sign in sign up
<<
Home , Apollo 8, Earthrise, Lovell (crater), Pioneer 4

F EATURE Deep Space Navigation: The VIII Mission

by Paul Ceruzzi things. I remember that one of the Missile Division. For a mission like things that I was concerned with at the Apollo, inertial navigation had several In the Apollo VIII time was whether our navigation was challenges. The first is that gyroscopes, mission took three from the sufficiently accurate, that we could, in which are at the heart of an inertial sys- to an orbit around the and fact, devise a trajectory that would get tem, tend to drift and give erroneous read- back again, safely. It was the first mission us around the Moon at the right dis- ings. The second is that the system needs to carry human beings far enough away tance without, say, hitting the Moon to account for the acceleration of gravity, from the Earth to be influenced primarily on the back side or something like which cannot be distinguished from the by the gravitational field of another heav- that, and if we lost communication accelerations due to a rocket’s thrust or enly body. That mission also marked a with Earth, for whatever reason, could any other accelerating force. number of other famous milestones: the we navigate by ourselves using celes- The drift problem, though serious, first human crew lofted by the V tial navigation. We thought we could, was manageable for intercontinental bal- booster, the Eve reading from but these were undemonstrated listic missiles (ICBM), whose rocket the Bible book of Genesis, and the skills.1 motors burn only for a few minutes. And famous photograph showing a insofar as the Earth’s gravity field was blue Earth rising above a barren and for- When President John F. Kennedy well-mapped, it was possible to factor out bidding lunar . challenged the nation to send a crew to gravity for missiles that remained within The mission was a triumph of long- the Moon and back before the end of the the Earth’s gravitational field. Neither distance space navigation. The flight plan decade, navigation was among the held true for Apollo, which had to travel called for the astronauts to arrive at the biggest of unknowns. At the beginning of for several days from Earth to the Moon, Moon and establish an orbit that swooped the , it was by no means clear that and which on arriving at the Moon, to an altitude of only about 110 kilome- navigating to the Moon would even be entered a gravitational field that was ters (60 nautical miles, as NASA pre- possible. The mission, poorly understood. ferred to measure it) above the lunar sur- launched in March 1959, indicated the Engineers had considered these face, after a journey of nearly 400,000 scope of the problem. Its goal was to send problems and had some experience in kilometers (km). They achieved that a 30 kilogram (kg) probe close enough to dealing with them. For the 1957 study nearly perfectly. That was no small feat, the Moon to measure that body’s radia- done for the U.S. Air Force, the as the astronauts were well aware. If their tion field, if any. The probe achieved Instrumentation Lab proposed that the velocity was a few percent too low, they enough velocity to escape Earth orbit, but approximately 150 kg robotic Mars probe would have crashed into the Moon on its due to a timing error in the cutoff of the carry “a space to make periodic far side, out of contact with controllers booster, it passed by the Moon at a dis- navigation angle measurements between back on Earth. A few percent too fast, and tance of about 27,000 km, beyond the pairs of celestial objects: the Sun, the near they would swing into an erratic orbit, range of its onboard sensors. planets, and selected stars.” Also in the unable to get back to Earth. In an inter- One of the first contracts NASA let 1950s, the Northrop Aircraft Corporation view conducted in 2001 by Steven E. in preparation for a lunar voyage was to developed a long-range navigation sys- Ambrose and Douglas Brinkley, Neil the MIT (Massachusetts Institute of tem for its robotic “SNARK” guided mis- Armstrong noted that as he and his fellow Technology) Instrumentation Labor- sile, an air-breathing, atmospheric astronauts were being introduced to the atory, under the direction of Charles Stark weapon that could navigate across the basics of a mission to the Moon, the nav- Draper, for a navigation system. The Atlantic Ocean to targets in the Soviet igation problem was one of the biggest Instrumentation Lab had an acknowl- Union using an automatic star tracker. concerns that he had: edged expertise in inertial guidance tech- The Instrumentation Laboratory had niques, having already developed inertial developed another inertial system that Well I suppose that everyone navigation systems for aircraft and sub- operated for long periods of time, with would have concerns, but I don’t marines. For space, the lab had done corrections for gyroscopic drift. That was know that they’d all be the same. some preliminary work on a Mars probe, for ballistic missile submarines, which People would worry about different never flown, for the Air Force Ballistic remained submerged and hidden as much

Q U E S T 17:4 2010 8 as possible, thus precluding periodic sightings on stars, as sur- face ships might navigate. For these submarines, MIT designed a system called SINS: Submarine Inertial Navigation System. Because the SINS gyros would drift over time, the Navy devel- oped the TRANSIT system, which allowed the sub to get a fix on its position by a brief ascent and deployment of an antenna. No optical sighting of stars was necessary. TRANSIT was a radio, not optical system. It was one of the ancestors of today’s satellite-based geolocation devices, although its method of providing a fix, by measuring Doppler shift, did not form the basis for the Global Positioning System and other modern satel- lite navigation systems.

