Laser Ranging Togps Satellites with Centimeter Accuracy

INNOVATION

The DoD's goals for the GPSILRE exper­ Laser Ranging iment are essentially the same as those of the Advanced Clock Ranging Experiment, which NRL originally proposed to DoD's Tri-Ser­ toGPS vice Space Test Program in 1986. In his pro­ posal, which later became the GPS/LRE, NRL principal investigator Ron Beard states Satellites with that a primary objective of the experiment is to provide an independent, high-precision Centimeter measurement of satellite position that, when compared with GPS pseudoranges, can unambiguously separate satellite-position Accuracy errors from on board atomic-clock errors. The accurate determination of satellite position and clock characteristics is essential to pre­ dicting GPS performance and reducing errors John J. Degnan in both the short and long term. The laser Erricos C. Pavlis measurements also support ongoing atomic­ clock evaluations within DoD and their effect Laboratory for Terrestrial Physics on overall GPS system performance. NASA Goddard Space Flight Center NASA's interest in the experiment stems from the agency 's current and future use of In 1960, Theodore H. Maiman, of the Hughes advances in GPS technology and its GPS in a variety of geophysics applications Aircraft Company, successfully operated the applications as well as the fundamentals of that previously had been carried out exclu­ first device to generate an intense beam of GPS positioning. The column is coordinated sively by two more mature space geodetic highly coherent monochromatic radiation. He by Richard Langley and Alfred Kleusberg of techniques: satellite laser ranging and very called his device a laser - for light amplifica­ the Department of Geodesy and Geomatics long baseline interferometry (VLBI). tion by the stimulated emission of radiation. Engineering at the University of New NASA's involvement in all three techniques The laser has become ubiquitous, with literally Brunswick. We appreciate receiving your is pervasive. The Jet Propulsion Laboratory hundreds of uses ranging from optical surgery comments as well as suggested topics for (JPL) in Pasadena, California - with fund­ to precision machining. Lecturers use laser future columns. ing from NASA- serves as both the Central pointers; surveyors use laser distance­ Bureau and an analysis center for the Interna­ measuring devices; police officers use laser On August 30, 1993, the Department of tional GPS Service for Geodynamics (IGS) radar units to catch speeders. Most of us Defense (DoD) launched the GPS-35 and has also been a leader in the engineering unwittingly use a laser each time we listen to (SVN35, PRN05) satellite into orbit, placing development of precise geodetic receivers for our CD players- the light reflected from the it in the GPS satellite constellation's B-4 both ground and spacecraft use. The Goddard microscopic pits on the CD is generated by a orbital plane position. This particular Block Space Flight Center (GSFC), which serves as precisely positioned laser. IIA satellite differs slightly from its prede­ an IGS Global Data Center, is also the pri­ One application of the laser that is not so cessors in that it carries a small panel of opti­ mary Coordination and Data Analysis Center well known is satellite laser ranging. In this cal retroreflectors, enabling it to be tracked for both SLR and VLBI, operating 10 of month's column, John Degnan and Erricos by an international network of centimeter­ approximately 43 stations in the global SLR Pavlis, from the Laboratory for Terrestrial accuracy satellite laser ranging (SLR) sta­ network and three of approximately 30 VLBI Physics (LTP) at NASA's Goddard Space tions. Laser tracking of GPS-35 began on field stations. GSFC hosts IGS GPS receivers Flight Center (GSFC), in Greenbelt, October 17, 1993. On March 10, 1994, GPS- at most of these sites. Maryland, introduce us to satellite laser 36 (SVN36, PRN06) , carrying an identical As GPS demonstrated its ability to mea­ ranging and describe the efforts to track two retroreflector package, was launched into the sure short- to medium-length baselines with of the Navstar GPS satellites using this C-1 orbital slot. Laser tracking of GPS-36 comparable accuracy in the late 1980s, it technique. Dr. Degnan is the head of LTP 's was initiated on April21 , 1994. gradually displaced the more-expensive Space Geodesy and Altimetry Projects Office. The GPS Laser Retroreflector Experiment mobile SLR and VLBI systems as the tech­ He has been employed at GSFC since 1964 (GPS/LRE), which focuses on these two nique of choice in internationally