Uplink - Downlink: A History of the Deep Space Network 1957-1997, Mariner, Viking, Voyager, Galileo, Cassini Eras, DSN as a Scientific Instrument (NASA SP-2001-4227)
National Aeronautics and Space Administration (NASA), World Spaceflight News, Douglas J. Mudgway
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Uplink - Downlink: A History of the Deep Space Network - 1957-1997
NASA SP-2001-4227
Douglas J. Mudgway
The NASA History Series
National Aeronautics and Space Administration * Office of External Relations * Washington, DC 2001
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Chapter 2—The Mariner Era: 1961—1974
Chapter 3—The Viking Era: 1974—1978
Chapter 4—The Voyager Era: 1977—1986
Chapter 5—The Galileo Era: 1986—1996
Chapter 6—The Cassini Era: 1996—1997
Chapter 7—The Advance of Technology in the Deep Space Network
Chapter 8—The Deep Space Network as a Scientific Instrument
Chapter 9—The Deep Space Network as an Organization in Change
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ACKNOWLEDGMENTS
To have been part of the history recorded here is sufficient reason in itself to acknowledge my indebtedness to many colleagues in the DSN who supported and assisted me throughout my long and rewarding career at the Jet Propulsion Laboratory (JPL). That said, there remains the need to recognize the important contributions that were made to the writing of this history by many of those engineers and scientists, and by other persons less directly associated with my life in the Deep Space Network.
I must begin with Nicholas A. Renzetti, for it was he who brought me from Australia to the United States in 1962 to begin a career at the Deep Space Instrumentation Facility (DSIF), and it was he who, in 1996, after I had retired from JPL, stimulated my personal interest in producing a history of the Deep Space Network. When the project began to falter for lack of funding support, MacGregor Reid provided much needed encouragement, and Paul Westmoreland responded with the limited resources available to him to keep it going.
Later, when those resources expired, the project came to the attention of the NASA Chief Historian, Roger Launius. His encouragement, backed with adequate resources, moved the project rapidly forward to completion. Without his enthusiastic support it is unlikely this book would have been published. Along the way, I was ably assisted by Louise Alstork and other members of the NASA History Office staff.
As the work got under way, my access to historical documents, files, and photographs was eased immeasurably by the generous help of members of the JPL Archives and Records section, notably John Bluth, Elizabeth Moorthy, and Robin Morris.
At all times, Shirley Wolff of the Telecommunications and Missions Operations (TMO) Outreach Office was my lifeline to the daily pulse of the Network. I came to depend on her patience and energy for transmitting documents and other technical material provided, at my request, by various engineers and scientists associated with the Network. She, too, helped in bringing this project to life.
Last and longest, but by no means least, were the contributions in the form of interviews, discussions, technical briefings and materials, narrative reviews, and encouragement on various DSN-related topics that were provided by: Catherine Thornton on geodesy; Michael Klein on the search for extraterrestrial intelligence; Martin Slade on radar astronomy; James Hodder on network operations; Thomas Kuiper, Marvin Wick, and Pamela Wolken on radio astronomy; George Textor on Voyager; Leslie Deutsch on DSN telemetry for Galileo; Joseph Wackley for DSN systems; Joseph Statman on the Big Viterbi Decoder; Robert Wallace on 34-m antennas; Dan Bathker on microwaves; Robert Clauss on masers; Charles Stelzried on system noise temperature, Venus Balloon, and Giotto; Bob Preston and John Ovnick on orbiting VLBI; Fred McLaughlin on the 70-m antennas; Dale Wells on 70-m antenna maintenance; James Layland on coding and arraying; Patrick Beyer on Galileo; Dennis Enari on Ulysses and Mars Pathfinder; Marvin Traxler on Mars Observer; Allen Berman on Magellan; Robert White on 34-m antennas; Thomas Wynne for photographs; Ronald Gillette on Cassini; John McKinney on Mars Missions; Ed Massey on Voyager and Ulysses; Nick Fanelli and Joe Goodwin on the Earth orbiters and reimbursables; Bob Ryan and David Lozier (Ames Research Center) on the Pioneers; Michael Stewart on Magellan; Don Mischel, Tom Reid, Richard Mallis, and Robertson Stevens for early background material; George Schultz for an early draft; and finally, Olivia Tyler, Bobby Buckmaster, and Lynda McKinley for miscellaneous but nevertheless indispensable help.
The onerous task of reviewing the draft version of the book was undertaken by Larry Dumas, Michael Hooks, Roger Launius, MacGregor Reid, Gael Squibb, and Jose Urech. Their insightful comments and suggestions greatly enhanced the accuracy, consistency, and quality of the narrative.
The families of Nicholas Renzetti and William Merrick kindly provided background material on the personal lives of these two important figures in the history of the Deep Space Network. Their contributions are gratefully acknowledged.
Douglas J. Mudgway
Sonoma, California
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FOREWORD
From the very beginning of its association with NASA in 1958, the Jet Propulsion Laboratory (JPL) received its fair share of public recognition for its successes and failures in pursuing the exploration of deep space. It started with the Explorers, the first American satellites to orbit Earth. Later there came the Rangers, the first spacecraft to reach the surface of the Moon; the Mariner spacecraft, first to visit Venus and Mars; and the Voyagers that pushed the boundaries of deep space communication further out to Jupiter and Saturn, and eventually to Uranus and Neptune. There were other spacecraft that put landers, probes, or orbiters into planetary orbits or atmospheres, or onto planetary surfaces. There were probes whose mission was to explore the composition and dynamics of the interplanetary medium, and probes to observe the physics of the Sun. There were the huge missions, such as Viking to Mars, Galileo to Jupiter, and Cassini to Saturn, and there were small missions like Pathfinder to Mars and the New Millennium missions to asteroids and comets. There was also science that did not require a spacecraft for its experiments such as radio astronomy, radar astronomy, and the search for extraterrestrial intelligence.
