That satellite, launched on September 28, 1963, in­ cycles) so that disturbances produced by the Birke­ to a nearly circular polar orbit at an altitude of land currents were observed on almost every orbit. 1100 km, carried an array of particle detectors and The importance of Birkeland currents to the a fluxgate magnetometer. 7 coupling between outer space and the auroral at­ 5 6 Zmuda • discovered that disturbances normal to mosphere is emphasized by their total intensity, the main geomagnetic field were often observed which ranges between 10 6 and 10 7 A, and by the with 5E-l in the same region of the earth associ­ energy they dissipate in the upper atmosphere, ated with a variety of auroral phenomena. It was which can exceed by a considerable factor the soon realized that the transverse disturbances could energy deposition associated with the dramatic, be interpreted as being caused by electric currents visual, auroral forms. The Birkeland currents are flowing along the geomagnetic field lines, and thus also the critical ingredient in a variety of com­ the first study of the global characteristics of plicated plasma phenomena that have important Birkeland currents was begun at APL. applications to earth (associated with substorms Recognizing the importance of sensitive satellite and the aurora), to Jupiter (associated with 10- magnetic field measurements to the study of Birke­ related radio emissions), ~.)O and to the galaxy land currents, Armstrong and Zmuda8 suggested a (related to comets and double radio sources). ))

228 johns Hopkins APL Technical DiJ!,esl MAGNETIC DISTURBANCES AND r:::::=:::J Current into ionosphere FIELD-ALIGNED CURRENTS r:::::=:::J Current away from ionosphere The magnitude and direction of equivalent cur­ IALI < 100 nT IALI> 100 nT rents associated with magnetic disturbances, AB, are determined from the relationship f = (1//1-0) curl AB. The disturbances occur predominantly in the east-west (geomagnetic) direction so that J = (l//1-o)(J/Jx) My, where J is the density of the field-aligned current, x is positive in the northward direction, and My is the observed disturbance in 12 the east-west direction. ,13 In this case, the formula is the same as that for the transverse magnetic dis­ turbance due to current sheets of infinite longitudi­ nal extent. At the altitude of Triad, a change in By equal to 100 nT over 10 of latitude is equivalent to /1-A/m2. Fig. 2-Statistical distributions of the location and flow a field-aligned current density of 0.64 The directions of large-scale Birkeland currents during weakly magnitude of the current intensity, Vdx, is given disturbed geomagnetic conditions (a) and during dis­ directly by the amplitude of the magnetic distur­ turbed periods (b) (from Ref. 16). bance My by Vdx = (11/1-0) My. A 100 nT distur­ bance corresponds to a current intensity of 0.08 Region 1 located at the poleward side and Region 2 Aim. located at the equatorward side. The Region 1 An example of a transverse magnetic disturbance Birkeland currents flow into the ionosphere in the often observed with Triad is shown in Fig. 1 (from morning sector and away from the ionosphere in Ref. 13) with the deduced field-aligned current den­ the evening sector, whereas the Region 2 currents sity and intensity. flow in the opposite direction at any given local time. The basic flow pattern of Birkeland currents PRINCIPAL CHARACTERISTICS OF remains unchanged during geomagnetically inactive BIRKELAND CURRENTS and active periods (Fig. 2), although the regions Large-scale Birkeland currents are concentrated widen and shift to lower latitudes during disturbed in two principal areas that encircle the geomagnetic periods (similar to optical auroral forms by Feld­ I7 pole. 12· 16 Figure 2 is a summary of the average stein ), spatial distribution in the northern high-latitude Figure 3 (from Ref. 18) is a schematic diagram region determined from 493 Triad passes during of the large-scale Birkeland currents that flow into weakly disturbed geomagnetic conditions and 366 and away from the auroral regions shown in Fig. 2 Triad passes during an active period (from Ref. and form cone-shaped regions. The current flow re­ 16.) The Birkeland current flow regions have been verses direction near midnight and near noon. This arbitrarily designated by Iijima and Potemra 14 , 15 as current system is permanent, with the total ampli­ tude ranging between 10 6 and 10 7 A, depending on the solar-terrestrial conditions. The striking visual forms of the aurora occur only when the incoming Ionospheric electric field I_southward) particles are energetic enough to penetrate to low

1540 UT May 4, 1973 altitudes and excite the atmospheric gas. Current out of Current into Analyses of Triad magnetometer data acquired over the south polar regions and recorded at McMurdo, Antarctica, indicate that the basic flow trfTft tTl'Trt pattern shown in Fig. 2 for the north pole is the same for the south pole. 19 At any given local time J = 5 x 10-7 A/m2 II and invariant latitude location, the field-aligned III = 8 x 10-2 Aim current flow is the same. 65 75 The average values of field-aligned current densi­ Invariant latitude, A Ideg) ty and intensity in the various regions and sectors discussed above are listed in Table 1 (from Ref. Fig. 1-An example of a magnetic disturbance transverse 16). The total current, estimated by integrating the to the geomagnetic field measured by Triad on May 4, 1973 (from Zmuda and Armstrong 13). The density and flow observed current intensity over the local time extent direction of the field-aligned currents producing these of the appropriate region, is also listed in Table 1. disturbances are determined from Maxwell's equation The total amount of current flowing into the iono­ J = (1//-'0) curl /lEI in the manner described in the text. sphere (including Regions 1 and 2) is always equal The direction of the appropriate electric field, also shown, to the total current flowing away (within 5OJo) for can produce a southward current flowing horizontally in the ionosphere to couple the upward and downward flow­ every level of magnetic activity. The fact that cur­ ing field-aligned currents. rent continuity is always preserved is important to

Volume I, Number 3,1980 229 Fig. 3-Cone·shaped regions formed by Birkeland currents flowing into and away from the auroral region. The cur­ rents on a given sheet reverse direction near midnight and noon (from Ref. 18).

Table 1 MAGSAT OBSERVATIONS OF A VERAGE CHARACTERISTICS OF LARGE-SCALE BIRKELAND CURRENTS FIELD-ALIGNED CURRENTS The Magsat spacecraft was launched into a Afternoon to Midnight to dawn-dusk synchronous orbit, whereas Triad is in a Midnight Forenoon polar orbit and appears to precess westward with Parameter Region 1 Region 2 Region 1 Region 2 respect to the earth-sun line at a rate of approx­ imately a degree per day. The orbital planes of the Current density two spacecraft were nearly coincident in November 2 (jJ.A/m ) 1979, when magnetic field measurements were Quiet -0.9 0.3 0.9 - 0.5 made at nearly the same point in time and space Disturbed -1.4 0.7 1.5 - 1.1 (separated only by their altitude difference and the Total current time differential related to their orbital periods - a (10 6 A) value that cannot exceed one-half the orbit period, Quiet - 1.3 0.8 1.5 - 1.1 or approximately 50 minutes, when they are mov­ Disturbed - 2.8 2.6 3.2 - 3.3 ing in the same direction). Data were acquired by Magsat and Triad during a disturbed period on A positive value denotes current flow into the ionosphere. November 7, 1979 at nearly the same place in the A negative value denotes flow away from the ionosphere. north auroral region. Segments of their orbital Values are averages over specified regions. tracks are shown in Fig. 4 superimposed on the statistical distribution of Birkeland currents shown the development of the models of the three-dimen­ in Fig. 2 determined for disturbed conditions. The sional current system that couples the magneto­ Triad observations were made at 17 : 12:30 UT, and sphere with the lower atmosphere and ionosphere. the Magsat observations 25.5 minutes later (at It is also significant that the total field-aligned cur­ 17:38 UT) . Regions of upward- and downward­ rent in Region I is significantly larger than the flowing Birkeland currents detected by Magsat and Region 2 total current during relatively quiet condi­ Triad are shown by the shaded areas of Fig. 4. tions, while during active periods the total current They are coincident and are close to the statistical flow in both regions becomes nearly equal. distribution. The orientations of the transverse

230 Juhlls Hopkins APL Technical Diges{ magnetometer axes (denoted A and B) for the two Theoretical magnetic field values (computed spacecraft with respect to their velocity vectors are from the 1965 Model International Geomagnetic shown in the upper right corner of Fig. 4. The A Reference Field) extrapolated to the period of and B axes for Triad are directed 135 and 45°, observation were subtracted from the Triad and respectively, from the velocity vector, and for Mag­ Magsat observations. The resulting disturbance vec­ sat are parallel and normal, respectively, to the tors were rotated from the spacecraft coordinate velocity vector. systems to the geomagnetic reference frame. The components in the geomagnetic east-west direction (i.e., tangent to the circles in Figs. 2 and 4) from

Triad/Magsat Triad and Magsat are shown in Fig. 5. These pre­ Nov. 7, 1979 liminary data show the amazing similarity of the (17:12:30 UT/17 :38:00 UTI details in the transverse magnetic disturbances mea­ IALI > 100 nT sured by different spacecraft. The disturbances are 12 over 1000 nT and can be interpreted as resulting 14 from a pair of intense Birkeland currents flowing into the ionosphere between about 67 and 65° magnetic latitude and away from the ionosphere between about 65 and 61 ° magnetic latitude. The density of these currents is about 2 {lA/m2 and their intensity is about 0.8 Aim. The processing of the Magsat data is in a prelim­ inary stage and work is only beginning on the in­ vestigation of disturbances associated with Birke­ 18r+~~-r----~-----+------~--~+-~~~6 land currents. The spatially separated measure­ ments provided by Magsat and Triad will permit the determination of the spatial extent of the Birkeland currents and of the differences, if any, in the intensities of these currents over sunlit and dark portions of the polar ionosphere. The improved sensitivity of the Magsat magnetometer and its lower altitude (in comparison to Triad) will also 22 2 provide data on the potential field contribution of ionospheric auroral currents that flow at an alti­ o tude of about 100 km (the magnetic effects of c::::::::::::J Currents into ionosphere which have been observed on the ground on a regu­ c=::::J Currents away from ionosphere lar basis since Birkeland's original work). The ulti­ mate objective of these studies is to understand and Fig. 4-0rbital tracks of Triad and Magsat superimposed define the variable high-latitude magnetic distur­ on the statistical pattern of Birkeland currents shown in Fig. 2. The locations and flow directions of the Birkeland bances to assist in the Magsat program's goal of currents detected by the two satellites are shown by the obtaining an accurate, up-to-date quantitative de­ shaded areas. scription of the earth's main geomagnetic field.

UT 63300 63360 63420 63480 63540 MLT 0426 0450 0504 0513 0520 MLA 78.71 74.92 71.05 67.12 63.18 500r-----,-----,-----,------,-----.-----,-----.------.----. l­ e: Triad/Chatanika 11/7/79 17:09 (UT) Magsat 11/7/79 17 :25 (UT)

t.l e:'" -eOJ .~ Fig. 5-The geomagnetic east· "0 t.l west component determined .;::; from the Magsat and Triad '"e: C> observations. The horizontal E -500 scale lists universal time (UT) UT - Universal time (s) in seconds, magnetic local ML T - Magnetic local time (h, min) time (MLT) in hours and MLA - Magnetic latitude (deg) minutes, and magnetic latitude (MLA) in degrees.