sponsored when, as a coop student from Drexel satellites, is a joint effort by several groups campaigns to measure regional crustal defor­ University, he participated in the first laser­ working in cooperation with the Navstar GPS mation. These early mobile campaigns were ranging experiments to the Beacon Explorer B Joint Program Office. These include the principally conducted in the United States, satellite. Dr. Pavlis is a senior geodesist in Naval Research Laboratory (NRL) in Wash­ . Canada, Mexico, and Europe. the LTP and is affiliated with the Department ington, D.C.; the National Aeronautics and NASA investigators are now comparing of Astronomy at the University of Maryland. Space Administration (NASA) through its global GPS network solutions with those His research interests include satellite Goddard Space Flight Center (GSFC) in obtained from SLR and VLBI. Following orbital dynamics and the analysis of space Greenbelt, Maryland; the University of an appropriate, seven-parameter coordinate geodetic data. Maryland at College Park, Maryland; and the transformation (three translational parame­ "Innovation" is a regular column in Russian Institute for Space Device Engineer­ ters, three rotational, and one for scale), GPS World,featuring discussions on recent ing in Moscow. global station positions obtained by the three

62 GPS WORLD Seplember 1994 ~ /~

...... ([;;)\~',.tf .~ Reflected / / Satellite time transfer. Another source by the receiver telescope. pulse ,./ ,./ optical of en·or in computing orbits Only a handful of the outgoing photons ..•. .-· ...... •• reflector •• ••·····Tran smitted for the GPS satellites is the makes it back. The telescope focuses the Tracking telescope ,.. •• ··' pulse description of attitude returning radiation onto a high-gain, hi gh­ and detector "' changes, especially during speed (subnanosecond) photodetector th at eclipse seasons when a satel­ stops the time-interval counter. A modern lite spends a significant frac­ SLR counter has a time resolution on the tion of its orbit in the earth's order of 20 picoseconds or less, which corre­ shadow. Better handling of sponds to a single-shot range resolution of 3 the nonconservati ve forces millimeters or less. The epoch time of depar­ Figure 1. A simplified block diagram of a typical satel­ lite laser ranging (SLR) station. By measuring the acting on the satellites (such ture for the laser pulse is also recorded. elapsed time between the transmission of a laser pulse as solar radiation pressure) Because the station master clock is typically and the reception of the pulse reflected by the satellite, can be achieved by using pre­ a cesium standard, periodically updated by a the range to the satellite can be determined. cise SLR tracking and the GPS timing receiver, epoch time is accurate resulting orbits to "tune" to a small fraction of a microsecond. models of these forces to Typically, the photodetector is a photo­ techniques agree at the one- to two-centime­ each satellite individually. multiplier tube. In a photomultiplier tube, ter level at high-quality, collocated sites. This Geodetic positioning can benefit from the incident photons strike a photosensitive sur­ agreement is at a level consistent with the increased orbit accuracy of retroreflector ~ face, causing it to emit electrons through the estimated modeling uncertainties for each carrying GPS satellites in two ways: the photoelectric effect. These so-called photo­ technique. However, there are important geo­ better orbits result directly in better position­ electrons are accelerated by a potential dif­ physical phenomena where the expected ing; and they can also help in the reliable ference and st1ike an electrode, causing it to rates of change of position are on the order of resolution and "fixing" of the ambiguities of dislodge additional electrons. These elec­ a few millimeters per year or less. Such phe­ doubly differenced canier-phase measure­ trons are attracted to another electrode, which nomena include changes in mean sea level, ments. The latter strengthens the estimation in turn ejects more electrons. This process postglacial rebound, and deformations due to procedure because it converts, in effect, the may be repeated 10 or more times, generating atmospheric loading. A better understanding very precise but relative measure of change­ a cun·ent that can readily be measured. of the residual error sources and their magni­ in-range to absolute range (for more on GPS­ When fit to a short orbital arc (less than tudes for each of the positioning techniques canier-phase measurements, see "The GPS one orbital revolution), the