The public accolades that were engendered by the bountiful science returned from all of these NASA projects were shared by NASA and the scientists whose exquisite instruments and innovative interpretation of the data produced the new knowledge reflected in their results. However, what the press conferences, news releases, and media coverage did not reveal was the incredibly complex infrastructure that made each of these marvelous deep space missions possible. This infrastructure, which had been built over the years at JPL, included the Deep Space Network (DSN), an essential, integral part of every mission. There was, in effect, a relationship between the planetary missions, the spacecraft that carried them out, and the Deep Space Network that enabled such missions to be planned in the first place.
Without the remarkable improvement in performance of the DSN, scientific missions to the distant planets would have been impossible. In 1964, when Mariner IV flew past Mars and took a few photographs, the limitation of the communication link meant that it took eight hours to return to Earth a single photograph from the Red Planet. By 1989, when Voyager observed Neptune, the DSN capability had increased so much that almost real-time video could be received from the much more distant planet, Neptune.
It is timely that, some 40 years after its inception, the Deep Space Network should be recognized for its remarkable litany of progress in radio communications over vast distances, thereby allowing planetary scientists to collect data from sites throughout the solar system. This book succeeds in bringing the history of the DSN forward for the attention of curious, generally informed, or technical specialist readers.
NASA is to be commended for commissioning this book as part of its History Series, for the NASA/JPL Deep Space Network is the world's largest and most advanced facility for tracking, navigating, and acquiring data from interplanetary spacecraft. Worldwide in scope, and international in concept, the DSN has supported not only NASA space missions, but also those of the space agencies of Japan, Germany, Russia, France, and Canada. Also, in concert with a NASA policy of international cooperation, the resources of the Network have been made available to support qualified enterprises from all nations.
In planning this book, New Zealand-born author Douglas Mudgway was faced with the formidable problem of making an extremely complex technology comprehensible to the curious or generally informed reader, while at the same time presenting for the specialist reader a historically accurate account of the advance of the technology that made possible more sophisticated planetary missions. The task was further complicated by the fact that the DSN was in a constant state of change, changes that were in consonance with the requirements of the space missions it was supporting.
For the former type of reader, Mudgway describes what the DSN actually did, within the framework of several overarching eras, each corresponding to a period of time during which a major NASA deep space mission dominated the public scene. The Mariner, Viking, Voyager, and Galileo Eras are examples. He provides an inside view of what it took to design, build, and operate those tenuous radio communication links between the controllers, engineers, and scientists at computer terminals at JPL and billion dollar spacecraft about to land on Mars, orbit around Jupiter, or fly through the rings of Saturn.
For the latter reader, the author provides an excellent review of the growth of the specialized technology that underlaid the remarkable expansion of Network capability that enabled the design of increasingly ambitious planetary missions. A comprehensive appendix provides help for the technical researcher.
The unique capabilities of the great antennas of the DSN, together with a generous NASA policy of making them available for non-NASA scientific research, attracted radio and radar astronomers from the United States and many other countries to the extent that the requests for observing time far exceeded the time that could be made available for these ground-based scientific purposes. Chapter 8, "The Deep Space Network as a Scientific Instrument," brings this important aspect of the work of the DSN to the attention of the reader, and provides a basic review of the published work that resulted.
Uplink-Downlink transforms the technical records of a major NASA facility, unique in the world, into a viable historical narrative covering 40 years of its critical involvement in the United States space program. The Deep Space Network emerges from this study not only as a complex, human-machine system of worldwide dimensions, but also, more convincingly, as a focus for the aspirations of the NASA scientists for ever-bigger science, and of the JPL engineers for ever-greater innovation and enterprise in navigating to distant targets and communicating at ever-greater data-rates, in spite of the fluctuations in available NASA funding for both, driven in some measure by the conflicting priorities of the piloted versus unpiloted programs within NASA itself.
Throughout the narrative we observe the interaction of these powerful currents, not from the lofty heights of a dispassionate historical observer, but from the eye-level of a dedicated participant, for the author's long career at JPL was played out at the vortex of these often conflicting currents of self-interest. As a consequence, the engineers, technicians, scientists, and managers at NASA, JPL, and its partner institutions in Spain and Australia, whose names appear from time to time in this book, are presented as real-life people contributing their various talents to the milieu in which they found themselves. It was the totality of these individual efforts, driven by a common inquiring interest in space and focused toward the common goal of its exploration, that produced the remarkable entity known throughout the world as the NASA Deep Space Network.
I am confident that this work will be a valuable addition to the documented history of the United States space program.
W H. Pickering
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INTRODUCTION
Surely there can be few people in this day and age who are not familiar with the television images of a NASA spacecraft blasting off from Cape Kennedy in a splendid gout of smoke, fire, and steam to a rendezvous with an asteroid, comet, or planet in some remote corner of the solar system. From the comfort of our living rooms we watch the launch rocket rise majestically off the pad, pass through the local cloud cover at the launch site and soon become an ever-diminishing point of light in the center of the television screen. Sometimes we observe the booster rockets burn out and drop away before the main rocket stage engines accelerate the vehicle and its delicate planetary payload out of sight. The Titan-Centaur launch of Voyager 1 was a typical example. What happened next? you may ask and, Where did that whole thing go?