-1000~----~----~----~----~-----L----~----~------L---~ UT 61800 61860 61920 61980 62040 62100 62160 62220 ML T 0448 0515 0536 0552 0604 0615 0623 0631 MLA 72.87 70.02 66.98 63.80 60.54 57.22 53.85 50.45

Volume I, Number 3, 1980 231 REFERENCES and NOTES of Field-Aligned Currems at 800 km in Ihe Auroral Region: Initial I K. Birkeland, The Norwegian Aurora Polaris Expedilion, 1902 - 03, ResulI s," J_ Geophys. Res. 78, p. 6802 (1973). Aschehoug, Oslo, Norway (1908). 9p. A. Cioulier, R. E. Daniell, Jr., A. 1. Dessler, and T. W . Hill, "A 2E. H. Vestine and S. Chapman, "The Eleclric Currelll Syslem of Comelary Ionosphere Model for [0," ASlrophys. Space Sci. 55, p. 93 Geomagneti c DislUrbances," Terr. Magn. Almos. EleCIr. 43 , p. 35 1 (1978). (1938) . tOA. J. Dess ler and T. W . Hill , "Jovian Longitudinal Control of 10- 3N. Fukushima, " Equivalence in Ground Geomagnetic Effect of Related Radio Emissions," ASlrophys. J. 227, p. 664 (1978 ). Chapman-Vesline's and Birkeland-Alfven's Electric Currelll Syslems I I H . Alfve n, "Elecnic Currents in Cosmic Plasmas," Rev. Geophys. for Polar Magnelic Storms," Rep. [o//os. Space Res. Jpn. 23 , p. 219 Space Phys. 15 , p. 271 (1977). (1969). 12A. J . Zmuda and J. C. Armstrong, "The Diurnal Varialion of Ihe 4N . Fukushima, "Generalized Theorem for No Ground Magnelic Effect Region with VeClor Magnelic Field Changes Associated with Field­ of Venical Currelll Connecled with Pedersen Currents in the Uniform­ Aligned C urrents," J. Geophys. Res. 79, p. 2501 (1974). ConduClivit y Ionosphere," Rep. hmos. Space Res. Jpn . 30, p. 35 13 A. J. Zmuda and J . C. Armstrong, " The Diurnal Flow Pall ern of (1976). Field-A li gned Currents," J. Geophys. Res. 79, p. 461 I (1974). 5A. J. Zmuda, 1. H. Manin, and F. T. Heuring, "Transverse Magnetic 14T. lijima a nd T. A. Potemra, "The Amplitude Dislribution of Field­ Dislurbances al 1100 km in Ihe Auroral Region," J. Geophys. Res. 71, Aligned Currems at Nonhern High Latitudes Observed by TR[AD," p. 5033 (1966). J. Geophys. Res. 81, p. 2165 (1976). 6A. J. Zmuda, F. T. Heuring, and J. H. Manin, "Day Side Magnetic 15T. [ijima and T. A. POIemra, "Field-Aligned Currenls in Ihe Dayside DislUrbances al 1100 km in Ihe Auroral Oval," J. Geophys. Res. 72 , C usp Observed by TRIAD," J. Geophys. Res. 81 , p. 597 I (1976). p. 1115 (1967). 16T. lijima and T. A. Potemra, "Large-Scale Characteristics of Field­ 7The 5E series of satellites was designed by APL in the period 1962-64 Al ig ned C urrents Associated with SubsLOrms," J. Geophys. Res. 83, p. to make scienti fi c and engineering measuremelllS of the environment 599 (1978). of the Transi l navigalion satellites. Four were launched piggyback on 17 y. 1. Feldstein, "Peculiarities in the Auroral Distribution and Transit Type 5 satellites, and Ihree achieved orbi!. SlUdies of the 5E Magnetic Di sturbance Distribution in High Latitudes Caused by the spacecraft data were carried out by scientis ts around the world, and 43 Asymmetrical Form of the Magnetosphere," Planel. Space Sci. 14, anicles were eventually published on Iheir findings in profess ional p.121 (1966). journals. The 5E- 1 satellite became one of the most productive ever 1ST . A. POlemra, " Aurora Borealis: The Greatest Light Show on laun ched , still producing data in ils eleventh year (a full solar cycle), Eanh," Smilhsonian 7, p. 64 (1977). when a sy mposium was held at APL on September 27, 1974, t9T. A. Pote mra, S. Favin , and T. lijima, "Field-Aligned Currellls in celebrating the outstanding success of the 5E program. the North and South Auroral Regions Measured with TRIAD," EOS, SJ . C. Armstrong and A. J. Zmuda, " Triaxial Magnelic Measuremellls 56, p. 617 (1975).

232 Johns Hopkins APL Technical Digesl ______SPECIALTOPIC

FREDERICK S. BILLIG

CHINA-- AS VIEWED BY AN AEROSPACE ENGINEER

From November 5 through 18, 1979, a group of cipal educational institution in aeronautical eight U.S. scientists and engineers (Fig. 1) visited engineering in China. Currently there are 3000 ramjet and turbojet engine facilities in the People's undergraduate and 200 graduate students. Its prin­ Republic of China. cipal areas of study are aerodynamics, propulsion, We took the train from Hong Kong to Canton structures, radio (electrical) engineering, and instru­ on November 5, stayed overnight in Canton, and mentation. flew to Beijing (Peking) the next morning (Fig. 2) . After a warm greeting and a terse overview of The next five days were spent in Beijing at the In­ the Institute, we were taken on a tour of the stitute of Aeronautics and Astronautics, the Insti­ facilities. The first of two air supply systems that tute of Mechanics of the Chinese Academy of Sci­ we saw consists of 3500 hp compressors that can ences, and the Power Plant Research Laboratory of pump 14 kg/ s of air to 25 atm. The air storage the China Precision Machinery Corporation. On system comprises 220 tanks with a total volume of 3 November 9, the delegation boarded the night train 224 m • The maximum operating pressure of the to Shenyang (Mukden), visited the Shenyang Aero­ facility is 8 atm. Air is heated in a combustion engine Factory and its Development Center on heater, without oxygen makeup, to a maximum November 10 and 11 , and returned by night train. temperature of 600 °C and is available for compres­ Mr. Wilson of the Mar.quardt Co. and I left the sor, combustor, and nozzle test rigs and a blow­ delegation in Beijing to revisit the aforementioned down cold-flow wind tunnel with run times of facilities, while the remainder of the delegation about 2 min. The facility exhauster consists of two went to the Sian Aeroengine Factory and the North air-to-air ejectors that provide altitude simulation Western Engineering University, and then to to 9 km. The wind tunnel has cross-section dimen­ Chiang-Yu via Cheng Tu to visit the Aeronautical sions of 540 x 470 mm. Five contoured nozzles Research Establishment. The delegation was re­ range in Mach from 1.5 to 2.75 and in Reynolds 6 united in Beijing on November 18, flew to Canton, numbers to as high as 2 x 10 . Models are and left for Hong Kong the following day. mounted in a 3.8 mm diameter, six-component BEIJING INSTITUTE OF strain gauge balance that can provide continuously varying angles of attack of - 6 to + 8 0. A mini­ AERONAUTICS AND ASTRONAUTICS computer built in China (2 /J-S, 16K 16-bit words) is Our first visit was to the Beijing Institute of used in conjunction with a 200 sample/s data ac­ Aeronautics and Astronautics, which is the prin- quisition system.

Fig. 1-Delegation members and hosts in front of the Institute of Mechanics, Chinese Academy of Sciences. Left to right, first row: L. O'Brien, United Technologies Research Center; D. Wilson, The Marquardt Co .; C. Swan, Boeing Aero· space Co.; Wu Chung Hua, of the Institute; J. G. Mitchell, Arnold Air Force Station; Yao Peng, Avco Lycoming; Zhu Shouxin (the interpreter), Chinese aeronautical establishment. Staggered second/third row: Mr. Wu and Chen Nai Xing of the Institute; Dr. J . E. Bubb (delegation head), F. M. Fox and Assoc.; Tsien Shou Hua and Huang Chao Hsia of the Institute; the writer; Ko Shau Yen and S. Yue of the Institute. (E. WolI , General Electric Co. , joined the delegation later.)

Volu me I, Number 3,1980 233 Beijing: Institute of Aeronautics and Astronautics puter facility that featured a 256K 32-bit word, Institute of Mechanics, Chinese Academy of Sciences 200,000 computations-per-second computer "Felix" Power Plant Research Laboratory, China Precision Machinery Corp. built in Romania under French license. Peripherals Shenyang: Aeroengine Factory and Development Center included eight disk drives, five tape drives, two Sian: Sian Aeroengine Factory high speed card readers, and a high speed printer. North Western Engineering University We saw a Continental jet engine with a maxi­ Chiang-Yu: Aeronautical Research Establishment mum thrust of 870 kg that was used by students to measure thrust, specific fuel consumption, pres­ sures, temperatures, etc. Quarter segments of sever­ al other engines, including the Rolls Royce HS 125, the GE 147, and engines for the MiG 15 and 17, were also in use as teaching aids. Following our tour, each member of the delega­ tion met with a particular group of 20 to 30 pro­ fessors, graduate students, and staff for technical discussions. As it turned out, this format of "one on many" was to be typical of many subsequent sessions at this Institute and elsewhere. In some cases, an interpreter was needed, but generally the host scientist filled that role. The discussion usually focused on reports on the technical activities in the U.S., but a notable exception was a discussion by Professor 1. S. Chin, a graduate of the University JoHong Kong of Cranfield, England, on the enhancement of igni­ tion in afterburners with the addition of small amounts of oxygen (see, e.g., Ref. 1). Fig. 2-ltlnerary of the U.S. delegation to the People's Undergraduate students at the Institute are Republic of China. selected on the basis of nationwide examinations, which result in tuition-free university level educa­ tion for only about 10070 of the relevant age group Another test rig is used to study supersonic mix­ in the population. A disproportionate 50% of those ing. Here we saw a 16-channel oscillograph record­ admitted are from urban secondary schools, where­ er, a precisely machined spherical pressure probe as these students represent only 20% of the total about 0.8 mm in diameter containing eight separate population. The undergraduate curriculum has tops, and Schlieren/ shadowgraph systems, all made been extended from three years2 to four years. in China. Neither undergraduate nor graduate degrees are The second air storage system has a volume of granted (graduate students spend either two or six 24 m3 at 150 atm, 500 m3 at 55 atm, and 1000 m 3 additional years in the Institute), but the recogni­ at 8 atm. A silica gel dryer is used in this system, tion that titles are important in dealing with the and again air-to-air ejectors are used for the ex­ outside world may cause the issuance of degrees hauster. Connected to the air supply is a manually within one to two years. controlled variable Mach number (1.5 to 3.5) wind tunnel with a 100 x 100 mm test section. Maxi­ mum total pressure in this tunnel is 8 atm and INSTITUTE OF MECHANICS, maximum run time is 5 to 8 min. A normal shock inlet about 50 mm in diameter was being tested. CHINESE ACADEMY OF SCIENCES Next we saw a connected pipe test setup of a On November 6, the delegation visited the In­ ducted rocket. A rubber-based propellant with 20070 stitute of Mechanics, Chinese Academy of Sciences aluminum and 3% magnesium was used on the gas in Beijing. I made a second visit to the Institute on generator. Air was brought to the secondary November 15 to discuss catalytic combustion with combustion chamber in four ducts about 50 mm in Huang Chao Hsiang and his colleagues. diameter each. The entrance angle was 90° and the The Chinese Academy of Sciences, formed in flow rate was about 5 kg/so The secondary com­ 1949, is the national academic organization for re­ bustor was about 150 mm in diameter and 760 mm search in science and technology in China. It con­ in length, and the nozzle throat area was about tains four science and engineering universities, the 50% of the duct area. On our second visit, we were Ministry of Education, and 112 research institutes, shown a video tape of a motor firing and were told 36 of which are located in Beijing. The staff has that the air/rocket flow ratio was 3: 1, the rocket grown from 300 in 1949 to 36,000 at present and pressure 70 atm, the burn time 20 s, and the aver­ includes 23,000 research scientists and engineers, age specific impulse (Isp ) 389 S. 1600 research professors or associate professors, We were then shown a modern large-scale com- plus supporting personnel.