Space Navigation, before Apollo To return to Apollo: what else was known about naviga- tion in space in 1961, at the time the contract with MIT was signed? The theory of space navigation for piloted began with the same individual who pioneered in adapting sea- faring navigation for aeronautical use: Philip Van Horn Weems (1889–1979). A graduate of the U.S. Naval Academy in 1912, “Earth Path Indicator,” . Weems worked with Charles A. Lindbergh to develop methods Credit: National Air & Space Museum of that Lindbergh and his wife, , applied in their charting of trans-Pacific air required, but Weems proposed, as he had done successfully for routes. He is perhaps best known for his innovations in modify- air navigation, to precompute solutions in advance and provide ing chronometers, , and other marine navigation devices the with that data as printed tables or graphs. Weems and techniques for use onboard aircraft: innovations that saw was reluctant to employ an onboard electronic computer as at wide use among U.S. air forces during World War II. that time, circa 1961, such devices were neither reliable nor Weems retired twice from the Navy, but as the compact. (The SNARK guidance system, mentioned above, did began in the aftermath of Sputnik, he was recalled to active duty have onboard computational ability, though it did not carry a at the rank of captain, and tasked with developing a course on digital computer. The device used vacuum tubes.) space navigation for the U. S. Naval Academy.2 The text that Among the navigational aids mentioned by Weems was an accompanies that course, the Space Navigation Handbook, was electromechanical “Position Finder” designed by Edwin Collen the work of many collaborators, but its overall tone, and much of the Kearfott Division of General Precision. (Collen called it of the writing, is probably due to Weems. The course itself, a an “Astronavigator,” and that term will be used in this essay.) four-week course at the graduate level, was first convened in the The device contained a small globe, a transparent hemispherical summer of 1961, a few months after cosmonaut star chart that depicted stars as dots of luminescent paint, and an Yuri Gagarin’s flight marked the of human exploration of ingenious system of lenses and mirrors that allowed the astro- space. Weems developed aeronautical navigation as an exten- naut to superimpose his observations of Earth and star field with sion of what seamen had done, taking into account the airplane’s those of the small globe and painted star field. When the two third dimension of altitude, its faster speeds, and other factors were aligned, the astronaut could determine a position in space that precluded a simple extrapolation of existing techniques. For and altitude above Earth. Collen proposed that Apollo astronauts his first attempt at developing a theory of space navigation, carry such a device as an emergency backup to the main Apollo Weems did the same, starting with what was known about air- guidance and navigation system. NASA rejected the proposal, craft navigation and extending it into space. although it did develop a set of emergency procedures, some of The techniques proposed in that course were quite differ- which were used during the Apollo XIII mission.3 For the early ent from those proposed by the MIT Instrumentation Lab for the Soyuz missions, Soviet cosmonauts carried a similar device Mars probe, or by Northrop for SNARK. Nor were they used for onboard to help them locate their position in orbit.4 Apollo missions, even though they were centered on the astro- For Project Mercury, NASA adopted an even simpler naut’s making observations of Earth, Moon, and stars from the mechanical navigator, intended to assist the astronaut in return- spacecraft. One principle he developed was to determine ing to Earth. It consisted of a small globe rotated by mechanical absolute distance from the Earth (or Moon) by measuring the clockwork, with an icon depicting the Mercury capsule suspend- size of the observed disk, and to further determine the space- ed above it. On achieving a stable orbit, the astronaut would set craft’s position in space by observing the star field behind the the capsule’s inclination and period of the orbit, as radioed up Earth of the Moon during such an observation. For trajectories from the ground. The globe rotated under the icon to follow the close to the Earth, the spacecraft’s altitude could be inferred by rotation of the Earth, and another mechanism adjusted the orbit observing the amount of Earth’s curvature. No inertial devices to account for the precession of the capsule’s orbital plane. Thus were proposed. Some amount of onboard computation was the astronaut could follow a path as the icon passed over the Q U E S T 17:4 2010 9 gy made earlier in that decade, most notably Apollo’s use of integrated cir- cuits in its guidance computer. But in some cases the Apollo VIII mission car- ried with it some of the legacy of deci- sions made earlier in the decade based on conditions that had changed.

The MIT Instrumentation Labora- tory’s Approach Weems’s work laid a foundation for thinking about navigation in space at a time when only a few steps had been taken into that realm. By the late summer of 1961, when NASA awarded the first contract to the MIT Instrumentation Lab for Apollo navigation and control, a dif- ferent approach to navigation emerged.7 Central to Weems’s approach was the determination of one’s distance from the SNARK guidance system, with automatic star-tracker. Earth by measuring the angle of the Credit: National Air & Space Museum Earth’s disc as seen from the spacecraft. The Instrumentation Lab’s approach was globe. carried this “Earth based clocks in use in the 1950s. Finally, more traditional: to measure the angle Path Indicator” on the Friendship 7 cap- with each Mercury and Gemini flight, between the Earth’s horizon and a star or 8 sule in February 1962, but during that NASA gained valuable data on the per- between two stars. These techniques flight Glenn’s naked-eye observations of formance of navigation techniques by were initially developed under an Air the Earth were so good that NASA found real-world experience. This was true Force contract for a robotic mission to the device redundant. It was carried throughout the decade but was especially Mars, never flown. After 1958, the new onboard the following Mercury flight, pronounced as the Gemini program tran- civilian agency NASA took over plan- piloted by M. Scott Carpenter in May sitioned to Apollo. ning for deep space missions from the 1962, and not used again.5 With President Kennedy’s end of Air Force. In November 1959, NASA These examples of early space the decade goal rapidly approaching, directed the Instrumentation Lab to work navigation