single-shot data would be helpful, as would, if possible, the Observables," GPS World, April1993). from ranging to passive orbiting geodetic ability to make reliable measurements with satellites, produced by state-of-the-art SLR millimeter accuracy, especially in the vertical SLR PRINCIPLES field systems, typically exhibit a root-mean­ direction. This rather ambitious goal can be A satellite laser ranging system is, in simple square (rms) scatter of 6 to 10 millimeters. If accomplished convincingly only through terms, an optical radar. Figure 1 is a simpli­ one forms normal points by averaging indi­ routine and painstaking comparisons of the fied block diagram of a typical SLR system. vidual range measurements over a two­ geodetic results obtained by different tech­ The modern laser transmitter uses a mode­ minute time interval to reduce random en·ors, niques at collocated sites. locked Nd:YAG laser (a solid-state, pulsing the rms scatter about the short arc is reduced The GPS/LRE experiment provides a laser using neodymium as a lasing impurity to between 1 and 3 millimeters. Normal totally new way to compare GPS and SLR, in a host lattice of yttrium aluminum garnet) points are fanned at the field sites after each which we expect will highlight the inherent with output frequency doubled to produce satellite pass and transmitted electronically to strengths of each technique in making impor­ green light with a wavelength of 532 three SLR analysis centers at the GSFC, at tant, new, scientific measurements. Analyses nanometers. The laser, which operates at a the University of Texas, and at the Technical with a dense, global SLR tracking network repetition rate of between 5 and 10 Hz, University of Delft in The Netherlands. - when it becomes available- can give us produces an ultrashort pulse with a pulse These "quick-look" analysis centers rou­ insight into the many sources of systematic width between 30 and 200 picoseconds (full­ tinely monitor station performance by fitting errors in the radiometric (canier phase and width-at-half-maximum) and a single pulse the global SLR data set to multiday arcs of pseudorange) data. The nature of these mea­ energy between 10 and 100 millijoules. (To the Laser Geodynamics Satellite (LAGEOS), surements (one way, using two, L-band put this amount of energy in context, a joule a passive geodetic satellite, orbiting at an radio-frequency [RF] carriers), makes it is the amount of energy required to illumi­ altitude of 6,000 kilometers, which was impossible to separate completely clock nate a single-watt flashlight bulb for one launched by the United States in 1976. errors from orbital errors. Residual iono­ second or to heat a half-teaspoon of water by Although data quality varies widely among spheric enors and errors from tropospheric­ one-tenth of a Celsius degree. Laser pointers the approximately 43 international stations refraction mismodeling result in additional have a power output of a few millijoules that make up the global SLR network, the degradation. The reflection of RF signals per second.) The outgoing laser pulse, con­ weighted rms-fit to the multiday LAGEOS near the antenna site (multipath) is an addi­ taining about 10 17 photons, is sampled by arc is typically on the order of 2 centimeters tional source of error that varies from site to the range receiver, which, in turn, starts a and of high enough quality to pinpoint site and is quite difficult to quantify or elimi­ time-of-flight measurement. The laser pulse systematic data problems rapidly , at the nate completely. A proper characterization of propagates through the atmosphere, is few-centimeter level, at individual stations. the enor spectrum of the frequency standards reflected by a retroreflector array on board Systematic errors in the hardware are on board the satellites is also of interest, the satellite, returns through the atmosphere controlled at the few-millimeter level by fre­ especially to those using GPS signals for to the source, and is collected on the ground quent calibration runs using carefully