234 fohns Hopkins APL Technical Digest In 1960 the Power Laboratory was merged with several research institutes in gas dynamics that had been formed during 1956-60 into the Institute of Mechanics. At present there are 400 research scien­ tists and engineers and 56 professors and associate professors. The Institute has two major departments: Me­ chanics and Engineering Thermophysics. The Me­ chanics Department has five divisions: (a) High Speed Aerodynamics, (b) Solid Mechanics, (c) Ex­ plosion Mechanics, (d) Plasma Dynamics and Mag­ netohydrodynamics, and (e) Physical Mechanics. Fig. 3-Mach 6.6 to 18 shock tunnel at the Institute of The Engineering Thermophysics Department also Mechanics. has five divisions: (a) Conversion and Transmission measuring strain, crack propagation, etc., under of Energy, (b) the 1st Research Division (Engine constant and cyclical loading at ambient and ele­ Thermodynamics and Aerothermodynamics of Tur­ vated temperatures. bomachinery), (c) the 2nd Research Division (Aero­ Explosion Mechanics - Programs include theo­ thermodynamics of Heat Engines), (d) the 3rd retical and experimental studies of the effects of ex­ Research Division (Heat and Mass Transfer), and plosions and high velocity. (e) the 4th Research Division (Combustion). Professor Wu Chung Hua, Deputy Director, Plasma Dynamics and Magnetohydrodynamics. gave a brief description of the Institute and intro­ - Investigations are conducted of the stability of magnetically confined high temperature plasmas duced the division heads, each of whom described and the industrial applications of low temperature his technical activities and showed us the test plasma torches. facilities. Physical Mechanics - Divisional studies com­ The Mechanics Department prise fluid mechanics, biomechanics, mechanical High Speed Aerodynamics - The division is properties of matter, and cosmic gas dynamics. involved in experimental and theoretical studies of supersonic, hypersonic, and viscous flows. Its The Engineering Thermophysics facilities include: Department 1. Two supersonic blow-down wind tunnels with Conversion and Transmission oj Energy 200 X 200 x 300 mm test sections and Mach 1.6, This division studies basic academic education with 2.0, 2.5, 3.0, and 4.0 nozzles. The air supply particular emphasis on engineering thermodynam­ system comprises a 7 kg/s compressor system with ics , aerothermodynamics of heat engines, heat and a maximum pressure of 9 atm and two 600 m) mass transfer, and combustion. storage tanks with an 8 atm capability. The tunnels 1st Research Division (Engine Thermodynamics were built in 1958. Future plans call for a 30 atm, and Aerothermodynamics oj Turbomachinery) - 7 kg/s air capability. The exhauster system is a The division has 7 senior scientists, 18 junior scien­ single 600 m) vacuum tank. The run durations are tists, and 23 engineers. Professor Chen, who speaks 3 min at Mach 4 and continuous at Mach 1.6. excellent English with a Russian accent reflecting 2. A Mach 6.6 to 18 shock tunnel with a test his academic training in the Soviet Union, gave a section diameter of 1.2 m (Fig. 3). The driver gas is rather detailed description of analytical work con­ a mixture of 6.5070 oxygen and 93.5070 hydrogen ig­ cerning an extension of Professor Wu's renowned nited by a 10 kV spark discharge. The driven gas is work on turbomachinery flow theory. His work a mixture of hydrogen and nitrogen. Maximum uses non orthogonal curvilinear coordinates and pressure is 800 atm and the run time is 5 to 11 ms. non orthogonal velocity components to provide a 3. A Mach 8 to 12 shock tunnel with a test sec­ design procedure for axial flow turbomachine tion diameter of 0.5 m. Driver and driven gas blades. Streamline extension techniques, matrix characteristics are similar to the larger tunnel. direct solution, and line relaxation methods are 4. A gun tunnel with a 200 x 200 mm test sec­ employed in the computer routines. By use of fic­ tion and Mach 1.6 to 4 nozzles. An aluminum pis­ titious grid points, the storage requirement (::; 32K) ton is used, and the maximum pressure is 1000 atm in the matrix direct solution is reduced by about with run times of 20 to 30 ms. one-half. 5. An 800 kW arc-heated tunnel for ablation Figure 4 shows a M ~ 1.3 transonic cascade tun­ testing of reentry bodies. nel that has a 220 x 290 mm 5-blade cascade test Solid Mechanics - This division studies light­ section. The cylindrical tube running transverse to weight structures, computational methods of stress the tunnel axis houses the Schlieren/ shadowgraph analysis, vibration fatigue, fracture mechanics, optics and provides an optical path that is free crack growth, and modeling of elastic and plastic from room disturbances. Maximum airflow is 21 6 deformations. Its laboratories are well equipped for kg/s; maximum Reynolds number is 3 X 10 ; and

Volume I, Number 3, 1980 235 quite interested in supersonic combustion, but at present the work in this area is minimal. Nonethe­ less, many of the Chinese scientists were interested in my lectures on the subject and asked many ques­ tions regarding details from the contemporary U.S. literature. POWER PLANT RESEARCH LABORATORY, CHINA PRECISION MACHINER Y CORP. On November 7, the entire delegation visited the Power Plant Research Laboratory located at Yung­ ang, southwest of Beijing in the Fengtai District. (En route we crossed the Yongding River on a new bridge adjacent to the old Marco Polo bridge, pass­ \ ing into an area of restricted access to foreigners.) Fig. 4-Mach 1.3 cascade tunnel at the Institute of The Laboratory is responsible for the development Mechanics. of propulsion systems, primarily ramjets, some testing of turbojets, and some research for the maximum run time is 200 s. We were also shown a China Precision Machinery Corp. The Laboratory 500 kW rotating cascade driven by a 3000 kW is 35 to 40 years old and the airbreathing test motor at 750 rpm. facilities were built in the late 1950's and early 2nd Research Division (Aerothermodynamics 1960's. Its activity during the Cultural Revolution of Heat Engines) - The division has three senior (1966-69) was limited, but it is currently very busy. scientists, seven junior scientists, and seven engi­ Most of the staff of 600 are engineers. neers. The facilities include two centrifugal com­ Mr. Shu Shen Wang, the Director, described the pressor test rigs, an axial turbine test rig, and a activities of the Laboratory and led the tour of the pressure wave supercharger engine. The compressor test facilities (shown schematically in Fig. 5). Two rigs are 250 kW and 850 kW with maximum speeds 3500 kW centrifugal first stage compressors and of 37,000 rpm. Under construction is a 3000 kW two 1500 kW second stage superchargers comprise axial compressor test rig. The pressure wave super­ the pumping system. The air is stored in a large charger is attached to a 150 hp rotary combustion number of cylindrical tanks having a total volume 3 3 engine. of 6840 m • Of this volume, 6600 m is stored at 3rd Research Division (Heat and Mass Trans­ 22 atm, the remainder at 220 atm. Air is piped to fer) - The division has three senior scientists, both the five test cells and two-stage air-air ejectors nine junior scientists, and ten engineers. Work in­ (Fig. 5) that are used to exhaust cells 2 through 4. cludes experimental and theoretical studies of film In contrast to most U.S. airbreathing test facilities, cooling of turbine blades, analysis of flowfields in which use common exhausters, each cell has its turbojet and ramjet combustors, tests and analysis own ejector system. The maximum flow to the test of sodium heat pipes to 800°C for cooling electrical cells is 230 kg/s, which can be heated directly in equipment, techniques for harnessing solar energy, one of two vitiation heaters (cells 2 and 3) or in­ and heat transfer to improve the cooling of air-and directly in a tube-and-shell heat exchanger. For water-cooled engines. In the tour of the heat trans­ direct heating, the air is subdivided, with half of it fer laboratory, we saw film cooling tests on flat passing into 16 kerosene-fueled turbojet can com­ plates and blade cascades, low and high tempera­ bustors and the other half bypassed for subsequent ture heat pipes, and a rotary combustion engine. mixing. No provision is made for oxygen makeup. This 450 cc, 49 hp air-cooled engine had been run The vitiation heater is used for temperatures up to the equivalent of traveling 17,000 km without re­ 350°C. The tube-and-shell heat exchanger is used pair at an average specific fuel consumption of 25 for the range of 350 to 700°C, so it is possible to gal hp-h. simulate flight Mach numbers up to 4.2 in the 4th Research Division (Combustion) - Five tropopause with unvitiated air. senior scientists, fifteen junior scientists, and four Five test cells are in operation: a sea level cell, engineers comprise the division. Tasks include tests two direct-connect high altitude cells, a supersonic and analysis of annular gas turbine combustors and high altitude free-jet cell, and a ram-air-turbine test afterburners with emphasis on durability, oscilla­ cell. A sixth cell for testing solid ducted rockets is tory mixing and flow stabilization, the use of cat­ currently under design. Test Cell 1 is approximately alysts, the ignition of lean mixtures, combustion of 2.6 m in diameter and 12 m in length, with a hy­ hydrogen and air in supersonic flow, and mixing of draulically actuated cell closure similar to Cell 8 at parallel flows in a ramjet combustor. During the the Marquardt Co. Both ramjet and turbojet en­ early 1960's, Professor Wu and his colleagues were gines, with inlet diameters up to 800 mm and 1000

236 Johns Hopkins APL Technical Digesl Air compressors Air storage located in the vertical plane is used to view the Density Volume Mass flowfield via camera and TV coverage. Maximum 2 centrifugal, 3500 kW each (atm) (kg) 2 2nd stage superchargers, r--- ~ flow rate to this cell was stated to be 115 kg/s, 1500 kW each 22 6600 177,000 which would permit sea level testing at Mach 2 and 220 240 64,500 altitude simulation of 1.3 and 2.1 km at Mach 3

Maximum pressure, and 4, respectively. Run times are typically 3 min. 25 atm Instrumentation consists primarily of strain gauge pressure transducers and thermocouples connected to a 300-channel data acquisition system. or~ Cell 4 was not shown to the delegation. Cell 5 is 115 k9/S~ 115 kg/s 230 kg/s 160 kg/s a smaller test cell used for the development of ram­ Vitiation heater Tube and shell air turbines and pumps. Maximum airflow to this 16 kerosene·fueled heat exchanger cell is 4 kg/ s at pressures of 10 atm and tempera­ turbojet can combustors 1200 tubes tures of 600°C. We were shown a turbine/ pump that produces 180 hp at 4000 rpm with a pump ~ T max = 700°C (M 4.2, outlet pressure of 100 atm at a flow rate of 10 lis. I Mixer I tropopause) The cell is also used for testing seals and bearings. T max = 350°C Under development at the facility were a Mach 3.5 to 4.0 ramjet, similar in design to the Typhon Test cells that was developed in the U.S. in the early 1960's, 1. Turbojet/ ramjet sea level maximum dia., 80 cm 2. Direct connect maximum altitude (Zmax) = 20 km and a solid ducted rocket. We were also shown a maximum flow, 70 kg/s wind tunnel test model of an aft-inlet ramjet in­ 3. Supersonic free·jet M 2- 4, 42 cm x 42 cm test section 4. Direct connect (not shown to delegation) stalled on a variable flow throttle support. The 5. Ram air turbine test 1I4-scale model was about 88 mm in diameter, had 6. Pl anned solid ducted rocket test cell been designed by method of characteristics, and was very precisely machined and assembled. It had Air·air ejectors been tested in a 1 x 1 m wind tunnel at the In­ 2·stage parallel: stitute of Mechanics in Beijing at angles of attack 80 kg/s primary Ieach 40 kg/s secondary stage upt06°. Mr. Wang also described briefly other research Fig. 5-Schematic diagram of the ramjet test facility at efforts at the Laboratory, including: analysis of the Power Plant Research Laboratory. bridging on spike and cone inlets; evaporation of droplets; injection, miXing, and atomization; mm, respectively, can be installed on a load cell studies of atomization from centrifugal injectors by balance for testing, with a maximum static thrust the "wax" method; low temperature ignition of limit of 5700 kg. kerosene and high density fuels; and catalytic igni­ Test Cell 2, completed in 1965, is a high altitude tion and combustion. direct-connect facility with a maximum simulated altitude of 20 km and a maximum flow of 70 kg/so SHENYANG AEROENGINE FACTORY This cell is about 1.8 m in diameter and 4.6 m in length; cell closure is accomplished by hoisting a AND DEVELOPMENT CENTER half cylinder cap in place. Cell 3, completed in The U.S. delegation took the overnight train on 1969, is a Mach 2 to 4 free-jet facility. Five nozzles November 9 that arrived in Shenyang the following with nominal Mach numbers of 2.0, 2.5, 3.0, 3.5, morning. Shenyang, an industrial center of 4 and 4.0 are used. Minor adjustments can be made million people located in Manchuria, was occupied in the nozzle throat blocks (not during a run) to by the Japanese from 1931 until VJ Day. In­ obtain ±0.25 M change from the nominal design teresting was the widespread use of Pinyin (the Mach number in the test section. The nozzle exits Chinese phonetic alphabet) on street signs and are square, 420 mm on a side, and the major frac­ buildings. The State Council has decreed that the tion of the air not captured by a test engine inlet, Roman alphabet will be used to spell out Chinese generally circular in cross section, passes through a names and places according to standard Beijing bypass diffuser to the ejector system. The cowl of pronunciation beginning January 1, 1979. the engine inlet is then centered in a cut-out of the On the morning of November 10, we visited the downstream end of the test cabin, which is sized to Aeroengine Factory where J6 and J7 engines for provide a small gap that permits clearance at angles the MiG fighters are manufactured. The factory of attack. The maximum engine diameter tested to was built in the 1950's with the help of the Soviets. date is 300 mm, which corresponds to 40070 of the There are about 10,000 employees, 10070 of whom nozzle discharge area. Engines are mounted on a are technical (500 engineers, 500 graduates - a 1000 kg maximum thrust stand that can be rotated graduate must have 6 to 7 years experience before in the horizontal plane from - 2 to + 10°. A long­ he is designated an engineer). The factory has .2000 focal-length Schlieren/shadowgraph optical system "precision" machine tools and turns out 1000