Months or even years after a launch, an item on the evening television news covering a NASA press release about an impending rendezvous of a NASA spacecraft with Mars, Jupiter, Saturn, or an asteroid, followed by a video clip or the latest set of pictures from the planetary object, might have caught the attention of viewers. For certain, the pictures showed areas and details never before seen and will be said to have risen new questions about origins and development of the solar system, and possibilities for water, maybe even life, on the planet or one of its satellites. The Voyager 1 view of Jupiter as the spacecraft approached the planet is a prime example of science data return with high value for public interest.
There will be some with enquiring minds who may connect new scientific evidence with a spacecraft launch, recalled from the distant past and ask the question, How did "it" get from there to what I am seeing now?
Well, the answer depends on what is meant by "it," but an essential part of the answer is the subject of this book, namely, the Deep Space Network. To properly understand that part of the answer, some explanation of the nature of the problem and the terminology used to describe it will be necessary. The discussion that follows is directed to that end, to readers who have a general interest in the subject but whose field of professional interest lies elsewhere. For the expert reader, numerous technical references are provided throughout the book for followup on topics of specific interest.
Before beginning a discussion of radio communication with spacecraft traveling in deep space, we shall briefly review the environment of space and the motions of planetary spacecraft within that environment. This review and the discussion of space communications that follows it will explain various technical terms commonly used in describing the environment of space, the motions of Earth and spacecraft, and space communications. It is hoped that the general reader, who may not be familiar with many of these terms, will find this helpful in understanding the terminology used in the main chapters of the book.
The Solar System
NOTE: The following discussion makes extensive use of material contained in "The Basics of Space Flight," a learning document produced by the Jet Propulsion Laboratory for use in its spaceflight operations training program.
The solar system has been studied for religious or scientific reasons from the very earliest times. For most of that time, studies of the solar system have had to rely on indirect measurements of the various objects in the solar system such as the visible light emitted by, or reflected from the objects, or later, by the radio waves emitted by the Sun. However, with the emergence of space flight, instruments can be sent to many objects in the solar system to make direct measurements of their physical properties and dynamics, at close range. Data collected from such measurements have resulted in an unprecedented increase in our knowledge of the solar system.
Size and Composition
The solar system consists of an "average" star we call the Sun and the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It also includes numerous satellites of the planets, comets, asteroids, meteoroids and of course, the interplanetary medium. The planets, most of the satellites of the planets, and the asteroids, revolve around the Sun in the same direction, in nearly circular orbits. The Sun and the planets rotate on their individual axes. Except for Pluto, all the planets orbit the Sun in or near the same plane, called the plane of the ecliptic.
The most common unit of measurement for distances within the solar system is the astronomical unit (AU). One AU equals the mean distance of the Sun from Earth, about 150,000,000 km. Prompted by the need for a more accurate figure for spacecraft navigation purposes, the DSN refined the value of the AU in the 1960s using radar echoes from Venus. Distances within the solar system, the distance between a DSN antenna on Earth and a planetary spacecraft for instance, are often indicated in terms of the distance light travels in a unit of time at the speed of 300,000 km per second.
For example:
Light Time * Light Travels Approx. Distance
1 second * 300,000 km 0.75 Earth-Moon
1 minute * 18,000,000 km 0.125 AU
8.3 minutes * 150,000,000 km 1 AU, Earth-Sun
1 hour * 1,000,000,000 km 1.5x Sun-Jupiter
Although the Sun is characterized as a typical star, it dominates the gravitational field of the solar system; it contains 99.85 percent of the mass of the solar system. All the planets combined contain only 0.135 percent of the total mass with the satellites of the planets, asteroids, meteoroids and interplanetary medium making up the balance of 0.015 percent. Even though the planets account for only a small portion of the total mass of the solar system, they retain the greater part of the angular momentum of the solar system. This storehouse of energy can be utilized by interplanetary spacecraft to make so-called "gravity assist" changes in their interplanetary trajectories.
In terms of volume, nearly all of the solar system appears to be an empty void. Far from being just nothingness, however, this void comprises the interplanetary medium and includes various forms of energy and at least two material components: interplanetary dust and interplanetary gas. The dust consists of microscopic particles which can be measured by special instruments carried by interplanetary spacecraft. Interplanetary gas is a tenuous flow of gas and charged particles called plasma, which when streaming from the Sun is called solar wind. The solar wind can also be measured by interplanetary spacecraft. The point at which the solar wind meets the interstellar medium, that is the stellar wind streaming from other stars, is called the heliopause. This boundary, which marks the edge of the Sun's influence, is theorized to lie perhaps 100 AU from the Sun. At the time of this writing, the Voyager and Pioneer spacecraft were making their way toward this remote limit of the solar system. The magnetic field of the Sun extends outward into interplanetary space and it, too, can be measured by instruments carried on planetary spacecraft.
The Terrestrial Planets
The four terrestrial planets of the solar system—Mercury, Venus, Earth, and Mars— have a firm rocky surface similar to that of Earth. None of the terrestrial planets has rings. Earth has a layer of rapidly moving charged particles known as the Van Allen Belt, first detected by the JPL Explorer 1 space probe in 1958.
The Jovian Planets
The four Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are all much larger than the terrestrial planets and have gaseous natures like that of Jupiter. They all have satellites and rings, although the size and number of each varies considerably between them.