64 GPS WORLD Seplem ber 1994 Over the past three decades, 1.27 kilograms. At normal light incidence, the range-measurement pre­ the aperture of an individual retroreflector is cision has improved by about a rectilinear hexagon equivalent in area to a three orders of magnitude - circle 28.6 millimeters in diameter. from a few meters in 1964 to Figure 3 illustrates the relative locations of a few millimeters in 1994. the GPS satellite center of gravity (CG), the Over the same period, the effective laser array center of reflection, and international SLR network the phase center of the L-band transmitting­ has grown to more than 40 antenna array. The positive-Z coordinate axis NASA MOBLAS·7 and a transportable SLR system were stations, including seven is in the direction of satellite nadir. The coor­ collocated at the Goddard Space Flight Center. A highly mobile systems (see dinates in millimeters of the nominal CG in collocation is the most effective method of testing an Figure 2) . the body-fixed reference frame are (0, 0, SLR system for systematic errors. Laser-ranging observa- 1,011.4). This is really an average position tions have proved their scien­ because, over the life of the mission, the CG surveyed target monuments on the ground. tific worth in a number of different areas . In is expected to move by about 4.6 millimeters Also, before deployment, new systems are addition to precise orbit determination, SLR in the negative-Z direction, from Z = 1,013.7 usually collocated with a network standard, (together with VLBI) pioneered measure­ millimeters to Z = 1,009.1 millimeters. In the such as the NASA Mobile Laser (MOBLAS) ments of global tectonic plate motion, same body-fixed reference frame, the coordi­ -7 system in Greenbelt, Maryland. In a collo­ regional crustal deformation near plate nates in millimeters of the antenna array cation, the SLR system under test is placed boundaries, and earth orientation. SLR phase center are (279.4, 0, 1,967.9), and the within 60 meters of the reference system, and uniquely defines the position of the earth's coordinates in millimeters of the effective the relative locations of the reference centers center of mass, which is the coordinate origin reflection center for the laser array are for the two systems are surveyed in, with an of the International Earth Rotation Service (862.6, -524.5, 1,669.8). The effective accuracy of two millimeters or less. Careful Terrestrial Reference Frame. SLR has con­ reflection center lies approximately 30 mil­ attention is also paid to the station clocks and tributed to studies of the earth's gravity field limeters below the physical surface of the meteorological sensors for each system. The and has been used to calibrate microwave laser array. two systems then simultaneously track a altimeters carried by oceanographic satellites variety of satellites, and, in a successful col­ such as ERS-1 and TOPEX/Poseidon, in SLR TRACKING OF GPS location, the measured orbits will not differ addition to defining their operational orbits. Figure 4 shows a typical OMC (observed by more than a few millimeters peak to peak. Lunar laser ranging (LLR) to retroreflectors minus calculated) plot of the laser-range The photo above shows a collocation left on the moon's surface by three of the residuals for a calculated best-fit GPS short­ between the larger (75-centimeter telescope U.S . Apollo missions and to retroreflectors arc orbit of approximately 40 minutes ' dura­ aperture) NASA MOBLAS-7 station in the on two Soviet Lunokhod remotely controlled tion. The data were taken on March 9, 1994, background with a smaller (28-centimeter rovers supplied exciting new scientific infor­ by the NASA MOBLAS-4 SLR station in telescope aperture) transportable SLR system mation on earth-moon interactions and the Quincy, California. Single-shot, laser-range in the foreground. dynamics and physical makeup of the moon measurements are represented by open The absolute accuracy of the laser-range and provided unique tests of competing theo­ squares and show an rms -scatter about the measurement is currently limited by residual ries of general relativity. short-arc orbit of 1.16 centimeters. Normal uncertainties in the atmospheric propagation points, representing laser-range data aver­ delay. However, unlike space geodetic tech­ GPS RETROREFLECTOR ARRAY aged over five-minute time intervals to niques that operate at microwave frequencies The retroreflector arrays used on GPS-35 and remove random range errors, are indicated by (such as VLBI and GPS), SLR measure­ 36 were built by the Russian Institute for black squares on the same figure and have an ments are relatively unaffected by the two Space Device Engineering in Moscow under rms scatter of about two millimeters. This most dynamic components of the atmo­ contract to Professor Carroll Alley at the short-arc performance is only slightly sphere: the ionosphere and the "wet" tropo­ University of Maryland. The GPS array, con­ degraded (by about 30 percent) from what is sphere. Although local, high-accuracy structed under the supervision of Dr. Victor typically achieved on short arcs with measurements of the relevant meteorological