Volume I, Number 3,1980 237 engines per year on a two-shift, six-day (the stan­ want further exchanges. We were impressed by dard work week in China) operation. The equip­ both their technical capability and their strong ment varied from 1930-vintage drop forges to commitment to the training of aerospace engineers computer-driven lathes for finish machining of and to the development of facilities for testing and compressor blades. manufacturing advanced jet engines. Our hosts Following a visit to the manufacturing plant, we were very familiar with U.S. technical literature; it witnessed a "green run-in" of a 17 engine. With was not uncommon to be asked very detailed ques­ the afterburner operating, the exhaust nozzle was tions regarding derivations of one's equations or bright yellow and the three actuators and supports methods of solution, or explanations of a particu­ were a dull red, a most impressive sight. Normal lar figure where the questioner had a photostatic procedure is to operate the engine for two hours at copy of the material in hand. On many occasions, sea level conditions at various throttle settings, shut the Chinese described their own equipment as in­ it down, disassemble and check all parts, and reas­ ferior and their technology as primitive. The dele­ semble it for three hours of additional testing be­ gation felt quite differently. We were impressed by fore shipment to the aircraft assembly plant. We both their technical capability and their ability to also saw an advanced 9070 kg bypass engine about manufacture complex equipment. Obviously, al­ 1.15 m in diameter that was under development at though their resources at present are limited, they the Development Center, which we visited in the are focused on specific goals. There is little doubt afternoon. that their national commitment to "The Great The Development Center is a large facility used Goal of Four Modernizations," viz., achieving for engine development, including component test­ modernization of agriculture, industry, national ing of the compressor, combustor, turbine, after­ defense, and science and technology by the end of burner, and exhaust nozzle. Power is supplied to this century, will result in world prominence in the facility by two 18,000 kVA transformers that aerospace sciences and technology. 3 step down the voltage to the 6300 V compressor We were quite impressed by the apparent ade­ motors. The air supply consists of six centrifugal quacy, if not abundance, of food. The fields of compressors driven by lO50 kW synchronous South China were lush, interwoven with myriad ir­ motors that pump 5 kg/s each to a pressure of 8 rigation channels. In North China it was harvest atm. This machinery was built in the Soviet Union time for bok toi, the white cabbage, which was and installed in Shenyang in the 1950's. In addition stacked like cordwood on many of the sidewalks of to the aforementioned bypass engine, we saw a Beijing and Shenyang. It sells for about four cents 20,000 rpm spool test rig, several combustor test a kilogram and can be used during the entire win­ rigs emphasizing reignition capability, a set-up for ter. Wages range from $40 to $70 per month. All bypass duct burning that was instrumented with an able-bodied adults work a six-day, 48-hour week, IR spectrometer, and a complete structural testing with five public holidays and one week vacation laboratory. each year. Housing is very modest, being assigned by the state and consisting of one to two rooms per CONCLUSIONS, COMMENTS, family at a cost of $2 to $5 per month. Most people live within walking or bicycling distance of AND TRIVIA their work. Private ownership of automobiles is There is no doubt that the delegation members nonexistent, but higher officials are assigned were truly "guests of the People's Republic of vehicles with drivers. Most of the automobiles are China," which is how we were treated and greeted made in China and are eastern European in appear­ on every occasion. The food was superb, the beer ance. Both city and rural roads are crowded with excellent, the wine sweet, and the mao tai (a liquor buses (many imported), automobiles, trucks, vans, distilled from sorghum) strong. We entered and left mechanized or animal-drawn farm vehicles, bicy­ China without clearing customs. We were met at cles, and pedestrians. each city by two or more interpreters, escorted to Primary and secondary education for all is pro­ VIP waiting rooms, and chauffeured in vans or vided by the government but university admission automobiles to guest houses or hotels, and were the is limited and very competitive. Nationwide en­ guests at a banquet in each city. Our hosts could trance examinations are the basis for admittance. not have been more gracious. University life has returned to normal following the There is no question that the principal objective acute disruptions of the Cultural Revolution, which of the delegation's visit was met, viz., "to promote created a large void in the population of those with cooperation through the exchange of scientific in­ advanced education. Many people with technical formation." The discussions were free and open. inclinations had to be trained on the job during Questions in areas that would be regarded as sen­ that period; they represent a significant part of the sitive were avoided for the most part, but when technical staff at the Power Plant Research Labo­ they arose, the disinclination to answer was un­ ratory and similar facilities. Such people, who are hesitatingly accepted. The Chinese were apprecia­ anxious for opportunities to study abroad, inun­ tive of the technical advice they were given and dated the writer with resumes and requests for the

238 Johns Hopkins APL Technical Diges{ names of people to contact in the U.S. Similarly, English and one on computer programming (in there were many expressions by university people English). We were also told that English is taught for establishing programs to exchange graduate by radio throughout the nation. students. Finally, although our sightseeing was limited, the Manufactured goods of all types were available places we visited - the Forbidden City, the Great in stores with prices comparable to the United Wall, the Ming Tombs, the Summer Palace, Beihai States, e.g., good quality bicycles selling for $120 Park, and others - were magnificent. and color television sets for $500. However, not only does such a purchase represent many months REFERENCES I J . S. C hin , " The Effect o f Oxygen Addition on the Minimum Ignitio n of saving, but for many goods a ration coupon Energy in a H o mogeneous C ombustible Mixture," Report H-B429, Bei­ issued by the state is a prerequisite for purchase. jin g Institute of Aeronautics and Ast ronautics, p. 3 (1979) . The state-run television system broadcasts only a 2G. Corning, "An Aeronautical Vis it to C hina," Aeronaut. Astronauf. , pp. 16- 19(1974). few hours a day, with heavy emphasis on educa­ 3Eco nomic Reporter, Eng li sh Supplement . July-September 1979, tion. In Beijing there is a daily program teaching Economic Information & Agency, Hong Kong.

Volume I, Number 3,1980 239 PUBLICATIONS

F. J. Adrian, "Principles of the Radical mati on System," Comput. Mag., pp. G. L. Dugger and F. K. Hill, "Use of Pair Mechanism of Chemically Induced 42 - 50 (Nov 1979). Satellite-Derived Sea Surface Tempera­ Nuclear and Electron Spin Polariza­ D. W. Bockemeier (MDAC/St. Louis), tures by Cruising OTEC Plants," Proc. tion," Rev. Chem. Intermed. 3, No. 3, W. C. Caywood, P. J . Waltrup, R. D. 6th OTEC Conf., Vol. II (1980) . pp. 3 - 43 (1979). Stockbridge, and F. S. Billig (APL), G. L. Dugger, F. E. Naef, and J. E. F. J. Adrian and L. Monchick, "Analytic and R. S. Burford (Hercules/ ABL), Snyder Ill, "Ocean Thermal Energy Formula for Chemically Induced Mag­ "Conceptual Design of the Hypersonic Conversion," Chap. 19, Solar Energy netic Polarization by S-T ± I Mixing in Wide Area Defense Missile," CPIA Handbook, J. Kreider and F. Kreith a Strong Magnetic Field," J. Chem. Pub. 315, Vol. II, pp. 387 - 449. (eds.), McGraw-Hill (1980). Phys. 72, No. 10, pp. 5786 - 5787 J. Bohandy and B. F. Kim, "A Normal C. Feldman, C. H. Arrington III, F. G. (1980). Mode Analysis of Free Base Por­ Satkiewicz, and N. A. Blum, "Vacuum W. H . Avery, "Ocean Thermal Energy phyrin," Spectrochim. Acta 36A, pp. Deposited Polycrystalline Silicon Films Conversion Contribution to the Energy 463 - 466 (1980) . for Solar Cell Applications," Solar Needs of the United States," Johns H. Bouver (APL) and R. E. Bargmann Research Institute Quarterly Report, 15 Hopkins APL Tech . Dig. I, No.2, pp. (Univ. Georgia), "Comparison of Com­ Sep - 31 Dec 1979, SERIIXS9/82781-1 101 - 107 (1980) . putational Algorithms for the Evalua­ (Mar 1980). C. B. Bargeron, "High Vacuum Scanning tion of the Univariate and Bivariate S. N. Foner and R. W. Hart, "The Mil­ Electron Microscopy as a Tool in Sur­ Normal Distribution," Computer ton S. Eisenhower Research Center: Its face Analysis," Johns Hopkins APL Science and Statistics: 12th Ann. Symp. Objectives and Activities," Johns Tech. Dig. I, No. I, pp. 38 - 44 (1980). on the Interface, pp. 344 - 348 (1979). Hopkins APL Tech. Dig. I, No. I, pp. R. C. Beal, "The Potential of Spaceborne N. J . Brown (Univ. California) and D. M. 8-33 (1980). Synthetic Aperture Radar for Oceanog­ Silver (APL), "Comparison of Reactive E. J . Francis and G. L. Dugger, "Promis­ raphy," Johns Hopkins APL Tech. and Inelastic Scattering of H 2 + D2 ing Applications of OTEC," Proc. 7th Dig. I, No.2, pp. 148 -156 (1980). Using Four Semiempirical Potential Energy Technology ConI., pp. R. C. Beal, "Spaceborne Imaging Radar: Energy Surfaces," J. Chem. Phys. 72, 1272-1286(1980). Monitoring of Ocean Waves," Science No. 7, pp. 3869 - 3879 (1980) . A. B. Fraser, R. E. Walker, and F. C. 208, No. 4450, pp. 1373 -1375 (1980) . J. L. Calkins (JHMI) and B. F. Jurgens, "Spatial and Temporal Cor­ W. G. Berl, "Albert Einstein - A Moral Hochheimer (APL), "Retinal Light Ex­ relation of Underwater Sunlight Fluc­ Visionary in a Distraught World," J. posure from Operation Microscopes," tuations in the Sea," IEEE J. Oceanic Wash. A cad. Sci. 69, No.3, pp. Arch. Ophthalmol. 97, pp. 2363 - 2367 Eng. OE-S, No. 3, pp. 195 - 198 (1980). 101-108 (1979). (1979). R. K. Frazer, "Duplication of Radome W. G. Berl and B. M. Halpin, "Human J. F. Carbary, "Periodicities in the Jovian Aerodynamic Heating Using the Central Fatalities from Unwanted Fires," Johns Magnetosphere: Magnetodisc Models Receiver Test Facility Solar Furnace," Hopkins APL Tech. Dig. I, No. 2, pp. after Voyager," Geophys. Res. Lett. 7, Proc. 15th Symp. on Electromagnetic 129 - 134 (1980) . No. I, pp. 29 - 32 (1980). Windows (1980) . F. S. Billig, P. J. Waltrup, and R. D. J. F. Carbary and S. M. Krimigis, M. H. Friedman, C. B. Bargeron, O. 1. Stockbridge, "The Integral-Rocket, "Energetic Particle Activity at 5-min Deters, and F. F. Mark (APL) and G. Dual-Combustion Ramjet: A New Pro­ and IO-s Time Resolution in the Mag­ M. Hutchins (JHMI), "Hemodynamic pulsion Concept," J. Spacecr. Rockets netotail and Its Relation to Auroral Ac­ Measurements in Human Arterial Casts: 17, No.5, pp. 416-424 (1980). tivity," J. Geophys. Res. 84, No. AI2, Geometric Effects and Morphological J. F. Bird, "Sonomagnetic Pulses from pp. 7123-7137 (1979) . Correlates," Proc. 31st ASME Biomech. Underwater Explosions and Implo­ L. L. Cronvich and H. P. Liepman, "Ad­ Symp. (1979); also in Proc. XII Int. sions," J. Acoust. Soc. Am. 67, No.2, vanced Missile Technology - A Review ConI. on Medical and Biological Engi­ pp. 491 - 495 (1980). of Technology Improvement Areas for neering (1979) . J. F. Bird, R. W. Flower, and G. H. Cruise Missiles," NASA CR 3187 R. M. Fristrom, "Report of the Task Mowbray, "Analysis of the Retina via (1979) . Force on the Flammability of Solid Suprafusion Electroretinography," Bio­ O. J. Deters and C. B. Bargeron (APL), Polymer Cable Dielectrics, Final phys. J. 29, pp. 379-396(1980). G. M. Hutchins (JHMI), and F. F. Report," National Research Council H. D. Black, "The Transit System, 1977: Mark and M. H. Friedman (APL), EL-1263, pp. 2-1/2-27 (Nov 1979). Performance, Plans, and Potential," "Velocities in Unsteady Flow through J. Goldhirsh, "A Review on the Applica­ Philos. Trans. R. Soc. London A294, Casts of Human Arteries (Abstract)", tion of Nonattenuating Frequency pp. 217 - 236 (1980). Proc. 32nd Annual ConI. on Engineer­ Radars for Estimating Rain Attenuation R. W. Blevins, H. L. Donnelly, J . T. ing in Medicine and Biology (1979). and Space-Diversity Performance," Stadter, R. O. Weiss (APL), and L. O. 1. Deters and M. H . Friedman, "Sim­ IEEE Trans. Geosci. Electron. GE-17, Perez y Perez (DOE), "At-Sea Test of a ple Directionally Sensitive Two-Com­ No.4, pp.218 - 239(1979). Large Diameter Steel, Cold Water ponent Laser Doppler Anemometer," 1. Goldhirsh, "Comparison of Radar Pipe," Ocean Engineering for OTEC Appl. Opt. 19, No.8, p. 1221 (1980). Derived Slant Path Rain Attenuations OED-9, ASME, pp. 47 - 65 (1980). G. L. Dugger, "Is There a Chance for with the COMST AR Beacon Fades at B. 1. Blum (APL) and C. J. Johns, E. E. OTEC?" Astronaut. Aeronaut. 17, No. 28 .56 GHz for Summer and Winter McColligan, C. R. Smith, and D. M. 11 , pp. 36-42 (1979) . Periods," IEEE Trans. Antennas Prop­ Steinwachs (JHMl), "A Low Cost Am­ G. L. Dugger, E. J. Francis, and W. H. agoAP-28, No.4, pp. 577 - 580 (1980) . bulatory Medical Information System," Avery, "Comparisons of Estimates, J. Goldhirsh, "The Use of Radar at Non­ J. Clinical Eng. 4, No.4, pp. 372 - 378 Sharing Potentials, Subsidies, and Uses attenuating Wavelengths as a Tool for (1979). for OTEC Facilities and Plantships," the Estimation of Rain Attenuation at B. 1. Blum (APL) and R. E. Lenhard, Jr. Expanded Abstracts, Proc. 7th Ocean Frequencies above 10 GHz," EASCON (JHMI), "An Oncology Clinical Infor- Energy Conf., paper mC/ I (1980) . '79 Record AES I, pp. 48-55 (1979) .