Inner and Outer Planets
Frequently, the planets whose orbits lie inside Earth's orbit—namely, Venus and Mercury—are referred to as the inner planets. Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are generally known as the outer planets.
Asteroids
Asteroids are rocky objects orbiting the Sun at a distance of about 2.7 AU, between the orbits of Mars and Jupiter, and moving in the same direction as the planets. They vary from the size of pebbles to objects measured in hundreds of kilometers. Some have orbits which cross the orbit of Earth from time to time. The DSN Solar System Radar at Goldstone is used to investigate the properties of asteroids with Earth crossing orbits.
Comets
Comets are believed to be composed of rocky material and water ice. Their highly elliptical orbits bring them very close to the Sun and swing them deeply into space often beyond the orbit of Pluto. Comet structures are diverse and very dynamic, but they all develop a cloud of diffuse material called a coma, that usually grows in size and brightness as the comet approaches the Sun. A small bright nucleus is usually visible in the middle of the coma. The coma and nucleus together constitute the head of the comet. As a comet approaches the sun it develops an enormous tail of luminous dust material that extends for millions of kilometers from the head away from the sun. It also develops a tail of charged particles and an envelope of hydrogen. Several comets have been investigated by planetary spacecraft flying close to or through the coma.
Earth and Its Reference Systems
Without a system of coordinates to consistently identify the positions of observers, planets, and interplanetary spacecraft, exploration of the solar system would not be possible. Because space is observed from an Earth platform, a system of Earth coordinates is required to establish the position of the observer. The locations of DSN antennas on Earth's surface are specified in terms of latitude and longitude.
To establish a coordinate system for the sky, the concept of a celestial sphere whose center is at the center of the Earth, is used. The celestial sphere has an imaginary radius larger than the distance to the farthest observable object in the sky, that is to say, it extends far beyond the limits of the solar system. The extended axis of Earth intersects the north and south poles of the celestial sphere. The direction of a spacecraft, planet, or star, or any other celestial object, can be specified in two dimensions on the inside of this sphere using a system of coordinates analogous to Earth's latitude and longitude system. Although the reference origins are different, the analogous terms latitude and longitude on the celestial sphere are declination (DEC, latitude) and right ascension (RA, longitude). When used in connection with a specific location such as a DSN antenna, at a particular time of day, the term RA is replaced by a different term called hour angle or HA.
A somewhat simpler system for describing the position of a distant spacecraft relative to a particular antenna and time of day uses the local horizon and true north as its references. Its measurements are azimuth (AZ), measured in degrees clockwise around the horizon from true north and elevation (EL), measured in degrees above the local horizontal datum. Optical telescopes, radio telescopes, and the DSN antennas are designed with mountings that make best advantage of either the HA-Dec or Az-El coordinate systems.
In a HA-Dec system the HA axis is parallel to Earth's axis of rotation. Thus an antenna built on a HA-Dec mount has the advantage that motion is required mostly in only one axis, HA, to track an object like a spacecraft, as Earth rotates. The disadvantage is that it requires an asymmetrical design, which is unsuited to the support of very heavy structures. By contrast, Az-El mounted antennas are basically symmetrical structures which can support heavy weights, but to track celestial objects they require driving in two axes, AZ and EL simultaneously. However, the advent of high speed computers has obviated the problems formerly associated with converting coordinates from one system to the other.
Earth Motions
Earth rotates on an axis inclined at 23.5 degrees to the plane of its orbit around the Sun. Its period of rotation is 24.0 hours mean solar time. This motion is of prime importance to the configuration of the Deep Space Network. Due to the rotation of Earth, a spacecraft moving away from Earth along its trajectory in space appears to an observer on Earth to set toward the west. To an observer further west, the spacecraft will appear to rise in the east and travel across the sky until, 8 to 10 hours later, it sets. The process repeats until 24 hours later the spacecraft appears, rising in the East for the first observer. It follows that three observers, each located 120 degrees of longitude apart on the surface of Earth, would have the spacecraft continuously in view as the view period passed from one to the other. This is the basic idea underlying the location of the three Deep Space Communication Complexes of the Network at Goldstone, California, near Madrid, Spain, and near Canberra, Australia. From these locations, approximately 120 degrees of longitude apart, their great antennas, some HA-Dec, others Az-El, are able to maintain continuous radio view of planetary spacecraft as Earth performs its daily rotations.
The ability of the DSN antennas to view the planetary spacecraft is not changed by the annual rotation of Earth around the Sun. It does, however, introduce a cyclic change to the distance between Earth and spacecraft depending on the orientation of the spacecraft trajectory relative to Earth's orbit.
Time Conventions
In addition to local time and Greenwich Mean Time, there are several other systems for measurement of time that are used for tracking planetary spacecraft in the Deep Space Network. Three of the most important are Universal Time Coordinated (UTC), Earth Received Time (ERT), and Round-Trip Light Time (RTLT).
UTC is a worldwide scientific standard of timekeeping based upon carefully maintained atomic clocks that are accurate to a few microseconds via the addition or subtraction of leap seconds as necessary at two opportunities every year. Its reference point lies on the Earth prime meridian at Greenwich, England. All spacecraft operations in the DSN are conducted on the basis of UTC.
The time (in UTC) at which a DSN tracking station observes an event associated with a planetary spacecraft is called the Earth Received Time (ERT). The time (in UTC) that a tracking station observed the loss of signal due to a spacecraft occultation by a planet would be an example of such an event.