devices illustrate an important Apollo engineers had to make decisions with the Jet Propulsion Laboratory in point: the early 1960s was a time of enor- about the craft’s design and “freeze” it Pasadena, , to plan deep space, mous advances in the science and tech- even as, for example, the Gemini XI mis- robotic missions. But according to David nology of spaceflight, including naviga- sion, in September 1966, set an altitude Hoag of the Instrumentation Lab, the two tion. In those few years, engineers record by coupling a Gemini spacecraft laboratories diverged in their approaches improved gyroscope technology and to an Agena and boosting its orbit to the to navigation; with JPL focusing less on reduced drift. The mathematics of celes- edges of cislunar space. Thus, although onboard techniques and more on navigat- tial mechanics, already well developed the Gemini inertial platform used four ing by precise radiometric techniques 9 since the time of Newton, was directed gimbals, Apollo used only three, a sim- from the ground. This divergence would toward practical spacecraft mission plan- pler system that however could lead to a reappear during the human Apollo mis- ning. Computers made a transition from condition known as “gimbal lock.”6 sions out of Earth orbit, as we shall see. vacuum tubes to solid state and became Under normal conditions the crew could The approach adopted by the more powerful and more reliable. avoid gimbal lock, although in the Apollo Instrumentation Laboratory is best Ground-based computers remained XIII mission, gimbal lock was a persist- described as an inertial system, periodi- large, but the development of high-level ent and recurring threat. Note that astro- cally corrected by star sightings. The programming languages like FORTRAN naut James , who flew on both inertial component was based on the made them more capable of handling Apollo VIII and XIII, was also a veteran Lab’s work on the guidance system for complex mathematical problems. Large- of two Gemini missions: Gemini VII and the Polaris submarine-launched ballistic diameter radio antennas were placed into XII; thus he had extensive experience missile, which did not need stellar cor- service and provided an ability to track with both designs, and clearly favored rection, as it was designed to operate for spacecraft into deep space. Atomic the four-gimbal system. In most a brief period of time. For Apollo, the clocks were developed that were many instances, however, Apollo was able to position given by the inertial system times more accurate than the quartz- take advantage of advances in technolo- would be corrected, perhaps twice a day,

Q U E S T 17:4 2010 10 by sightings on the Earth, Moon, or the President Kennedy grew more stars. For that purpose Apollo carried a urgent. Like many of the sys- sextant, manufactured by the Kollsman tems that made up Apollo, the Instrument Corporation, which operated development of the guidance like a traditional maritime sextant in that computer was not easy. In par- the astronaut would adjust the device ticular, the software being writ- until the image of a known star was ten for the missions kept grow- aligned with the image of, say, the Earth’s ing, overwhelming the memo- or Moon’s horizon. That angular datum ry capacity of the computer. was then fed into the Apollo Guidance Many of the accounts of the Computer (AGC), which computed the development of the system spacecraft’s position. The sextant did not focus on the hardware, correct- look like a classic marine or even aircraft ly emphasizing the break- sextant, but that term was appropriate. It through of using the newly had two telescopes: one at 28 power and developed integrated circuit, the other at unity power but with a wider and the heroic efforts made to field of view. The optics were also ensure reliability, while avoid- mechanically constrained in their move- ing the complexity and weight ment because of the need for the tele- of having redundant hardware scopes to penetrate the pressure hull.10 (as the IBM-designed Launch The Instrumentation Lab devel- Vehicle Digital Computer on Collen “Astronavigator” oped a basic technique to correct for drift. the had).12 It turned out that a Credit: National Air & Space Museum The astronauts would key in the coded pacing element was the writing and test- number of a given star, and the computer ing of the software, especially as the Regardless of that decision, the would then orient the spacecraft’s optics AGC was tasked not only with naviga- need for onboard navigation ability so that the star was centered in the eye- tion but also with controlling the service remained, at least to navigate if commu- piece of the telescope. Depending on module rocket motor engines digitally nications with Earth-based stations were how much the inertial system had drifted, (the Saturn V’s rockets were controlled lost or degraded. There were also two the star would be found a short distance by a separate, analog computer). major components of the Apollo mis- away from the crosshairs. The astronaut The crisis came to a head in May sions that had to be controlled onboard. would center the image of the star, press 1966, at a meeting held at MIT and The first was to reduce the velocity of the a button, and the computer would note chaired by Howard W. (“Bill”) Tindall of Apollo configuration to enter into a sta- the amount of drift. The process could be NASA.13 As a result of this and other ble orbit around the Moon. The physics repeated against a second star if needed following meetings, a number of tasks of orbital transfer dictated that this be to realign the gyros. for the AGC were eliminated, to save performed by a carefully controlled burn Thus, although the Apollo naviga- memory. And the question of providing a of the ’s engine behind the tion system was far removed from what redundancy in the event of a computer Moon, out of contact with Earth. As men- Weems had envisioned a few years earli- failure was resolved by designating tioned above, the resulting orbit was to er, it replicated nautical techniques in use ground-based navigation techniques as have a pericynthion (the lunar equivalent for centuries. It used a sextant. Both the back-up to the onboard computer. of perigee for Earth orbit) of approxi- sailors and Apollo astronauts carried star This finessed the on-going debate about mately 60 nautical miles, so this maneu- charts. Sailors carried books of mathe- whether to have the computer repairable ver had to be executed correctly. The sec- matical tables to assist them in convert- inflight by the crew, who would be pro- ond function was the landing itself. The ing observations into latitude and longi- vided with spare modules that they could time delay of radio signals from Earth to