240 Johns Hopkins APL Technical Digest ______~ ______DEPARTMENTS

B. L. Gotwols and G. B. Irani, "Optical Line Source of Conical Waves," J. Potemra, "The Role of Vibrationally Determination of the Phase Velocity of Sound Vib . 67, No. 4, pp. 523 - 531 Excited Oxygen in Auroral Excitation Short Gravity Waves," J. Geophys. (1979). _ of 02 (I Ag)," J. Geophys. Res. 85, No. Res. 85, No. C7, pp. 3964-3970 (1980) . A. N. Jette and J . G. Parker, "Surface A4, pp.1792 - 1794(1980). W. M. Gray and R. W. Witte, "Guidance Displacements Accompanying the Prop­ M. L. Moon, "Environmental Impact of System Evaluation Laboratory," Johns agation of Acoustic Waves within an Salt Drift from a Natural Draft Cooling Hopkins APL Tech. Dig. 1, No.2, pp. Underground Pipe," J. Sound Vib . 69, Tower," Johns Hopkins APL Tech. 144 -147 (1980). No. 2, pp. 265 - 274 (1980). Dig. 1, No.2, pp. 120 - 128 (1980). R. A. Greenwald (Max-Planck Inst. Aero­ S. M. Krimigis, "Observations of Particle R. R. Newton, "On the Fractions of nomie), T. A. Potemra (APL), and N. Acceleration in the Earth's Magneto­ Degrees in an Ancient Star Catalogue," A. Saflekos (Boston College), "Stare tail," AlP Conf. Proc., No. 56, pp. Q. J. R. Astron. Soc. 20, pp. 383 - 394 and Triad Observations of Field­ 179-197 (1979). (1979). Aligned Current Closure and Joule S. M. Krimigis (APL), T. P. Armstrong V. O'Brien, "Fully Developed Forced Heating in the Vicinity of the Harang (Univ. Kansas), W. I. Axford (Max­ Convection in Rectangular Ducts and Discontinuity," J. Geophys. Res. 85, Planck Inst. Aeronomie), C. O. Illustrations of Some General In­ No. A2, pp. 563 - 568 (1980) . Bostrom (APL), c. Y. Fan (Univ. equalities," J. Appl. Math. Phys. G. Gucer (JHMI) and L. J. Viernstein Arizona), G. Gloeckler (Univ. (ZAMP) 30, pp. 913 - 928 (1979). (APL), "Clinical Evaluation of Long­ Maryland), L. J. Lanzerotti (Bell Labs), V. O'Brien, "Pulsatile Flow through a Term Epidural Monitoring of Intra­ D. C. Hamilton (Univ. Maryland), and Constricted Artery," Bull. Am. Phys. cranial Pressure," Surg. Neurol. 12, pp. R. D. Zwick I (Los Alamos Scientific Soc. 24, No.8, p. 1130 (1979). 373 - 377 (1979). Lab.), "Energetic ('" 100keV) Tailward V. O'Brien and L. W. Ehrlich, "Pulsatile Directed Ion Beam Outside the Jovian G. Gucer (JHMI) and L. J. Viernstein Flow through a Constricted Artery," Plasma Boundary," Geophys. Res. (APL), "Continuous Recording of ICP Proc. 2nd Mid-Atlantic Conf. on Bio­ Lett. 7, No.1, pp. 13 - 16 (1980). in the Normal Monkey," Intracranial Fluid Mechanisms, pp. 497 - 516 (1980) . S. M. Krimigis (APL) and E. T. Sarris Pressure IV, (K. Shulman et. aI., eds.), F. C. Paddison and K. Yu, "Use of (Democritos Univ. Thrace), "Energetic Springer - Verlag, Berlin (1980). Geothermal Energy in the Eastern G. Gucer (JHMI) and L. J. Viernstein Particle Bursts in the Earth's Environ­ United States," Johns Hopkins APL (APL), "Intracranial Pressure in the ment," in Dynamics of the Magneto­ Tech. Dig. 1, No.2, pp. 88 - 100 sphere, D. Reidel, pp. 599 - 630 (1979). Normal Monkey while Awake and (1980). Asleep," J. Neurosurg. 51, pp. J. R. Kuttler and V. G. Sigillito, "Upper D. A. Payne, " Backlund Transformations 206 - 210 (1979). and Lower Bounds for Frequencies of in Several Variables," G. Gucer (JHMI), L. J . Viernstein and J . Clamped Rhombical Plates," J. Sound J. Math. Phys. G. Chubbuck (APL), and A. E. Walker Vib. 68, No.4, pp. 597 - 607 (1980). 21, No. 7, pp.1593 - 1602(1980). (JHMI), "Clinical Evaluation of Long A. T. Y. Lui, "Observations on Plasma T . E. Phillips (APL) and B. M. Hoffman, Term Epidural Monitoring of Intra­ Sheet Dynamics during Magnetospheric C. J. Schramm, and S. K. Wright cranial Pressure," Surg. Neurol. 12, pp. Substorms," in Dynamics of the Mag­ (Northwestern Univ.), "Conductive 373 - 377 (1979). netosphere, D. Reidel, pp. 563 - 597 Molecular Crystals: Metallic Behavior in Partially Oxidized Porphyrin, G. Gucer (JHMI), L. J. Viernstein and (1979). W. H . Guier (APL), "A Hemodynamic M. M. Labes, M. Jones, H. Kao, Tetrabenzporphyrin, and Phthalo­ Model for Relating Phasic Pressure and L. Nichols, and C. Hsu (Temple Univ.) cyanine," Molecular Metals, Plenum Flow in the Large Arteries," IEEE and T. O. Poehler (APL), "Conductivi­ Press, p. 393 (1979). Trans. Biomed. Eng. BME-27, No.8, ty and Optical Properties of a T. O. Poehler (APL) and M. E. Hawley, pp. 479 - 482 (1980). Polyiodine Canal Complex (Ben­ W. A. Bryden, J. P . Stokes, A. N . Bloch, and D. O. Cowan (JHU), G. Gucer (JHMI), L. J. Viernstein (APL), zophenone)9 (KI)zhCHCl)," Mol. "Charge Transfer Salts of Fluorinated and A. E. Walker (JHMI), "Continu­ Cryst. Liq. Cryst. 52, p. 115 (1979). TCNQ: Mott Insulators Isostructural ous Intracranial Pressure Recording in M . Maxfield, D. O. Cowan, and A. N. with Organic Conductors," Proc. Conf. Adult Hydrocephalus, Surg. Neurol., Bloch (JHU) and T. O. Poehler (APL), Organic Conductors and Semiconduc­ 13, pp. 323 - 328 (1980). "Synthesis of 13,13,14,14-Tetracyano- tors, Springer-Verlag, Berlin (1979) . M. E. Hawley, W. A. Bryden, A. N. 4,5,9, 10-Tetra hyd ropyrenoquino­ Bloch, and D. O. Cowan (JHU), T. O. dimethane, an Electron Acceptor for T. O. Poehler and R. S. Potember (APL) Poehler (APL), and J. P. Stokes Organic Metals," Nouv. J. Chimie 3, and D. O. Cowan and A. N. Bloch (JHU), "Mott Transition and Magnetic No. II, p. 647 (1979). (JHU), "Switching and Memory Properties of HMTSF (TCNQ)x C.-I. Meng, "Polar Cap Variations and Phenomena in Semiconducting Charge­ (TCNQF4)J - x," Bull. Am. Phys. Soc. the Interplanetary Magnetic Field," in transfer Complexes, Proc. Electrochem. 24, No. 3, p. 232 (1979). Dynamics of the Magnetosphere, D. Soc. 80-1, pp. 189-190(1980). B. F. Hochheimer (APL) and S. A. Reidel, pp. 23 - 46 (1979) . I. P. Pollack (JHMI), L. J. Viernstein D'Anna and J. L. Calkins (JHMI), R. A. Meyer, M. E. Zaruba, and G. M. (APL), and R. L. Radius (JHMI), "An "Retinal Damage from Light," Am. J. McKhann, "Flow Cytometry of Iso­ Instrument for Constant-Pressure Ophthalmol. 88, pp. 1039 - 1044 (1979). lated Cells from the Brain," Anal. Tonography," Exp. Eye Res. 29, pp. H. Hopfield, "Improvements in the Quant. Cytol. J. 2, No. I, pp. 66 - 74 579 - 585 (1979). Tropospheric Refraction Correction for (1980). R. S. Potember and T. O. Poehler (APL) Range Measurement," Philos. Trans. L. Monchick, "Surface Diffusion and and A. Rappa, D. O. Cowan, and A. R. Soc. London A294, pp. 341 - 352 Spin Polarization: Two Dimensional N. Bloch (JHU), "A Reversible Field (1980) . CIDEP," J. Chem. Phys. 72, No. II, Induced Phase Transition in Semicon­ A. N. Jette and J. G. Parker, "Excitation pp. 6258 - 6264 (1980). ducting Films of Silver and Copper of an Elastic Half-Space by a Buried L. Monchick, J . G. Parker, and T. A. TNAP Radical-Ion Salts," J. Am.