The elapsed time that it takes a radio signal (traveling at the speed of light) to travel from a DSN station to a spacecraft or planetary body, and after retransmission by the spacecraft or reflection from the body, return to the tracking station, is known as the Round-Trip Light Time (RTLT). It is used by the DSN to measure spacecraft range and other navigational and scientific parameters. The RTLT from Earth to the Moon is about 3 seconds; to the Sun, about 17 minutes. In 1993, the RTLT for the Voyager 1 spacecraft was 14 hours, 13 minutes. By the end of 1998, this had increased to 20 hours, 18 minutes, making Voyager 1 the most distant spacecraft in the solar system.
Interplanetary Trajectories
A spacecraft sitting atop a launch vehicle at Cape Canaveral can, in one sense, be considered as already in orbit around the Sun by virtue of the motion of the Earth orbit around the Sun. To send such a spacecraft to an outer planet, Mars for example, the spacecraft's existing orbit must be adjusted to cause it to intercept the orbit of Mars at a single point. The portion of the new orbit between Earth and Mars is called the interplanetary trajectory.
To achieve such a trajectory, the spacecraft is lifted off the pad by the launch vehicle, rises above Earth's atmosphere, orients itself to the right attitude and is accelerated in the direction of Earth's orbit around the Sun to the extent that it becomes free of Earth's gravitational effect. The magnitude and direction of the accelerating force is carefully calculated to achieve a new orbit which will have an aphelion (farthest distance) equal to that of the orbit of Mars. After injection into its new orbit, the spacecraft simply "coasts" the rest of the way to its destination. Of course, to get to Mars itself, rather than just to Mars' orbit, the spacecraft must be inserted into its interplanetary trajectory at precisely the right time to reach the orbit of Mars at the same time that the planet itself reaches the point where the spacecraft will intercept the orbit of Mars. This calls for very precise timing indeed, and results in constrained opportunities for launch called "launch windows." A spacecraft interplanetary trajectory can be adjusted or trimmed, to a limited degree, by means of midcourse maneuvers which use the onboard thrusters to change the spacecraft speed and direction, or by means of a "gravity assist," which uses the gravitational force of a nearby planet for the same purpose.
The vital DSN operation of "initial acquisition" is performed during, or as close as possible to, the interplanetary trajectory injection maneuver right after launch. After that, spacecraft and mission controllers are entirely dependent upon the DSN for communications with the spacecraft throughout the life of the mission.
The foregoing paragraphs have provided a greatly simplified answer to the question of what happens to a spacecraft after it leaves the launch pad. They have also described the environment in which it will move for the rest of its life. An answer to the next question, How do spacecraft and mission controllers communicate with their spacecraft during its long passage through the environment of deep space? requires some understanding of the nature of deep space communications.
Deep Space Communications
The Deep Space Network makes use of electromagnetic radiation for communicating with interplanetary spacecraft. All planetary spacecraft are equipped with radio transmitters and receivers for sending signals to and receiving signals from the Earth-based antennas of the DSN. In addition to signals from spacecraft, DSN antennas and receivers are capable of detecting signals from natural emitters of electromagnetic radiation, such as the stars, the Sun, molecular clouds, and giant gas planets such as Jupiter. The "signals" from these sources appear as random noise to the sensitive receivers of the DSN, and their study and interpretation is the field of radio astronomy. The DSN supports many research projects in radio astronomy.
Bands and Frequencies
Electromagnetic radiation from the natural emitters combines with radiation from artificial sources to create a background level of electromagnetic noise from which the spacecraft signals must be detected. The ratio of the signal level to the noise level is known as the signal-to-noise ratio (SNR). SNR is one of most common terms used in the DSN to describe the quality of a communication link.
Electromagnetic radiation with frequencies between about 10 kHz and 100 GHz are referred to as radio frequency (RF) radiation. For convenience in managing its use, RF radiation is divided into groups called bands, such as S-band and X-band. The bands are further divided into small ranges of frequencies, or channels, some of which are allocated by international agreement, for the use of deep space telecommunications. The following table shows the approximate range of frequency and wavelength for each band.
Radio Communication Bands vs. Wavelength and Frequency
Band Name *Wavelength (cm) * Frequency (GHz)
L * 30-15 * 1-2
S * 15-7.5 * 2-4
C * 7.5-3.75 * 4-8
X * 3.75-2.4 * 8-12
K * 2.4-0.75 * 12-40
Early spacecraft used L-band for deep space communications. Within a few years, S-band replaced L-band, and more recently X-band came into general use for deep space communications. Experiments to demonstrate the advantages of telecommunications systems using K-band were in progress at the time of this writing (1997). Generally speaking, the higher frequency bands offer greater advantages for space communications, although these tend to be offset by other factors, such as increasing losses at the highest bands.
Doppler Effect
The Doppler effect is routinely observed as changes in the frequency of spacecraft radio signals received by a DSN tracking station. This is caused by the relative motion between a spacecraft and the tracking station due to the spacecraft trajectory, its orbit around a planet, Earth's orbit around the Sun, or the daily rotation of Earth about its axis. When the distance decreases, the frequency decreases proportionally to the rate of change of distance, and vice versa. If two widely-separated tracking stations observe a single spacecraft, they will have slightly different Doppler signatures. This information, described as the Doppler type of radio metric data, is generated at the DSN tracking stations and used by spacecraft navigators to describe the path of the spacecraft through deep space, very precisely and in three dimensions.