tude; Apollo astronauts carried similar install in the machine. Prior experiences, the Moon is not much, on the order of a mathematical data, in this case precom- especially Gordon Cooper’s Mercury few seconds, but that was too long to puted and stored in the onboard comput- MA-9 flight of May 1963, indicated that entertain any notion of controlling the er. One should not push the analogy too making repairs to onboard electronic landing from Earth. Strictly speaking, far. Apollo was fundamentally an inertial, equipment created as many problems as this second function was a control, not a not a celestial system. Still, the notion of it might solve.14 Apollo would fly with navigation problem, but the absolute deep space as “this new ocean,” in only one AGC, sealed from the elements, requirement for onboard guidance and President Kennedy’s words, must have and designed to work as reliably as pos- control capability meant that onboard been strong.11 sible. MIT engineers’ memoirs of the navigation ability had to be present. Apollo missions stress this fact: the AGC Although the AGC did not use Ground-Based Tracking had to work right the first time, and it did, redundant circuits, as the Saturn V As the 1960s progressed, the end- throughout the program.15 Digital Computer used, of-the-decade deadline imposed by Q U E S T 17:4 2010 11 these sites were used for the Earth-orbital or not worth the cost. Although tensions phase of the mission. To track the astro- remained high between the United States nauts after leaving Earth orbit, only three and the Soviet Union during the 1960s, as sites with larger, 26-meter antennas, the decade progressed NASA came to spaced approximately 120° longitude believe that that the Soviet Union was not apart, were needed, although the 30-foot going to directly threaten U.S. human stations were also used. These were co- missions to the Moon, and the fear of located near the three Deep Space jamming subsided. Network (DSN) sites managed by the Among the technical advances JPL for unpiloted missions away from behind the decision was the development Earth orbit.20 The DSN and the Manned for Apollo of a unified system that com- Space Flight Network (MSFN) had dif- bined all tracking, communications, ferent missions but were complementary: uplinked commands, telemetry, and other each used 26-meter (85-foot) antennas, control signals onto one band of frequen- and links were established so that the cies in the S-Band, around 2,300 MHz.23 DSN system could back up MSFN if nec- This system was not in full operation at A Soviet Soyuz navigator essary. The three stations were at the time of the Apollo VIII mission, but Credit: National Air & Space Museum Goldstone, California; Honeysuckle enough was in use to allow more func- Creek, near Canberra, Australia (about 30 tions to be performed by Apollo’s avion- there were redundancies onboard. The km away from the DSN Tidbinbilla sta- ics without incurring a weight penalty or lunar module (LM) carried an identical tion); and Fresnedillas, Spain, west of excessively complicating the spacecraft’s AGC, which could be used as a backup Madrid.21 design. A second technical factor was the on the outward leg of the journey. It was The MSFN did not duplicate the rapid improvement in the accuracy of unavailable for the return to Earth, as the much larger DSN 70-meter (230-foot) timing devices, beginning with the intro- LM ascent stage was jettisoned before antennas, which the DSN required for tra- duction of rubidium and cesium-based that leg of the journey. (Note also that the jectories far beyond the orbit of the frequency standards in the late 1950s, Apollo VIII mission carried no LM on Moon. The links between the two sys- which replaced quartz oscillators that had either leg of its mission.) The LM also tems, however, allowed NASA to use been in use since before World War II. By carried an Abort Guidance System them, in addition to large radio astrono- the late-1960s, cesium-based clocks (AGS), intended to be used only to get my dishes, such as the 64-meter (210- attained a stability of nearly 1:1013 the LM’s ascent stage off the Moon and foot) antenna at Parkes, Australia, if defined as the stability of the standard into quickly in the event of an needed—a need that did arise on several during a 24-hour period. That was nearly emergency. The AGS contained a small occasions.22 100,000 times improved over quartz digital computer of its own.16 NASA The decision to use this network as oscillators (and a million times better rejected electromechanical backup the primary cislunar navigation system than mechanical chronometers).24 For devices, such as Edwin Collen’s was because of more than just the limita- the Apollo missions, NASA’s MSFN Astronavigator, but astronauts could nav- tions of the onboard AGC. One factor used a rubidium standard, not as accurate igate using sightings out the windows, was political: by the mid 1960s, the fear as cesium but stable enough to give very with timing supplied by their mechanical that the Soviet Union would jam or other- accurate fixes.25 The best fixes were analog wristwatches. Nearly all of these wise interfere with communications obtained using the 26-meter antennas on procedures, including the use of the LM’s between astronauts and the ground during the ground, and the high-gain, S-band guidance computer, were used to bring a lunar mission abated. The antenna located on the Apollo service the Apollo XIII astronauts home safely. 17 was still on, and classified information module, with its distinctive array of four The ultimate outcome of the meet- about Soviet progress was communicated small parabolic dishes pointed toward ings between NASA and the at least to James Webb, NASA adminis- Earth. A fix could also be obtained using Instrumentation Laboratory was that trator, at the time. The decade began with the lower-power, omnidirectional anten- ground-based navigation techniques a sense that space would be a military na on the spacecraft, thus providing an would play a greater role.18 By 1966, theater much like air: with orbiting additional layer of redundancy.26 with the transition from Gemini to Apollo bombers poised to deliver bombs to tar- The early 1960s also was a time of fully underway, the policy went beyond gets on Earth, with interceptors shooting advances in radio communications and that: “The primary navigation system in enemy spacecraft, and so on. That did not signal processing, which further con- cislunar space is the ground system materialize. ICBMs travel through space tributed to meeting President Kennedy’s [emphasis added].”19 By that year, on their way to a target, and space did challenge. Around the time of Sputnik, NASA had established a world-circling become a theater for military reconnais- Eberhardt