Volume 1. Number 3. 1980 241 Chem. Soc. 102, Vol. 10, pp. (APL), "An Assistive Equipment Con­ Tech. Dig. I, No.2, pp. 135 - 143 3659 - 3660 (1980). troller for Quadriplegics," Johns (1980). T. A. Potemra, "Current Systems in the Hopkins Med. J. 145, No.3, pp. L. J. Viernstein (APL) and l. P . Pollack Earth's Magnetosphere," Rev. Geo­ 84-88 (1979). (JHMI), "Validity of the Imbert-Fick phys. Space Phys. 17, No. 4, pp. G. Schmeisser (JHMI) and W. Seamone Law for Constant-Pressure Tonog­ 640 - 656 (1979). (APL), "Low Cost Assistive Device raphy," Exp. Eye Res. 29, pp. T. A. Potemra (APL) , T. Iijima (Univ. Systems for a High Spinal Cord Injured 587 - 594 (1979) . Tokyo), and N. A. Saflekos (Boston Person in the Home Environment," J. L. Wagner (APL) and J. D. Anderson, College), "Large-Scale Characteristics Bull. Prosthetics Res. BPR 10 - 32, pp. Jr. (Univ. Maryland), "Laser Radiation of Birkeland Currents," in Dynamics of 212 - 223 (1979) . - Gasdynamic Coupling in the SF6-Air the Magnetosphere, D. Riedel, pp. D. M. Silver, " Interaction Energy be­ Laminar Boundary Layer," AIAA J. 165 -199 (1979) . tween Two Ground-State Helium 18, No. 3, pp. 333 - 334 (1980) . W. R. Powell, "Community Annual Stor­ Atoms Using Many-Body Perturbation L. T. Watson (Michigan State Univ.) and age Energy System," Johns Hopkins Theory," Phys. Rev. A 21, No. 4, pp. D. Fenner (APL), "Algorithm 555: APL Tech . Dig. I, No.2, pp. 108 - 113 1106 - 1117 (1980). Chow-Yorke Algorithm for Fixed (1980). D. M. Silver, "Rotationally Inelastic Col­ Points or Zeros of C2 Maps [C51." D. W. Rabenhorst, "Demonstration of a lisions of LiH with He. l. Ab Initio ACM Trans. Math. Software 6, No.2, Low Cost Flywheel in an Energy Stor­ Potential Energy Surface," J. Chem. pp. 252 - 259 (1980) . age System," Proc. 1979 Mechanical Phys. 72, No. 12, pp. 6445 - 6451 R. O. Weiss, R. W. Blevins, H. L. Don­ and Magnetic Energy Storage Contrac­ (1980). nelly, J. T. Stadter (APL), and L. tors' Review Meeting, pp. 408-414 D. M. Silver (APL) and N. J. Brown Perez y Perez (DOT), "At-Sea Test of a (1979). (Univ. California), "Valence Bond Large Diameter Steel, Cold Water D. W. Rabenhorst, "Energy Conservation Model Potential Energy Surface for Pipe," Ocean Engineering for OTEC, with Flywheels," Johns Hopkins APL H4," J. Chem. Phys. 72, No.7, pp. Proc. 1980 Energy-Sources Technology Tech. Dig. I, No.2, pp. 114 - 119 3859 - 3868 (1980). Conj., pp. 47 - 67 (1980) . (1980). R. O. Weiss and J. T. Stadter, "Analysis W. D. Stanbro and D. A. Pyrck, "Stabili­ R. P. Rich, "Mechanical Proof Testing," ty of Rhodamine WT in Saline of Contact through Finite Element Comput. Lang. 5, pp. 1-28 (1980). Waters," Water Resour. Res. IS, No. Gaps," Comput. Struct. 10, pp. 867 - 873 (1979). C. H. Ronnenburg and R. L. Trapp, 6, pp. 1631-1632(1979). "Automated Antenna Tests Save Time S. Wilson (Science Research Council, UK) J. P. Stokes, A. N. Bloch, W. A. Bryden, and Cut Costs, Microwaves 19, No. I, and D. M. Silver (APL), "On the Con­ D. O. Cowan, and M. E. Hawley pp. 66 -72 (1980). tribution of Higher-Order Excitations to (JHU) and T. O. Poeh ler (APL), L. J. Rueger and A. G. Bates, "Nova Correlation Energies: The Ground State "Mott Transition and Conductivity in Satellite Time Experiment," Radio Sci. of the Water Molecule," Theoret. the Organic Solid Solutions HMTSF 14, No. 4, pp. 707 -714 (1979). Chim. Acta 54, pp. 83 - 91 (1979). N. A. Saflekos and T. A. Potemra, "The (TCNQ)x (TCNQF4)I - x," Bull. Am. S. Wilson (Science Research Council, UK) 24, No.3, p. 232 (1979) . Orientation of Birkeland Current Sheets Phys. Soc. and D. M. Silver (APL), "Diagram­ in the Dayside Polar Region and Its A. M. Stone, "Geothermal Energy - An matic Perturbation Theory: An Appli­ Relationship to the lMF," J. Geophys. Overview," Johns Hopkins APL Tech: cation to the Nitrogen, Carbon Monox­ Res. 85, No. A5, pp. 1987-1994 (1980). Dig. 1, No.2, pp. 78 - 87 (1980) . ide, and Boron Fluoride Molecules Us­ F. G. Satkiewicz, "Secondary Ion 'Mass H. Sulzbacher and W. Baumjohann ing a Universal Even-Tempered Basis Spectrometry for the Study of Solids," (Univ. Munster, FRG) and T. A. Set," J. Chem. Phys. 72, No. 3, pp. Johns Hopkins APL Tech. Dig. I, No. Potemra (APL), "Coordinated Mag­ 2159 - 2165 (1980) . I, pp. 45 - 48 (1980) . netic Observations of Morning Sector S. K. Wright and C. J. Schramm (North­ J. A. Schetz, F. S. Billig, and S. Favin, Auroral Zone Currents with Triad and western Univ.), T. E. Phillips (APL), "Approximate Analysis ofAxisymmet­ the Scandinavian Magnetometer Array: and D. M. Scholler and B. M. Hoff­ ric Supersonic Base Flows with Injec­ A Case Study," J. Geophys. Res. 48 , man (Northwestern Univ.), "Conduc­ tion," AIAA J. 18, No.8, pp. pp. 7 -17 (1980) . tive Molecular Solids: Partial Iodine 867 - 868 (1980). L. J. Viernstein, "Intracranial Pressure Oxidation of Octaethylporphyrins," G. Schmeisser (JHMI) and W. Seamone Monitoring," Johns Hopkins APL Synth. Metals I, No. I, p. 43 (1979).

PRESENTATIONS

F. J. Adrian, "Nature of the Three­ R. H. Andreo and J. A. Krill, "Vector C. B. Bargeron, "Precursive Blistering in Electron Bond in Noble Gas Stochastic Variational Expressions for the Localized Corrosion of Alumi­ Monohalides as Revealed by ESR Spec­ Electromagnetic Wave Scattering from num," Corrosion Seminar, NSWC, troscopy," Meeting, Greater Random Magnetic Objects," Workshop White Oak, Md., 15 Jan 1980; also Washington ESR Discussion Group, on Wave Propagation in Random presented at Washington, D.C. Georgetown Univ., 22 Feb 1980. Media, VPI&SU, 25 Mar 1980. Chapter, Electrochemical Society, 3 Apr F. J. Adrian, "Radical Pair Mechanism R. H. Andreo and J. A. Krill, "Vector 1980. of Chemically Induced Magnetic Stochastic Variational Principles: J. L. Calkins (JHMI), B. F. Hochheimer Polarization," Seminar, Princeton Generalizations and Computational (APL), and S. A. D'Anna (JHMI), Univ., 23 Oct 1979. Features," 1980 CSL Scientific Con­ "Potential Hazards from Specific R. H. Andreo, "Stochastic Variational ference on Obscuration and Aerosol Ophthalmic Devices," Conference on Principles for Electromagnetic Wave Research, Aberdeen Proving Ground, Intense Light Hazards in Ophthalmic Scattering from Random Systems," 22 Ju11980. Diagnosis and Treatment, Houston, 25 Meeting on Recent Developments in W. H. Avery, "The OTEC Contribution Nov 1979. Classical Wave Scattering: Focus on the to Energy Needs of All Regions of the L. L. Cronvich, "Aerodynamics of Air­ T-Matrix Approach, Ohio State Univ., U.S.," IEEE 1980 Region 6 Conf., San Breathing Missiles for U.S. Navy 25 - 27 Jun 1979. Diego, 21 Feb 1980. Surface-to-Air Application," Workshop