Cassegrain Focus Antennas
Whether the mount is of the HA-Dec or Az-El type, all DSN antennas, irrespective of their size, use the Cassegrain focus system to concentrate the electromagnetic energy incident upon their surfaces. The incident energy may originate from a distant spacecraft, a celestial body, or the station's own uplink transmitter. It works equally well for either uplink or downlink.
In a Cassegrain antenna, a secondary reflector is added to the structure to fold the electromagnetic beam back to a prime focus near the primary reflector. Incoming electromagnetic waves are focused by the prime reflector on to the secondary reflector, or hyperboloid, which refocuses them into the receiver feed horn located at the prime focus. Usually, several feed horns are mounted on a single cone structure, and by rotating the slightly offset secondary reflector, the main beam can be directed to any receiver horn as required. For transmitting, the system works in the reverse way.
This design accommodates very large-diameter antennas and allows bulky, heavy receiving and transmitting equipment to be located near the center of gravity of the composite structure.
Uplink and Downlink
The radio signal transmitted from a DSN antenna to a distant spacecraft is known as an uplink. The radio signal transmitted by the spacecraft to the DSN is known as a downlink. Uplinks or downlinks may consist of a pure RF tone called a carrier, or a carrier which has been modulated to carry information in each direction, including commands to the spacecraft or telemetry data from the spacecraft. A spacecraft communications link that involves only a downlink is called a one-way link. When an uplink is being received by a spacecraft at the same time that a downlink is being received, the communications mode is said to be two-way. These two distinct modes of operation play a significant part in the operation of deep space communications.
Signal Power
Typically, a local broadcasting station with a transmitting power of 50,000 watts will deliver an acceptable radio signal to a portable receiver at a distance of 100 km. How then can a spacecraft transmitter, limited to a power of 20 watts, deliver an acceptable signal to a DSN receiver across hundreds of millions of kilometers of interplanetary space? The first step involves concentrating all of the available energy into an extremely narrow beam pointed in one direction, rather than spreading it in all directions, as in a broadcast station. At the spacecraft, this is done with a small parabolic dish antenna, typically one to five meters in diameter. Even so, when these concentrated signals reach Earth they have vanishingly small power, perhaps as small as 1 x 10-20 watts. The rest of the solution is provided by the receiving power of the large aperture antennas of the DSN and the extraordinary sensitivity of its cryogenically-cooled low-noise receivers. Aided by special coding and decoding schemes to discriminate against radio noise, the DSN can extract the science and engineering data from these unimaginably weak signals and deliver it in real time to the intended users. As this history will show, the ability of the DSN to do this has improved by many orders of magnitude over the forty or so years since its inception.
Coherence
In addition to its use as a conveyor of modulated telemetry data, the downlink carrier is also used by the DSN for tracking the spacecraft and for carrying out some types of radio science experiments. Each of these applications requires the detection of minute changes (fractions of 1 Hz) in many GHz of carrier frequency over many hours. This can only be accomplished when the frequency of the downlink carrier itself is extremely stable and is known with very great precision. Since the spacecraft itself could not carry the massive equipment needed to do this, it makes use of the uplink, which does have the requisite stability, as a frequency reference for the downlink. In effect, the spacecraft simply retransmits the DSN uplink after modulating part of it with the desired telemetry data. When this is done, the downlink is said to be two-way and coherent, that is, in-phase with the uplink.
At each Deep Space Complex, a hydrogen maser-based frequency standard provides the reference for generating an extremely stable uplink frequency for transmitting to the spacecraft. The resulting spacecraft downlink, based on and coherent with the uplink, has practically the same high frequency stability as the original reference frequency standard. Comparison of one with the other at the phase level of individual cycles produces the desired tracking or scientific data. The entire process ultimately depends for its success on the "phase coherence" of the uplink and downlink carriers, and the accuracy with which corrections can be made for the many sources of error in the end-to-end system.
Under some operational conditions the spacecraft may not have an uplink. To cover such cases, all spacecraft carry a small auxiliary oscillator, which serves as a reference for generating a non-coherent downlink in the absence of a DSN-generated uplink. It is not highly stable and its frequency is affected by temperature variations on the spacecraft. Nevertheless, there have been numerous instances in the long history of the DSN when a spacecraft's local oscillator played a major role in saving an otherwise doomed spacecraft. Some spacecraft also carry a thermally stabilized ultra-stable oscillator (USO) which is used for precision radio science experiments involving planetary occultations. Because of stringent frequency requirements for spacecraft tracking navigation and radio science, the DSN remains at the forefront of the technology for frequency and timing standards.
Decibels
Perhaps the most commonly used technical term in the DSN is the ubiquitous decibel. Since the term decibel (or dB) appears frequently in the following history, a simplified, non-rigorous explanation is in order. The decibel is a unit of measure used to describe the ratio between two power levels. For example, the power delivered by a 100-watt audio amplifier is said to be 10 dB higher than the power delivered by a small 10-watt amplifier. The ratio of the powers is 10 to 1. Inversely, where the ratio is 1 to 10 (1/10) the equivalent term would be -10 dB.
Because of its mathematical origin, the decibel scale is not linear, it is logarithmic; the 10 dB corresponds to a ratio of 10, 20 dB corresponds to a ratio of 100, 30 dB to a ratio of 1000 and so on. For ratios less than 10, 3 dB corresponds to a 2 to 1 power ratio, 6 dB to a ratio of 4, etc. When the ratios are inverted they are represented by a negative sign before the corresponding dB value, so that a ratio of 1 to 2 or 1/2 is represented by -3 dB.