Shargorodsky, is similar in design to those geodetic satellites such as LAGEOS and parameters (barometric pressure, tempera­ used successfully on all of the Russian Starlette (a smaller, .retroreflector-covered ture, and relative humidity) are routinely GLONASS satellites. However, the total satellite orbited by France in 1975). taken at laser sites and transmitted daily with reflecting area of the GPS arrays is much Although the quality of the GPS laser­ the range data for atmospheric refraction smaller due to limited mounting space on the tracking data taken to date has been excel­ models, there are still residual uncertainties nadir-viewing face of the Block IIA satel­ lent, the data yield , as predicted from that are believed to limit the absolute range lites. A GPS array, illustrated on page 68, theoretical analyses prior to launch, has accuracy to the 5- 12-millimeter level, espe­ consists of 32 individual, fused-quartz ·been relatively low . The low data yield cially at low-elevation angles (for more on retroreflectors. Each retroreflector is coated results from two factors - one program­ the problematic troposphere, see "Effect of on the back reflective surfaces with alu­ matic and the other technical. On the pro­ the Troposphere on GPS Measurements," minum, placed in a special holder, and grammatic side, GPS-35 and 36 are GPS World, January 1993). arranged in a flat panel in alternating rows of currently ranked 1Oth and 11th in tracking NASA's GSFC performed the first suc­ either five or four. The array measures 239 priority in a group of 13 laser-tracked satel­ cessful laser ranging to an artificial satellite millimeters in length by 194 millimeters in lites. On the technical side, the principal equipped with optical retroreflectors in 1964. width by 37 millimeters in height and weighs limitation is a relatively weak optical link

66 GPS WORLD September 1994 (one or two photoelectrons per range measurement) resulting from a combina­ Huahlne tion of long slant ranges • (greater than 20,000 kilo- meters) to a GPS satellite, • Fixed SLR site the small size of the • Mobile SLR occupation onboard retroreflector, and the engineering character- istics of most SLR stations, Figure 2. Locations of fixed (permanent) SLR stations and sites occupied by mobile SLR systems. which were designed prin- Data were collected at more than 40 sites worldwide in 1993. cipally for precise day and night tracking of LAGEOS and lower-orbit­ priority than high-orbiting satellites because tation for ranging to GPS satellites relatively ing satellites. of their shorter pass duration and potential easy. First, because these stations operate SLR tracking priorities are periodically capability to further our knowledge of the only at night, LLR thresholds are set to detect reviewed and set by the SLR Subcommission earth's gravity field and marine geoid. How­ single photoelectrons. Second, because the of CSTG (Coordination of Space Techniques ever, it is possible for interested GPS investi­ lunar ephemeris is well known and offset for Geodesy and Geodynamics, an interna­ gators to aiTange special GPS laser-tracking guiding based on lunar features or local star tional commission created under the auspices campaigns of limited duration to meet spe­ fields is possible, very nanow (about 2 arc of the International Association of Geodesy cific experimental needs. second), atmosphere-limited laser beam and the Committee on Space Research). At Lunar laser ranging stations, such as the divergence is used to transmit more light to the CSTG SLR Subcommission meeting in MLRS station at the McDonald Observatory the target and thereby increase the probabil­ Potsdam in October 1993, TOPEX/Poseidon near Fort Davis, Texas, and the Centre ity of detection. Finally, LLR stations use and ERS-1, two oceanographic missions that d'Etudes et de Recherches Geodynamiques specially constructed multistop event timers U3e lasers as the primary source of tracking et Astronomiques site in southern France, are to accommodate the relatively long 2.5-sec­ data, were assigned highest tracking p1iority. designed to detect weak laser signals from ond roundtrip time to the moon. These Among geodetic and other special satellites, retroreflectors on the moon and therefore devices allow the random multiplexing of low-orbiting satellites were assigned higher have several design features that make adap- start-and-stop pulses so that there can be JRC Unveils Advanced Technology Introducing an 8-channel GPS PACK receiver ideal for car navigation systems

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Circle 23 September 1994 GPS WORLD 67 INNO VAT ION