242 Johns Hopkins APL Technical Digesl on Aerodynamics of Missiles with Air­ Frequencies of Orthotropic Clamped J. A. Schetz, F. S. Billig, and S. Favin, breathing Propulsion, London, 8 - 9 Plates," Meeting, American Mathemati­ "Analysis of Mixing and Combustion May 1980. cal Society, San Antonio, 6 Jan 1980. in a Scramjet Combustor with Co-Axial G. L. Dugger, "Ocean Thermal Energy L. Monchick, "Anisotropic Forces and Fuel Jet," 16th AIAA/ SAE/ ASME Conversion," First International Symp. Molecular Collisions," Zentrum Inter­ Joint Propulsion Conf., Hartford, Jun on Non-Conventional Energy, Trieste, dis zip lin are Forschung, Univ. Bielefeld, 1980. 27 Aug - 21 Sep 1979, AEO-79-4I, Sep FRG, 20 Nov 1979; also presented at J. A. Schetz, F. S. Billig, and S. Favin, 1979. Inst. Theoretical Physics, Univ. "Approximate Analysis of Base Drag G. L. Dugger, E. J. Francis, and W. H. Erlangen-Niirnberg, 3 Dec 1979. Reduction by Base and/ or External Avery, "Projected Costs for Electricity K. Moorjani, "Disordered Magnets," Burning for Axisymmetric Supersonic and Products from OTEC Facilities and Physics Dept. Seminar, Univ. Mary­ Bodies," 16th AIAA/ SAE/ ASME Joint Plantships," IECEC '80, 15th Intersoci­ land, College Park, 27 Feb 1980. Propulsion Conf., Hartford, Jun 1980. ety Energy Conversion Engineering K. Moorjani, "Magnetic Glasses," D. M. Silver, "Electron Correlation in Conf., "Energy to the 21st Century," Groupe de Transition des Phases, Simple Chemical Systems," Univ . of Seattle, 18-22Aug 1980. C.N.R.S., Grenoble, France, 23 Jul Electro-Communications, Chofu-shi, W. L. Ebert, "Multiple Marker Bi-Plane 1979; also presented at 7th AIRAPT In­ Tokyo, 16 Nov 1979. Cineventriculogrammetry," Interdepart­ ternational Conf., Le Creusot, France, D. M. Silver, "Rotational Energy mental Seminars on Cardiovascular 30 Jul - 3 Aug 1979. Transfer in LiH-He Collisions: A Com­ Research, The Johns Hopkins Univ., 25 K. Moorjani (APL) and S. K. Ghatak (In­ parison between Theory and Experi­ Oct 1979. dian Inst. of Tech., Khargpur), "Com­ ment," Hebrew Univ., Jerusalem, 23 A. Eisner and S. M. Yionoulis, "Neutral peting Exchange Interactions in Ran­ Dec 1979. Density Variations in the 900-1200 km dom Magnets," American Physical J. E. Tillman, "Differentiation of Region of the Upper Atmosphere," In­ Society Meeting, Chicago, 20 - 21 Jan Mesozoic Fractures by Fluid Inclusion ternational Symp. on Space Geodesy 1980. Analysis," U.S. Geological Survey and Its Applications, Cannes, 18 - 21 J. C. Murphy, "Time Dependent Pro­ Seminar, Reston, Va., 7 Feb 1980. Nov 1979. cesses in Photoacoustic Spectroscopy," J. E. Tillman, "Hydrothermal and Uplift C. Feldman, "Vacuum Deposited Topical Meeting on Photoacomtic Spec­ Histories of the Northern Appalachian Polycrystalline Silicon Films for Solar troscopy, Univ. Iowa, 1-3 Aug 1979. Basin," Eastern Section Meeting, Cell Applications," 14th IEEE Photo­ V. O'Brien, "Pulsatile Blood Flow," Assoc. of Petroleum Geologists, voltaic Conf., San Diego, 7 - 10 Jan Bioengineering Seminar, The Johns Morgantown, W. Va., 1-40ct 1979. 1980. Hopkins Univ., 22 Oct 1979. J. E. Tillman, "Unraveling the Cenozoic D. W. Fox, "Bounds for Eigenfrequencies V. O'Brien, "Pulsatile Flow in Arteries," Tectonic History of the Eastern U.S.," of Reinforced Elastic Plates," Seminar, Fluid Dynamics Reviews, Univ. Mary­ Nuclear Regulatory Commission Dept. Mathematical Sciences, The land, College Park, 2 May 1980. Seminar, Bethesda, Md., 19 Feb 1980. Johns Hopkins Univ., 10 Apr 1980. V. O'Brien and L. Ehrlich, "Planar Entry P. J. Waitrup, "Observed Pressure J. W. Giles and H. A. Kues, "Photo­ Flow of Viscoelastic Fluid," Workshop Oscillations in Full-Scale Subsonic graphic Dye Studies of Underwater ofVEFlow, BrownUniv., 2Nov1979. Dump Combustors," and "Overview of Wakes," Ocean Optics VI, Naval T. O. Poehler, "Infrared Extinction in NA VSEA Air Breathing Propulsion Postgraduate School, 25 Nov 1979. Organic Compounds and Polymers," Programs," 1979 JANNAF Workshop J. F. George and D. Richards, "Design CSL Scientific Conf. on Obscuration on Pressure Oscillations in Ramjets, and Integration of a 40 MWe Floating and Aerosol Research, Aberdeen Prov­ Monterey, 7 - 8 Sep 1979. OTEC Pilot Plant," 7th Ocean Energy ing Ground, Md., 17 - 21 Sep 1979. P . J. Waitrup, F. S. Billig, and M. C. Conf., Washington, D.C., 2 - 4 Jun T. O. Poehler, "Switching and Memory Evans, "Critical Considerations in the 1980. Effects in Organic Semiconducting Design of Supersonic Combustion Ram­ J. F. George and D. Richards, "Prelimi­ Charge-Transfer Complexes," Chemis­ jet (Scramjet) Engines," 16th AIAA/ nary Design of a Cruising OTEC Mod­ try Div . Symp., Naval Res. Lab., SAE/ ASME Joint Propulsion Conf., ular Experiment," 14th Intersociety Washington, D.C., 13 Feb 1980. Hartford, 30 Jun - 2 Ju11980. Energy Conversion Engineering Conf., R. S. Potember and T. O. Poehler, G. P. Warman, J. C. Murphy, and L. C. Boston, 10 Aug 1979. "Switching Phenomena and Memory in Aamodt, "Surface Area Effects in G. Giicer (JHMI) and L. J. Viernstein Organic Semiconductors," Chemistry Photoacoustic Spectroscopy," Topical (APL), "Continuous Recording of ICP Dept. Colloq., Univ. Pennsylvania, 29 Meeting on Photoacoustic Spectros­ in the Normal Monkey," American Jan 1980. copy, Univ.lowa,I-3AugI979. Association of Neurological Surgeons, R. S. Potember and T. O. Poehler (APL) L. B. Weckesser, "DC93-104 Thermal Los Angeles, Apr 1979; also presented and A. Rappa, D. O. Cowan, and A. Modeling Efforts," JANNAF Ramjet at the 4th International Symp. on In­ N. Bloch (JHU), "Electrical Switching Combustor Insulator Thermal Modeling tracranial Pressure, Williamsburg, Va., and Memory Phenomena in Semicon­ Workshop, Monterey, 14 Mar 1980. Jun 1979. ducting Organic Charge-Transfer Com­ L. B. Weckesser, "Radome Aerodynamic A. N. Jette, "Interpretation of Hyperfine plexes," NATO Advanced Study Inst. Heating Effects on Boresight Error," Interaction and Structure of Defect on Physics and Chemistry of Low 15th Symp. on Electromagnetic Centers Using a Spin-Correlated Va­ Dimensional Solids, Tomar, Portugal, Windows, Atlanta, 18 - 20 Jun 1980. lence Bond Theory," Seminar, Univ. 27 Aug - 7 Sep 1979. L. B. Weckesser and L. L. Perini, "Ther­ Antwerp, 4 Mar 1980; also presented at R. S. Potember and T. O. Poehler (APL) mal Modeling of Combustor Insulation Seminar, Gesamthochschule Paderborn, and D. O. Cowan (JHU), "Switching DC93-104," JANNAF 1980 Propulsion FRG, 22Apr 1980. and Memory Phenomena in Semicon­ Meeting, Monterey, 11 - 13 Mar 1980. A. N. Jette, "Valence Bond Study of ducting Charge-Transfer Complexes," R. O. Weiss (APL), R. Barr (Hydro­ Hyperfine Interactions and Structure of Meeting, American Chemical Society, nautics), J. Gianotti (Giannotti and Hypervalent Radicals," Zentrum Inter­ Washington, D.C., 10 - 13 Sep 1979. Assoc.), W. Deuchler (Gibbs and Cox), diszipliniire Forschung, Univ. Bielefeld, J. C. W. Rogers, "Numerical Solution of R. Scotti (NOAA), J. Stadter (APL), FRG, 11 Mar 1980. Systems of Conservation Laws," Col­ and J. Walsh (VSE), "Report of the Ad H . A. Kues, "Retinal Nerve Fiber En­ loq., Dept. Mathematics, Univ. Wiscon­ Hoc OTEC CWP Committee: An As­ hancement Techniques," Wilmer Re­ sin, Madison, 1 Apr 1980. sessment of Existing Analytical Tools search Institute Meeting, Baltimore, 27 J. C. W. Rogers, "The Stefan Problem," for Predicting CWP Stresses," 7th Sep 1979. Analysis Seminar, Univ. Minnesota, 31 Ocean Energy Conf., Washington, J. R. Kuttler, "Estimating Characteristic Mar 1980. D.C., 2-4Jun 1980.

Volume 1, Number 3,1980 243 APL COLLOQUIA

Jan. 4, 1980 - "The Riddle of Tektites," Through the Garden of Elementary Par­ Apr. II - "A Visit to China," by F. S. by J. A. O'Keefe, NASA/Goddard ticles," by A., Pevsner, The Johns Billig, APL. Space Flight Center. Hopkins Univ. Apr. 18 - "Godel, Escher, Bach: An Eter­ Jan. 11- "Technical- Congressional In- Feb. 29 - "Gas Industry Perspective on nal Golden Braid," by D. R. teraction on Synthetic Fuel Produc­ Future Energy Resources," by A. C. Hofstadter, Univ. of Indiana. tion," by H. C. Hottel, Massachusetts Eberle, Columbia Gas System Service Apr. 25 - "National Laboratories: What Inst. of Technology. Corp. Are They? What Do They Do? Who Jan. 18 - "An Overview of Our Energy Mar. 7 - "Particle-Beam Weapons," by Cares?," by W. E. Massey, Argonne Future," by O. M. Phillips, The Johns K. Tsipis, Massachusetts Inst. of National Lab. Hopkins Univ. Technology. May 2 - "The Automatic Implantable Defibrillator from Inception to Clinical Jan. 25 - "The Acid Rain," by J. N. Mar. 14 - "Electrochemistry of Solid State Application," by M. Mirowski, Sinai Galloway, Univ. of Virginia. Batteries," by A. Schneider, Catalyst Hospital and The Johns Hopkins Univ. Research Corp. Feb. I - "Fireplaces and Wood burning May 9 - "The Piltdown Man Hoax: Stoves," by R. D. Thulman, Thulman Mar. 21 - " Photoacoustics - Principles Whodunit?," by J. Weiner, Univ. of Eastern Corp. and Recent Developments," by A. London. Feb. 8 -" The Physics of Violins," by N, Rosencwaig, Lawrence Livermore May 16 - "The Role of the Oceans in the Pickering, Southampton Hospital. Laboratory. Earth's Heat Balance," by A. H. Oort, Feb. IS - "Designing a Global Future: Mar. 28 - "The New Genetics," by D. Princeton Univ. Some Reflections on the New Social Nathans, The Johns Hopkins Univ. May 30 - "The Role of Oxygen in Paradigm," by D. Pirages, Uni" . of Apr. 4 - " Impress ions of Soviet Science Retinopathy: A 14-Year APL - Wilmer Maryland. and Technology," by A. S. Greenberg, Institute Cooperative Study," by R. W. Feb. 22 - "Quarks, Gluons, ... A Walk U.S. Dept. of State. Flower, APL.

244 johns Hopkins APL Technical Digest THE AUTHORS

lege, and The Polytechnic Institute of Brooklyn where he obtained the B.S.M.E. degree in 1954. He was an engineering instructor at the U.S. Naval Academy for three years prior to joining I. the Space Department of APL in 1960. He is a power systems specialist and has been responsible for the design and development of both solar and nuclear power systems for many spacecraft, in- cluding Transit, Geos, Triad, Transit Improvement Program, and Magsat. Mr. Allen is supervisor of the Space Power Systems Section and assistant supervisor of the Spacecraft Systems Group.

FREDERICK S. BILLIG was born in Pittsburgh in 1933. He did his under­ graduate work at The Johns Hopkins MARIO H. ACUNA was born in University and earned the M.S. (1957) (1964) and has done graduate work at 1940 in Cordoba, Argentina, where he and Ph.D. (1964) degrees in mechanical The Johns Hopkins University. In 1964 received his undergraduate degree from engineering from the University of Mary- he joined APL where he has been in­ the University. He earned the M.S.E.E. volved primarily in hardware and soft­ degree from the University of Tucuman ware design for digital electronic systems (1967) and the Ph.D. in space science associated with space, shipboard, and from The Catholic University of ground applications. In particular he America (1974) . In 1969, he joined the contributed to the design of the Seas at Goddard Space Flight Center, where his radar altimeter and the Transim satellite research interests have centered around navigation processor. Mr. Burek de­ instrumentation for geophysical and signed and programmed the LORAN-C space research as well as studies of navigation software used in the Hydro magnetic fields and plasmas in inter­ and Aries ocean experiments, and has planetary space and in magnetospheres. been responsible for the data storage Dr. Acuna has been involved with Ex­ subsystems for several recent spacecraft. plorers 47 and 50, Mariner 10, Pioneer He is working on a microprocessor­ 11, Voyagers 1 and 2, Magsat, the Inter­ based data handling system for the national Solar Polar Mission, Project Ampte spacecraft and is exploring the Firewheel, and Ampte as instrument en­ potential use of magnetic bubble mem­ gineer, principal investigator or en­ ories for space applications. gineer, or project scientist. He has received many awards in recognition of his contributions to NASA programs.

land. He joined APL in 1955 where he THOMAS B COUGHLIN was born participated in experimental programs in­ in Baltimore in 1941. He studied me­ volving hypersonic wind tunnels and ex­ chanical engineering at the University of ternal burning, and developed theoreti­ Maryland (B.S., 1964) and at Drexel cal/ experimental techniques for analyzing supersonic combustion. Subsequently Dr. Billig studied hypersonic propulsion ram­ jets, hydrogen and storable liquid fuels, the SCRAM combustor, ignition and combustion of fuels, and the penetration of liquid jets into supersonic streams. Since 1977, he has been assistant supervisor of the Aeronautics Division. He received the Distinguished Young Scientist Award of the Maryland Aca­ demy of Sciences in 1966 and the Silver Combustion Medal of The Combustion Institute in 1970. He is presently a direc­ tor of the AIAA.