Much of the technical progress in the DSN is reflected in terms of dB values for high power transmitters, gain of large antennas, threshold of receiving systems, gain of maser amplifiers, signal-to-noise ratios, bit-error rates for decoding systems, etc.
The remarkable improvement in downlink performance that took place during the Mariner, Viking, and Voyager Eras, the first three great eras of DSN growth. In the context of this figure, improved downlink performance implies an increased capability for a DSN tracking station to return more data at the same range, the same data at a greater range, or some other enhanced combination of data and range from a planetary spacecraft. The effect of changes in operating frequency, from L-band to S-band to X-band, is evident in the stepwise improvements, as is the contribution of the larger diameter antennas both individually and in array with other antennas. The parameter called system noise temperature is a measure of the sensitivity of the maser receiving systems that are a key element in the overall performance of the downlink.
After the early 1980s, improvements in terms of dB were less dramatic, not because of any less incentive for further improvement, but because by that time the technology of deep space communications was approaching the practical limits of physical realization for that era. More esoteric approaches, such as advanced data coding methods, were called upon, and although significant improvements continued to be made, they were much smaller in terms of dB. This issue, and how the DSN dealt with it, is discussed in chapter 7, "The Advance of Technology in the Deep Space Network."
Modulation and Demodulation
Scientific and engineering data is generated by a planetary spacecraft in digital form and modulated to an S-band or X-band carrier signal for transmission to the antennas of the DSN. Modulation is a two step process in which the bits of raw data are first applied to a subcarrier of much lower frequency. The data-modulated subcarrier is then applied to the RF carrier for radiation, via the spacecraft's transmitter and antenna, to Earth. At the DSN receiving station, the process is reversed. Sensitive receivers detect the carrier signal, extract the subcarrier and pass it to a subcarrier demodulator. The subcarrier demodulator extracts the data and conditions it for recording and forwarding to JPL. Most spacecraft apply a complex code to the raw data to protect it against noise-induced errors during transmission. Special equipment at the tracking stations performs the extra step of decoding the data before it is processed for recording and delivery. This end-to-end function of a deep space communication system, and the data associated with it, is called telemetry.
There is an analogous function that uses the S-band or X-band uplink for transmission of command data to a spacecraft. It is called command. In this case, the subcarrier modulated with a bit-stream of command data. At the spacecraft, the carrier is detected, the subcarrier is demodulated and the command data bits are eventually passed to the command system for immediate execution or for storage for later action.
The distance from Earth, or range, of a distant spacecraft can be measured by modulating the uplink with a special code or sequence of digital characters, which, when turned around by the spacecraft and remodulated on the downlink, allow the time delay between transmission and reception to be determined with great precision. The Earth/spacecraft range is then calculated from knowledge of the time delay and the propagation velocity of the modulated radio signal.
Spacecraft Radio System
Intentionally, the spacecraft itself was not among the topics discussed up to this point. While a planetary spacecraft is a miracle of modern technology, the history of planetary spacecraft development is beyond the scope of this book. It is important to note, however, that the spacecraft radio system is an integral part of a deep space communications system although not an integral part of the DSN organization during the period of time covered here. The spacecraft radio system was provided by the flight project organization as part of the spacecraft. Nevertheless, responsibility for RF compatibility between the spacecraft and the DSN was always a DSN responsibility. In the ensuing chapters, where a spacecraft radio system is discussed, the general explanations given above will suffice for an understanding of the matter.
An Essential Part of the Answer
In the preceding discussion, I elected to use generalities in describing some of the basic concepts of deep space communications. The reader will encounter all of these terms and topics many times as the history of the Deep Space Network unfolds. These concepts also allow us to answer the questions prompted by the launch pictures, and by the remarkable new images shown at the press conference. An essential part of the answer is provided by NASA's Deep Space Network. Worldwide in concept, continuous in operation, the DSN is the link between the two images.
In terms of the foregoing discussion, the Deep Space Network may be characterized as a scientific instrument of worldwide proportions that uses a single up- and downlink radio signal, in combination with a coherent spacecraft transponder, to perform three basic functions. Interplanetary spacecraft depend on the first two for their communications with Earth. The third and no less significant function brings the DSN into the science community in a more direct way. Each of these functions is associated with one of the unique types of data carried on the up- and downlinks.
The first, and perhaps the most important, function is that of generating radio metric data. Consisting of Doppler data, ranging data, and interferometric comparisons between two tracking stations, radio metric data are used by spacecraft navigators to determine the precise location of a spacecraft along its trajectory at all times. Radio metric data are also key to the task of pointing the DSN antennas in the right direction to establish, and continuously maintain, a radio link between spacecraft and Earth.
The second function, telecommunications, makes use of appropriate modulation impressed on the uplink (by a tracking station), and on the downlink (by a spacecraft), to connect a spacecraft with its Earth-based controllers. Command instructions are sent to the spacecraft on the uplink, while engineering and science data are telemetered back to Earth, on the downlink.
A third function relates to the use of the DSN as a scientific instrument for research in radio and radar astronomy. Scientists were quick to recognize the potential of the state-of-the-art capabilities of the DSN as a powerful new resource for advancing their experiments. Encouraged by a generous NASA policy for making time observation available, the large antennas that could be pointed with great precision, sensitive receivers, and extremely stable timing systems of the DSN soon attracted the attention of many of the world's leading researchers. Using spacecraft radio signals, Extra-Galactic Radio Sources or echoes from powerful radar transmitters at Goldstone, scientists continue to use the DSN as a scientific instrument to widen our knowledge of distant regions of the universe.