several pulses in transit to and from the moon If we examine the distribution of the at any given time. The stop pulses associated tracked segments in time and by station, with each start pulse are sorted out in soft­ we find that some of the sites have tracked ware. These multistop epoch timers have a only over certain periods of time in a non­ second important advantage: the occun-ence uniform way. This is because not all sites of a random noise pulse does not terminate have the capability to track targets at such The laser retroreflector array used on the time-of-flight measurement for a given long distances, in broad daylight. Most of the GPS-35 and GPS-36 is made up of 32 individual retroreflectors that reflect laser pulse as in the more common single­ systems are undergoing modifications that light back in exactly the same direction stop time interval units used by most SLR will allow them to track day or night in the from which it comes. systems. future. In the meantime, there are only short In contrast to lunar ranging, SLR detection periods of a day or so when ~1111111111~11111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111~ thresholds are typically maintained at the several sites were simultane- -x JrSpacecraft CG three- or four-photoelectron level so that ousl y successful in tracking satellite signals can be clearly distinguished GPS-35. .lk::;l..,.,..,.,.,../ against a strong daytime noise background. With minor exceptions, Unfortunately, the range returns from GPS we have adopted the models satellites consist of one or two photoelectrons and algorithms in a set of for most systems, so the probability of detec­ standards for processing SLR tion is relatively small. However, in recent data promulgated by the experiments at the NASA MOBLAS-7 sta­ International Earth Rotation Service to ensure as much I tion, the simple inclusion of a postdetection, ....______. ; ' wideband (6 GHz) amplifier, with a gain of compatibility as possible Center of _I/ _j approximately 8, increased the nighttime with the results from other retroreflector array laser data yield from GPS satellites by analysis centers. A long arc +X roughly an order of magnitude to about 100 (at this point its length is ranges per minute. In contrast, Russian about I 04 days), used to Figure 3. A sketch of a GPS satellite shows the loca­ GLONASS satellites, which are at roughly check the fidelity of the force tions of the retroreflector array, antenna array phase the same altitude as the GPS satellites, are models, is continuously center, and the satellite center of gravity (CG) . The tracked relatively easily by many SLR sta­ extended as new data positive Z-direction points out of the page. tions, without need for amplification, become available. The laser because the reflecting array on GLONASS data fit the arc with an rms satellites is roughly 30 times larger in area of 3 centimeters. Figure 5 6 r--,---,-- than that on GPS satellites. shows the residuals to thi s Additional experiments to enable daylight trajectory . As we mentioned SLR tracking of GPS satellites are under way before, there are several days at the MOBLAS-7 site. These focus on without any data, and in most reducing the spectral bandpass of the daytime cases, even on the days when range receiver by a factor of three and the data are available, their spatial field of view by a factor of four, for an geographical di stribution is approximate factor-of-12 reduction in day­ limited. In particular, Yan-a­ light-induced noise. This would allow the gadee started ranging to SLR systems to operate with the postdetec­ GPS-35 only recently, and Minutes into Pass tion amplifier in daylight. If the ongoing there is an obvious lack of Figure 4. Range residuals from the analysis of data experiments are successful, the receiver Southem Hemisphere track­ from a pass of GPS-35 collected by the MOBLAS-4 upgrade will be replicated at key SLR sites. ing that can induce signifi­ system at Quincy, California, on March 9, 1994, starting cant biases in our orbits. at 03:47 UT. ORBITAL ANALYSIS AND RESULTS Without uniform data con- At the time this article was written, GPS-36 trol, the quality of the fitted orbit is hard to inclusive. We fit another arc to the data over had been tracked for less than a month ; assess. On one occasion, however (Novem­ the 14-day period beginning on November therefore we will restrict our discussion ber 18, 1993), the SLR network managed to 18 . These two 14-day arcs have only one of data collection and analysis to GPS-35. acquire a fairl y large number of passes (10) day's worth of