Rechtin of JPL was working network of nine-meter (30-foot) antennas sance and signals intelligence, but for on jamming, and defense from jamming, for tracking, communications, and con- military activities the human presence of signals for guided missiles. In the trol at sites around the globe. For Apollo, was found to be unnecessary, impractical, course of that work he turned to the clas- Q U E S T 17:4 2010 12 sic mathematical work done by Norbert Wiener during World War II on the extraction of a meaningful signal in the presence of noise. Using that as a starting point, Rechtin and his colleagues came up with techniques that would allow one to track spacecraft with great accuracy from the ground, despite the weight and size restrictions that made it difficult for spacecraft to carry high-powered trans- mitters or large parabolic dishes onboard. One of the outcomes was the develop- ment of the Phase Lock Loop (PLL), which is now the standard method of reception employed by cell phones, GPS receivers, car radios, broadcast and satel- lite televisions, et cetera. In an interview conducted for the IEEE (Institute of Electrical and Electronics Engineers) in 1995, Rechtin recounted how experts told him that it would be impossible to receive James Lovell, sighting through the Apollo sextant, Apollo VIII. His right hand hovers over meaningful information from deep space, a button, which marks the sextant’s position as it is centered over a known star. On the as the weak signals would be swamped upper right is the ’s Display Keyboard (DSKY), one of two in by background noise. The PLL used a the Command Module. Credit: NASA narrow band receiver, which tracked the frequency of the transmitter even as that frequency shifted due to Doppler the Doppler shift of the S-band frequency because of the geometry of the Apollo’s effects.27 Rechtin was also among those gave the radial component of the space- trajectory as it orbited the Moon; perhaps who developed the concept of using a craft’s velocity relative to the ground sta- also to the irregularities of the Moon’s 30 sequence of “pseudo-random” num- tion. By combining readings from differ- gravitational field. bers—a sequence of digits that appeared ent ground stations, plus similar readings These techniques determined the to be random but that were known and taken during a span of time, the space- position of spacecraft beyond Earth’s specified in advance, to carry informa- craft’s velocity and position could be orbit with great accuracy. Two decades determined in all axes. later, the Global Positioning System tion. This, too, was an outgrowth of It was the precision of the Doppler (GPS) would use the same techniques Norbert Wiener’s work on the extraction measurements that tipped the balance in “inside-out” to determine the position of of signals from noise, and would be favor of ground techniques. By an ingen- a person or object on Earth. GPS uses developed by others into what has ious use of the telemetry codes, com- pseudo-random codes, and measures dis- become known as “spread-spectrum” bined with using more than one ground tance by measuring the time it takes a sig- communications in common use today. station to track the spacecraft and the nal to get to a receiver from space. But The ground-based navigation sys- rubidium frequency standard, NASA was the GPS system has the atomic clocks in tem used for Apollo employed these tech- able to determine range to around 30 space, not on the ground. And GPS niques. A pseudo-random code was sent meters, and velocity to within 0.2 feet per knows the position of the in to the spacecraft over the unified S-band. second. space, not the receiver on the ground, to It was received, and retransmitted back to Simulations done before the Apollo great accuracy. Earth on a slightly different frequency VIII mission showed that tracking by the 28 Many histories of the Apollo pro- also within the S-band. The time it ground-based MSFN was more accurate gram have focused on the remarkable took for the signal to go to and from the during the initial phase of a mission, until capabilities of the AGC, with its novel spacecraft, subtracting out the known about 35 hours after Trans Lunar interactive programming ability, its relia- times for the signals to travel within the Injection (TLI), when the spacecraft was bility, and its pioneering use of integrated equipment, divided by two, gave the about halfway between Earth and the circuits. Fewer have looked at the com- absolute distance of the spacecraft from Moon. From that point until the craft 29 puters used on the ground to support the the dish, to within 2 meters. The angle entered lunar orbit, the onboard system mission. NASA’s computing facilities of the antenna as it focused on the space- was slightly more accurate. MSFN-based grew out of a Naval Research Laboratory craft gave further information on the tracking was also more accurate in deter- facility on Pennsylvania Avenue in craft’s position, to a few milliradians mining position and velocity as the Washington, within sight of the Capitol. (about 1/10°). Finally, a measurement of spacecraft orbited the Moon. That was With the establishment of NASA in 1958,

Q U E S T 17:4 2010 13 Major units of the CM Guidance, Navigation, and Control System. Note the two DSKYs: one on the main panel, the other next to the scanning telescope and sextant. Credit: Laboratories

the Space Computing Center moved to could operate this way. By the mid- input/output functions), it not only served Greenbelt, Maryland, at the Goddard 1960s, NASA–Goddard continued to NASA well but also was offered as a Space Flight Center. Beginning in 1960, manage space communications, while product by IBM to other customers. Goddard computers, primarily IBM navigation and trajectory analysis were Creating an operating system for a (International Business Machines) main- transferred to the Real Time Computation machine as complex as the System/360 frames, calculated trajectories and orbits Center (RTCC) at the Manned was perhaps one of the most challenging for robotic and early human flights. Spaceflight Center (MSC) in . tasks in all of computer programming. NASA was one of IBM’s best customers, Initially the RTCC used a set of IBM The fact that NASA was able to do this so and it was able to do what few other IBM 7090-series mainframes, which were well, while its main focus was getting users could do, namely make fundamen- replaced beginning in 1966 by IBM’s human beings to the Moon, is testimony tal modifications to the systems IBM third-generation System/360 comput- to the space agency’s talent.32 These sys- supplied. IBM leased, not sold its com- ers.31 The term “third generation” tems created an effervescent atmosphere puters, and it did not allow its customers implied