WALTER E. ALLEN was born in RONALD K. BUREK was born in New York City in 1932. He attended Detroit in 1940. He received the B.E.E. Queens College, Indiana Technical Col- degree from the University of Detroit

Volume 1, Number 3,1980 245 University (M.S., 1967). He worked in magnetometer, Dr. Farthing is a co-in­ missile dynamics at the Martin Co. dur­ vestigator for the magnetic fields experi­ ing 1964-67 and in the mechanical design ments on Dynamics Explorer, and is sci­ ®d structural analysis of the ATS-6 and ence manager for the Space Environ­ Sert spacecraft at Fairchild Industries ment Monitor onboard Geos. during 1967-72. Since joining APL in 1972, Mr. Coughlin participated in the analysis and GLEN H. FOUNTAIN was born in testing of spacecraft mechanical systems. Hutchinson, Kans. in 1942 and received He is now supervisor of the Structural the B.S.E.E. and M.S.E.E. degrees from Analysis and Test Section of the Space Kansas State University in 1965 and Electromechanical Design Group. 1966. Since 1966 he has been employed by APL in the area of satellite attitude control and detection and system in­ strumentation. He has participated in JOHN R. DOZSA was born in Alpha, the design of control systems for a N.J. He received a B.S.E.E. from La­ number of satellites including the Small fayette College in 1963 and has done graduate work at the University of Maryland. Upon joining APL in 1963, he assisted in the installation of the Typhon Radar System aboard the USS where he specialized in thermal analysis Norton Sound. From 1964 to 1973 he of spacecraft. During 1961-63 he served was involved in the design of electronic as a branch manager in the Environmen­ flight and ground equipment for space tal Test Division at the Goddard Space satellite systems including portions of Flight Center. Returning to APL in 1963 the Dodge satellite TV cameras and as a project engineer, he has been super­ visor of the Space Systems Branch since 1967. During 1977-79, he also acted as program manager for the Magsat space­ craft. On January 1, 1981, he will be­ come Assistant Head of the Space De­ partment and Programs Manager.

WINFIELD H. FARTHING was born in North Carolina in 1934. He received his undergraduate education at North Carolina State University, and the Ph.D. Astronomy Satellites. Appointed assis­ from The Catholic University of Amer­ tant project scientist for Magsat in 1977 , ica. He was employed by th e National Mr. Fountain has been principally con­ cerned with the design of the attitude control and determination subsystems. Other professional interests have in­ cluded biomedical engineering and the study of coherence of the earth's mag­ netic noise field. other imaging camera systems, scan con­ verters, and displays. During 1973-78, Mr. Dozsa designed shipboard hardware units for the SEATIPS and AN/SYS-I automatic detection and tracking radars. In 1978 he became the cognizant engi­ neer for the Magsat standard s-band transponder. At present, he is a design engineer in the Ocean Engineering Pro­ gram Sensor Evaluation Laboratory of the Strategic Systems Department

LEWIS D. ECKARD, JR. was born Security Agency before transferring to in Philadelphia in 1923. He received the the Goddard Space Flight Center B.S.M.E. degree (aeronautical option) in (GSFC) in 1962. 1949 from the University of Maryland, He now is an electronics engineer with and has done graduate work in aeronau­ GSFC, where he has participated in the / tical engineering at the University of design and implementation of a variety Maryland and in engineering administra­ of scientific instruments for spaceflight. tion at George Washington University. He was project engineer of the scalar I In 1952 Mr. Eckard started working magnetometers flown on the Pogo satel­ for the Engineering Research Corpora­ lites and participated in the early efforts KEVIN J. HEFFERNAN was born in tion, where he performed analyses in to bring a follow-on mission such as Bryn Mawr, Pa. in 1952 and received his support of the development of electronic Magsat into being. In addition to being B.S.E.E. from Drexel University and his flight simulators. In 1957 he joined APL technical officer for the Magsat scalar M.S.E.E. from The Johns Hopkins Uni-

246 lohns Hopkins APL Technical Digest versity in 1975 and 1980 . Specializing in BRUCE C. MOORE was born in control theory and electronic design, he Hackensack, N.J., in 1943. He received joined APL's Attitude Control Systems the B.E.E. from Rensselaer Polytechnic Group in 1977. His principle responsibil­ Institute in 1965 and the M.S.E.E. from ities on Magsat were as program sponsor The Johns Hopkins University in 1978. of the IR scanner reaction wheel, co­ Prior to joining APL in 1975, he was designer of the microprocessor-based at­ employed as an engineer by RCA Astro titude signal processor, and participant Electronics Division where he worked on in the simulation and analysis of the such space programs as Tiros, Apollo, various pitch control modes for stability and Atmosphere Explorer. At APL he purposes. has been involved in the design of com­ Mr. Heffernan is currently designing mand and telemetry hardware on the another microprocessor-based system - Transit, Transit Improvement Program, a magnetometer signal processor - for and Magsat satellites. Mr. Moore is a the Air Force Defense Meteorological member of the Space Instrumentation Satellite Program. Systems Group and is providing techni­ cal support to the Fleet Systems Depart­ ment for telemetry pertaining to the Standard Missile program.

Geos, and Small Astronomy Satellite ve­ hicles. Mr. Lew has been the command system leader for the Triad, Transit Im­ provement Program, and Magsat space­ craft. Currently, he is working on another microprocess or-based design - the data system for the LAN experiment of the Solar Polar spacecraft. Mr. Lew is supervisor of the Systems Analysis and Development Section of the Space Instrumentation Systems Group.

ROBERT A. LANGEL was born in Pittsburgh in 1937 but spent most of his youth in Ohio. His undergraduate de­ gree was earned at Wheaton College (1959) and his Ph.D. in physics was GILBERT W. OUSLEY received his received from the University of Mary­ B.S.M.E. (1954) and M.S.M.E. (1958) land in 1973. After undergraduate from the University of Maryland. In school he worked for three years at the 1959 he joined the NASA Goddard Naval Research Laboratory before going Space Flight Center (GSFC) where he to Goddard Space Flight Center (GSFC) has held several line, staff, and project where he is presently a member of the manager positions . He spent two years Geophysics Branch. While at GSFC he in Paris in the mid-1960's as the first has participated in the magnetic field ex­ representative in Europe of NASA periments on the Ogo and Pogo satel­ Headquarters. He has been the Magsat lites, including pioneering studies of project manager from project initiation magnetic disturbances at high latitudes. During his professional career Dr. Langel has authored or co-authored 30 papers and is recognized as an authority FREDERICK F. MOBLEY was born in geomagnetism. He is presently project in Atlanta in 1932. He received a B.S. scientist for the Magsat project and a degree from the University of Illinois in leader in GSFC's research in the area of 1953 and an M.S. in aeronautical engi­ the earth's main magnetic field and neering from the Massachusetts Institute crustal magnetic anomalies. of Technology in 1958. He came to APL in 1955. Since 1960 Mr. Mobley has worked on satellite attitude control, in­ ARK L. LEW was born in Canton, cluding gravity-gradient stabilization of China, in 1941. He received the the Transit satellites and the use of B.S.E.E. degree from Case Institute of magnetic control systems in the Small Technology (1963) and the M.S.E. Astronomy Satellites and Magsat. He degree from The Johns Hopkins Univer­ was the APL project scientist on Magsat sity in 1968 . Since joining APL in 1963, and participated in the overall satellite he has worked in the areas of spacecraft design and its implementation. Mr. systems and electronics design. He has Mobley is supervisor of the Space At­ designed electronics for the Explorer, titude Control Systems Group.

Volume I, Number 3,1980 247 through in-orbit mission operations. He 1964. He is currently enrolled in the is now manager of NASA's Ampte pro­ M.S. program in technical management ject. of the Johns Hopkins Evening College. Mr. Ousley is the recipient of the Since joining APL in 1961, he has spent DoD Meritorious Civilian Service most of his time in the Space Depart­ Award, the NASA Exceptional Service ment where he has been responsible for Medal, the Medaille de Vermeil du mechanical and attitude control system CNES, the Bundesverdienstkreuz, as design, control system simulations, elec­ well as GSFC performance and achieve­ tronic and electromechanical hardware ment awards. development, and post-launch opera­ tions and analysis. Mr. Toss man has also been responsible for state-of-the-art THOMAS A. POTEMRA was born systems development of magnetic instru­ in Cleveland in 1938. He did both his mentation for the Strategic Systems undergraduate and graduate work in ,/ Department. He is currently program

/ for the Small Astronomy Satellite star mappers built at APL and was heavily involved in the development of the Atti­ tude Transfer Sys tem and optical me­ trology for Magsat. He is a senior member of the IEEE.

JAMES F. SMOLA was born in Chi­ cago in 1928 and obtained a B.S.M.E. at the Illinois Institute of Technology in

electrical engineering, receiving the B.S. manager for the Galileo Energetic Par­ from Case Institute of Technology ticles Detector Project and the LAN (1966), the M.E.E. from New York Uni­ project of the Solar Polar spacecraft. versity (1962) and the Ph.D. from Stan­ ford Uriiversity (1966). CLARENCE A. WINGATE was born Since joining APL in 1965 Dr. Potem­ in 1930 in Charlotte, N.C. He received a ra has engaged in research in ionosphere B.S.M.E. (1952) and an M.S.M.E. and space physics. He was principal in­ (1958) from North Carolina State Uni­ vestigator for the Triad magnetometer versity and did graduate work in physics experiment and co-investigator for the during 1960-64 at the Unive rsity of Atmospheric Explorer photoelectron Maryland. Upon joining APL in 1958 he spectrometer experiment. He is presently worked on ramjet performance analysis assistant group supervisor of the Space until 1967, when he became supervisor Physics and Instrumentation Group. of the Thermal Systems Design Section Among hi s many activities outside the of the Spacecraft Systems Group. Since Laboratory, he is associate editor of the then he has been directly involved in the Journal oj Geophysical Research and 1951 and an M.S. in numerical science design analysis and testing of 12 space­ secretary of the Aeronomy Section of from The Johns Hopkins University in craft and five spacecraft instruments. the American Geophysical Union. 1969. During 1955-58 he was directly in­ volved in the integration of the Terrier, FREDERICK W. SCHENKEL was Tartar, and Talos surface-to-air missiles born in Jersey City in 1932. He earned a aboard vessels of the U.S. Navy's sur­ B.S. E.E. degree at Fairleigh Dickinso n face fleet. During that period he was University in 1958, and has done gradu­ employed by the Vitro Corp. and the ate work at Johns Hopkins and the Uni­ Washington Technological Ass ociates. versity of Maryland. During 1955-63 he In 1958 Mr. Smola joined APL where was engaged in physical electronics R&D he has been in vo lved in the design, fab­ and engineering with the Allen B. Du­ rication and testing of mechanical, elec­ Mont and CBS Laboratories. tromechanical, and attitude control sys­ Mr. Schenkel joined APL in 1963 as tems for spacecraft. At present he is an section supervisor in the Microelectron­ assistant group supervisor and section ics Group. Since 1965 he has been with supervisor in the Space Attitude Systems the Space Department, where he has Group. been involved in electro-optic systems design and analysis, including the Dodge BARRY E. TOSSMAN was born in satelli te's color TV camera system, Baltimore in 1940 and recei ved his which secured the first color pictures of B.S.M.E. and M.S.M.E. degrees from the full earth. He was also responsible the University of Maryland in 1961 and

248 Jo hns Hopkins APL Techn ical Digesl The APL 6O-foot-diameter parabolic antenna for transmitting com­ mands to and receiving telemetry from some orbiting satellites.

Volume 1, Number 3,1980