data in common: November There are 12 sites that, with varied fre­ from the majority of the tracking sites. This 18. In fact, the data span on that day covers quency, have successfully tracked GPS-35 being the best day for data collection during the time from 11 :00 UT to 23:00 UT, so the over the past six months: Monument Peak, the period we were studying, we decided to common data in the overlapping segment of California; Greenbelt, Maryland; Quincy, use it as a test day to verify the quality of the two arcs span only 12 hours. The data fit California; Fort Davis, Texas; Haleakala our orbits and gain some insight into the either arc with an rms resi dual of about 1.9 (Maui) , Hawaii ; Yarragadee, Australia; level of agreement with the radiometric centimeters. Hestmonceux, England ; Graz, Austria; data-deteD"nined orbits that IGS is routinely Despite the fact that the SLR data distribu­ Wettzell, Germany ; Potsdam, Germany ; distributing. tion is not as optimal as we would prefer for Maidanak, Uzbekistan ; and Evpatoria, We fit a 15-day arc to the data that were a precise orbit determination, it is still worth­ Ukraine. collected over the period November 5-18 while to compare our orbits to the radiomet-

68 GPS WORlD September 1994 300

200 - E' ..s 100 observations with our data­ CONCLUSION (ij :::> analysis software package, About a dozen stations in the international ·u;-o 0 GEODYN, to reconstitute a laser-tracking network are currently tracking ~ Q) dynamic orbit fitting that the two GPS satellites equipped with retro­ Ol c -100 ro data. When this process con­ reflectors. Modifications to the ground a: verged, a trajectory file was receivers will allow for a further increase in -200 produced consistent with the tracking capabilities of several additional those from the SLR analyses sites and add some desperately needed 20 40 60 80 100 for subsequent comparisons. Southern Hemisphere tracking within the Days since epoch: November 5, 1993, 12:42:58 UT With only one day to coming months. When the data distribution compare, it is hard to come increases, further tests can explore the Figure s. Range residuals from a fit of SLR data to a to firm conclusions. We strengths of the SLR and GPS technologies, 10 4-day orbital arc for GPS-35 believe, however, that the to assess the quality of the models used in the comparisons show at least a data reduction process, to link the reference ric data-derived orbits distributed by IGS. compatibility of the SLR and IGS orbits at frames of each techniqu~ in a direct way, and Since the commencement of the operational about the decimeter level in the radial direc­ to investigate possible benefits in the geo­ phase of IGS on January 1, 1994, the official tion and 0.5 to 1.0 meter in the cross-track detic products from a combined data analy­ product, in terms of GPS orbits, consists of a and along-track directions. This is a very lim­ sis. The reduced orbits will also be used by weighted mean of the individual results from ited test, where neither technology has put our partners at NRL to study the short-term the contributing analysis centers. In anticipa­ forward its best accomplishments and capa­ and long-term behavior of the frequency tion of the operational phase, the analysis bilities. A much more uniform and extended standards on board GPS satellites. • centers had already begun testing the proce­ SLR data set will be required before we can dure for merging their results into a single, reliably determine the orbit at the few-cen­ reliable, and uniform orbit. The orbit for GPS timeter level of accuracy. On the other hand, For product information, turn to page 74 week 723 was one of the first to become reduction of GPS data directly within GEO­ and see Manufacturers. For reprints (250 available. Our test day, November 18, 1993, DYN will remove any inconsistencies in the minimum), contact Mary Clark, Marketing falls in that week. The IGS orbit was rotated standards and the reference frames used by Services, (503) 343-1200. into the inertial frame and used as pseudo- the IGS analysis centers and the SLR group. Setting the Standard in Network Timing Synchronize Your NetiiVork IIVith Datum Inc Time Servers & Board-Level Timing Modules Tap into the Global Positioning System satel lite constellation and use International Standard Time to synchronize your campus LAN for the most accurate network timing available. Today's network designers and man­ agers achieve 1-10 ms absolute timing accuracy, improved reliability, and reduced network overhead.

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70 GPS WORLD Seplember 1994 Circle 48