that they used integrated circuits in Houston, as mathematicians combined that freedom. In a typical mainframe (IC). The System/360 used a hybrid cir- centuries-old equations of celestial installation, programs were keypunched, cuit called “Solid Logic Technology,” mechanics developed by Isaac Newton, transferred from decks of punched cards which combined a number of discrete Pierre-Simon Laplace, Carl Friedrich to tape, and the tapes ran through the components onto a small ceramic sub- Gauss, and others, with new techniques computer to give an answer in printed strate. (The AGC circuits used silicon and tailored for space missions and taking full form. IBM made an exception to that rule were a direct ancestor of the ICs in use advantage of the number-crunching abil- for NASA, which needed to be able to today). The new-generation IBM main- ity of IBM’s hardware. run the computers in “real-time”: to enter frames were a large advance in the state in data directly and receive results as of computing art. They not only had Apollo VIII soon as the computer could calculate faster processing speeds and more mem- These concepts and simulations them. In other words, NASA modified ory, they also came with more sophisti- were put to the test on the morning of 21 the IBM machines to operate like a mod- cated software. Programmers at MSC December 1968, with the launch of a ern personal computer, even if the developed a customized operating sys- Saturn V rocket, carrying a crew of three machine cost a million dollars and tem that allowed Houston controllers to astronauts, from Pad 39A of the Kennedy required an air-conditioned room of its operate the computers in both batch and Space Center in Florida. About 38 min- own . real-time mode. Called HASP (Houston utes after launch, and while still in low- For Project Mercury, NASA creat- Automatic Spooling Priority; SPOOL Earth orbit, command module Pilot ed a “Mercury Monitor” system that was itself an acronym related to James A. Lovell Jr. jettisoned the protec-

Q U E S T 17:4 2010 14 Apollo Guidance Computer DSKY on the main panel, during a ground simulation. Credit: NASA

tive covers from the command module optics, in preparation for a preliminary alignment of the spacecraft’s inertial plat- form.33 When he looked through the tel- escope, he saw a field of bright particles that made it hard to locate stars. The par- ticles were probably small pieces of debris that came off the spacecraft when the covers were ejected. Later on during the mission there would be other sources of highly-reflective particles that would complicate the procedure. The crew also found it necessary to dim the cabin lights so that Lovell’s eyes could identify objects in space. It took some time, about 15 minutes, to sort out the procedures, but Lovell was able to locate two stars, center them in the optics, and command the onboard computer to realign the gyros.34 Lovell’s readings, plus the computer’s calculations, aligned the gyros to within .01° of what ground-based tracking indi- cated. “Pretty good for a beginner here,” crew to take readings on stars and check ed to ignore the readings. At this point a remarked lunar module Pilot William A. the Inertial Measurement Unit (IMU) second complication arose: the mission .35 alignment shortly after separation, but the plan called for the crew to rotate the com- The real test of the system came crew found that the third stage was flying mand service modules (CSM) slowly, to later, after the crew left Earth orbit. After in formation with them a little closer than even out the effect of solar radiation the launch and subsequent injection into expected, and they spent a bit of time (called “Passive Thermal Control,” better Earth orbit were determined to be suc- ensuring that there would not be a colli- known as “barbecue mode”). That had to cessful, Michael Collins, from his con- sion. That put them behind schedule, but be postponed, as it would conflict with sole at Mission Control in Houston, gave with the Moon several days away, it was the need to hold the CSM to a precise atti- the crew the go ahead to restart the not a critical delay. At about five hours tude to locate a star. 38 Saturn’s third stage and send the astro- into the mission, ground controllers Eventually all of this was worked nauts to the Moon. Collins gave the mes- directed the third stage to dump its resid- out. The crew managed to remove the sage “Apollo VIII, you are go for TLI ual propellants and other fluids, to pre- bias from the optics, find stars, and take [trans-lunar injection]” at about two vent a possible explosion. That had the good readings. Working with ground con- hours and 27 minutes after launch. At two additional effect of pushing the stage trollers, they learned how to roll the hours, 50 minutes, the third stage of the away, although that had to be carefully spacecraft for Passive Thermal Control, Saturn V fired again for about five min- choreographed. The procedure, however, and stop it when necessary to point the utes, which increased its velocity, and the dumped a lot of small particles into the telescope at a target. By about 12 hours command and service modules attached region around it, and as the Sun reflected into the mission, the onboard navigation to it, enough to reach the vicinity of the off them, the astronauts once again had calculations, carried out by the AGC, Moon.36 trouble distinguishing between the parti- were in close agreement with the ground- As might be expected for Apollo cles and stars. tracking data. All indications showed that VIII, the crew and mission controllers in Partly for that reason, a procedure when the astronauts arrived at the Moon, Houston had to adapt and modify their to calibrate the Apollo optics was they would skim above its surface--but plans as the mission proceeded. The TLI scrapped, and the crew proceeded direct- not hit it—and thus be able to fire the burn was executed perfectly, as was the ly to take sightings and feed that informa- service module’s engine to put them into separation of the Apollo command and tion to the IMU.37 The resulting readings a safe and stable orbit around the Moon. service modules from the Saturn V third were way off what the ground tracking As the crew got closer to the Moon, stage. The schedule then called for the had indicated. Mission controllers decid- their navigation readings got progressive- Q U E S T 17:4 2010 15 Comparison of ground-based (MSFN) and on-board (AGC) navigation accuracy at var- ious phases of a mission, projected in sim- ulations, 1966. (Ref: Proceedings, Apollo Lunar Landing Symposium, June 25-27, 1966, Houston, Texas, NASA, NASA-S-65- 10006; fig. 24. Credit: NASA

work.41 This test of the SPS engine was critical, on which the success of the mis- sion, and with it President Kennedy’s goal, depended. Reading between the lines of the conversations between the crew and Mission Control, one senses the tension that everyone was feeling. There was none of the good-natured banter as the crew prepared for this burn. Before the mid-course correction, it was neces- sary to get as accurate a fix on the space- craft’s position as possible, and the crew ly easier. Early in the journey, sunlight because Jim probably won’t even be able methodically took readings and entered reflected from the Earth washed out some to wear his comm. carrier [his communi- commands into the computer. They of the sightings through the telescope; cations headset] anymore, but that last set found, to their relief, that the resulting this diminished as the Earth receded. The of marks put your state vector right on top navigational fix from the onboard system 40 spacecraft continued to be surrounded by of the MSFN state vector.” This banter was in close agreement with the state stray particles, including ice crystals after between the crew and Mission Control vector computed on the ground. Now a urine dump, but it became easier to dis- reflected the fact that the Apollo VIII mis- came a critical decision for mid-course tinguish between these and the stars. Also sion, which began with so many correction: which to use? as the Earth receded and as Lovell gained unknowns and risks, was turning out to NASA decided that the tracking practice, he was more consistent in sight- be a huge success. It met all its objectives from the ground would take precedence. ing the Earth’s horizon, which was indis- and gave NASA confidence that At about 10 hours into the mission, and tinct because of its atmosphere. A reading President Kennedy’s goal to land a man an hour before the critical mid-course of the angle between the Earth’s horizon on the Moon would in fact be met. correction, Frank asked Mission and a star, taken at 17 hours, 53 minutes, However, there is more to this Control if it was time to realign the into the mission, matched perfectly with story, and to understand that we go back onboard platform. From his console in what the onboard computer indicated. to an exchange between the crew and Houston, Capsule Communicator Lovell exclaimed, “How’s that, sports Mission Control at about nine hours into Mattingly said, “That’s negative, Apollo fans? All balls.” To which Anders replied, the mission, as Apollo VIII was preparing VIII. We would like to update things first, “As soon as you’ve got an audience, you for its first mid-course correction. The and we’re going to give you an LM state do great.” (“All balls” was the astronaut’s spacecraft was on a very good course, but vector and then an external Delta-V.”42 way of saying the computer DSKY— mission planners decided to allow it What that meant was that NASA would data storage and keyboard—was display- deliberately to drift a little farther than transmit the craft’s state vector—the set ing all zeroes, that is, no error.)39 On the necessary before making a correction, so of numbers describing its position and return journey, the onboard MIT naviga- that the mid-course correction could be velocity—from ground-based, or exter- tion system continued to work perfectly. made with the service module’s SPS nal, data taken from its tracking network, The mid-course corrections were mini- (service propulsion system) engine, and store that into a portion of the AGC’s mal, and the onboard navigation readings rather than the small thrusters that would memory location that was available matched those from MSFN exactly. Near have sufficed for a small correction. This because there was no LM on this mission. the end of the mission, around the time of allowed NASA to test the engine, which That state vector would be used as the the final mid-course correction, had to perform a critical maneuver basis to compute the mid-course correc- Astronaut Jerry Carr, on duty as capsule behind the Moon twice: once to get tion. Borman keyed in the “Verb” P00 communicator at Mission Control, joked Apollo VIII into lunar orbit, and again to into the computer, which essentially told that Lovell’s head was getting swollen bring it home. Because Apollo VIII was it to suspend all other program execution. because of the good job he was doing: “I not carrying a lunar module, there was no He then flipped a switch, located just to hate to tell you this Frank [Borman], redundancy; the SPS engine had to Q U E S T 17:4 2010 16 star charts. This was prepared especially for the position of the stars, Earth, Sun, and Moon at the time of the Apollo 11 launch, July 1969. Slightly different charts were prepared for Apollo VIII and would have been carried onboard. Note the two-digit numbers for many of the stars—these numbers were keyed in as a “noun” into the Apollo Guidance Computer’s DSKY. Credit: NASA the left of the navigation station’s DSKY, beings have had to navigate outside the Museum, catalog number A 1993-0078- from “Block” to “Accept”: allowing the close range of Earth since Apollo XVII in 000. computer to receive and store the data 1972, but one may assume that such mis- 4. One of the devices, used by cosmonaut being sent from Earth. sions will resume eventually. When they Vladimir Shatalov and carried on Soyuz 4 in For the rest of the Apollo VIII mis- do, how will the crew navigate? Since 1969, is on display at the National Air and sion, and for all Apollo missions there- 1972 we have seen remarkable achieve- Space Museum. after, ground navigation data took priori- ments with unpiloted deep space probes 5. During his tenure in the Senate, Glenn ty for all navigation maneuvers outside hitting very precise targets at the displayed an Earth Path Indicator (EPI) on a . Ground navigation took of outer planets, asteroids, and comets, credenza behind his desk. The EPI in the priority on all subsequent missions. It was beginning with the mission to National Air and Space Museum collections, especially critical during the Apollo XIII Venus and Mercury in 1974. But we may catalog number A 1972-1170-000, was mission of April 1970, when an explosion also assume that there have been taken from an earlier, robotic Mercury flight. cut off power to the CM, leading to the advances in onboard, stellar-inertial tech- 6. David Hoag, “The History of Apollo On- powering down of the CM’s guidance niques also. Whether the dream of board Guidance, Navigation, and Control,” computer. The crew—which coinciden- autonomous can be Ernst A. Steinhoff, editor, The Eagle Has tally included James Lovell—used the revived remains to be seen . Returned (American Astronautical Society, LM computer for guidance, but not for supplement to Advances in the navigation. Using a combination of About the Author Astronautical Sciences, vol. 43, 1976), 277. ground commands, plus onboard Paul E. Ceruzzi is curator of aerospace 7. David A. Mindell, Digital Apollo: Human mechanical aids and mechanical wrist- electronics and computing at the National and Machine in Spaceflight (Cambridge, watches, they were able to safely reenter Air and Space Museum in Washington, Massachusetts: MIT Press 2008), 106–107. DC. Dr. Ceruzzi attended Yale University the Earth’s atmosphere and land within a 8. Hoag, “The History of Apollo,” 270–300. few kilometers of the intended landing and the University of Kansas, from which he received a PhD in American studies in 9. Hoag, “The History of Apollo,” 271. point.43 1981. He is the author or co-author of The Apollo VIII astronauts realized 10. Although it was not difficult to maneuver several books on the history of computing the dream of autonomous human space the stack while coasting, during an observa- and related topics. travel, in which spacefarers followed in tion the crew had to suspend the so-called Passive Thermal Control, or “barbeque the footsteps of Captain Cook, or the mode”: the slow rotation of the craft to even Lewis and Clark expedition.44 Although Notes 1. Neil A. Armstrong, interview with Dr. out the heating of solar radiation. they did not land on the Moon, their Stephen E. Ambrose and Dr. Douglas 11. President Kennedy used this phrase at a accomplishment will stand for all time as Brinkley, Quest 10, no. 1 (2003): 6–45; one of the greatest in the annals of human speech at Rice University in Houston on 12 quote on 29–30. September 1962. Although not the case exploration. But we must also remember with Apollo, note that the that the dream lasted only about seven 2. [U.S. Navy], Space Navigation Handbook, NAVPERS 92988 (Washington, DC: U.S. orbiters were named after ships of discov- hours: from the time the crew was given Government Printing Office, 1962), i. ery. The Shuttle Enterprise was named after a “Go for TLI,” at three hours into the a fictional spacecraft from the television mission, to flipping the switch to 3. Edwin Collen, private communication to series Star Trek. But that Enterprise was “Accept” at ten hours into the mission. the author. Collen donated the prototype itself named after a ship. Astronavigator to the National Air and Space As of this writing, no human 12. Eldon C. Hall, Journey to the Moon: The Q U E S T 17:4 2010 17 http://www.ieeeghn.org/wiki/ .php/Oral-History: Eberhardt_Rechtin.html. Reprinted in Quest 15, no. 2 (2008): 20–39. 28. The downlink carrier frequency was 240/221, or about 1.09 times the uplink frequency. Kayton, Navigation, 320. 29. Kayton, Navigation, 320. 30. Duncan, “Apollo Navigation, Guidance, and Control,” Proceedings, figures 23, 24, and 25. 31. Paul Ceruzzi, A History of Modern Computing, second edition (Cambridge, Massachusetts: MIT Press, 2003), 122–124. 32. It was a task that IBM itself failed at least once in doing; see Frederick P. Brooks Jr., The Mythical Man-Month: Essays on Software Engineering (Reading, One of the NASA 26-meter MSFN antennas collocated with similar 26-meter, and much Massachusetts: Addison Wesley, 1975). larger 70-meter antennas of the Deep . The DSN was developed for 33. Flight Journal, transcript, uncrewed missions far beyond the . A link established between the DSN http://history.nasa.gov/ap08fj/index.htm, antennas and the Manned Space Flight network, and this link was pressed into service accessed 25 January 2010; 54–55. to obtain live television images of the Apollo 11 landing, as well as emergency communi- cations during the Apollo XIII missions. Credit: NASA 34. The command was “P-52”: a two-digit “verb” that the AGC interpreted to mean to realign the platform based on the star sight- History of the Apollo Guidance Program, 21. Tsiao, “Read You Loud and Clear!”, ings. AIAA, 1996 146–147. 35. Apollo 8, transcript, 67, Mission 13. Hoag, “The History of Apollo,” 290. 22. Live television during the Apollo 11 Elapsed Time (MET) 000:56. 14. Chris Kraft, Flight: My Life in Mission landing was received by the large antennas 36. Apollo 8 Flight Journal, http://history. Control (New York, Dutton, 2001), in California and Australia. .gov/ap08fj/index.htm accessed 25 182–183. 23. Myron Kayton, editor, Navigation: Land, January 2010, also Robert Godwin, editor, 15. Hoag, “The History of Apollo,” 284. Sea, Air, and Space (New York: IEEE Press, Apollo 8: The NASA Mission Reports 1990), 317–331. (Ontario, California: , 2000), 16. James Tomayko, Computers in 178–179. Spaceflight: The NASA Experience (New 24. William G. Melbourne, “Navigation York: Marcel Dekker, 1987), 59–60. between the Planets,” Kayton, editor, 37. Apollo 8, transcript, 158, MET 5:08 and Navigation, 337. The author notes that proj- following. 17. John L. Goodman, “ Guidance, ect Mercury kept time using the National Navigation, and Control Challenges,” Bureau of Standards radio station WWV, 38. Apollo 8, transcript, 158, MET 5:36. Proceedings AIAA Space 2009 Conference accurate to about one millisecond. 39. Apollo 8, transcript, 273, MET 17:53:25. and Exposition, Pasadena, California, September 2009. 25. Rubidium clocks were estimated to 40. Apollo 8, transcript, 1056, MET keep time to within one second in 30 years, 123:31:14. 18. “More Apollo Guidance Flexibility about 1,000 times less accurate than Sought,” Aviation Week and Space cesium. 41. Kraft, Flight, 298. According the Kraft, Technology (16 November 1964): 71–72. the decision was his, after some dissent 26. Duncan, “Apollo Navigation, Guidance, among other NASA controllers. 19. Robert C. Duncan, “Apollo Navigation, and Control,” Proceedings. During the Guidance and Control,” Houston, Texas: Apollo XIII mission, the hi-gain antenna, 42. Kraft, Flight, 200; MET 9:54:35. NASA, Manned Spaceflight Center, located on the damaged service module, 43. Goodman, “Apollo 13 Guidance.” Proceedings and Compilation of Papers, was unusable. The astronauts communicat- 44. Note that this did not in any way dimin- Apollo Lunar Landing Symposium, 25-27 ed with low-power transmitters located on ish the importance of the AGC for control June 1966, unpaginated. Emphasis in the the command and lunar modules, through functions: that is, putting the craft into the original. the 70-meter DSN antennae pressed into proper attitude, or guidance functions: that emergency service. 20. Sunny Tsiao, “Read You Loud and is, ensuring during a rocket burn that the Clear!”: The Story of NASA’s Spaceflight 27. Eberhardt Rechtin, interview with thrust vector is properly aligned with the Tracking and Data Network, (Washington, Frederik Nebeker, IEEE Center for the craft’s center of gravity as desired for a DC: NASA, 2008), NASA SP-2007-4232, History of Electrical Engineering, 23 given trajectory. 146. February 1995, accessed at

Q U E S T 17:4 2010 18