What follows is an end-to-end scenario that brings all the pieces together to describe the deep space communications process for a typical planetary mission.
Soon after the launch vehicle was lost to view on the television screen, the first stage booster burned out and fell away, leaving the injection vehicle, with the spacecraft attached, to coast for a short time until it reached the exact position and time for the trajectory injection maneuver to begin. At about that same time, the injection vehicle had moved far enough around Earth to be in radio view for the DSN tracking stations in Australia. The Canberra stations tuned their receivers to the spacecraft's auxiliary oscillator frequency, making allowance for the one-way Doppler effect, and locked-on their receivers to automatically follow the spacecraft. The telemetry data on the one-way downlink carried engineering data that confirmed that the spacecraft survived the stresses of the launch events. At the appointed time, the injection vehicle engine was fired to accelerate the spacecraft free of Earth gravity and on to its interplanetary trajectory. The small initial acquisition antenna at Canberra had been following the spacecraft and now steered the more powerful, but less agile, 34-meter antenna to point at the spacecraft where it too, locked up its receivers.
Telemetry data, demodulated and decoded at the tracking station, continued to flow to mission controllers at JPL, but they needed to send commands to the spacecraft immediately to orient it to the Sun and to reconfigure it for its long cruise to the planet. Besides, spacecraft navigators were anxious to have the better, more-stable Doppler data that was associated with the two-way, coherent mode. The transmitter at the tracking station was turned on, and making allowance for the uplink Doppler offset, its ultra-stable S-band (or X-band) uplink was tuned across the frequency of the spacecraft receiver. On detecting the uplink, the spacecraft automatically switched over to the coherent mode of operation and began retransmitting the uplink back to the tracking station. The DSN receivers were then in the two-way, coherent mode, the two-way Doppler was reaching the navigators, and commands could be transmitted at any time as required.
As the spacecraft continued to move away from Earth on its new trajectory, Earth rotation brought the other DSN Complexes into view, and the stations there passed the spacecraft in turn, from one to the other. This involved a complicated operational procedure to avoid losing telemetry data, but the net result was that communication with the spacecraft continued, uninterrupted by Earth's rotation, for the approximately two remaining years of the mission.
During the long cruise period, the Navigation Team used two-way Doppler and ranging data generated at the tracking stations to verify, and when necessary adjust, the trajectory of the spacecraft to ensure that it reached the planet at the correct position and time for a successful encounter. Because the distance between the spacecraft and Earth had been steadily increasing, the power level of the carrier signal received by the 34-meter antennas gradually dropped to the point where the signal-to-noise ratio was too low for the DSN receivers to operate properly. At that point, the tracking stations changed over to the big 70-m antennas for tracking this spacecraft. The 70-m antennas allowed them to receive telemetry data at a higher data rate, a capability that would be needed at the greater range of the forthcoming planetary encounter.
Encounter day finally arrived. All of the complex sequences of operation that the spacecraft would carry out during encounter had been transmitted to the spacecraft from the DSN and were stored in its computer memories. For some time, the spacecraft cameras and other scientific instruments had been trained on the approaching planet; engineering and the telemetry data rate had been increased to the maximum possible. All the terrestrial communications circuits linking the three Deep Space Complexes to JPL were on high alert. The Deep Space Network was then doing what it does best: providing a real-time, two-way, coherent, digital data communications link between NASA's Jet Propulsion Laboratory in Pasadena, California, and a planetary spacecraft encountering one of the Jovian planets.
Responding to its stored commands, the spacecraft executed the encounter sequence of events according to a pre-determined plan that included all of the onboard science instruments. As the spacecraft entered occultation (was obscured by the planet), radio science experiments were carried out using the spacecraft USO. The immense amount of data collected in the short time of its actual encounter was stored by the spacecraft on its tape recorder for later playback via the downlink. Only the most important data could be sent back in real time, but this was sufficient to give anxious mission scientists their first glimpse of the planet, up close. Even after demodulating and decoding the downlink data stream that contained the images, the tracking stations could not view them. They had to be separated from the other science data and further processed by computers at JPL before they could be analyzed by the imaging team and presented for public viewing.
It was those pictures that the public first saw on television at the early NASA press conferences from JPL. Later, as the remainder of the encounter data were retrieved by the DSN, scientists conducted much more detailed analyses of their data and their findings were ultimately presented to the world at NASA press conferences in Washington. The DSN stations continued to track the spacecraft as it pursued its orbit around the Sun, returning science data at an ever decreasing rate and conducting radio science experiments as the opportunity arose. So long as the spacecraft continued to function properly, scientists continued to press for more data from deep space, and for the support of the Deep Space Network to obtain it.
The foregoing end-to-end scenario established the DSN as an essential part of the answer to the questions invoked by a search for a less obvious connection between the launch of a spacecraft and science data presented at a NASA press conference. Perhaps that answer begets further the question of how the DSN came into being and how it developed. With that question in mind, what a timely open to the history of the Deep Space Network.
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PREFACE
Although the subtitle for this book, "A History of the Deep Space Network" has an air of finality about it, the suggestion of a task ended or a work completed, that does not represent the true state of affairs. The Deep Space Network (DSN) is really a work in progress. It had a beginning, of course, and a life that, at the time this book was written, had spanned four decades. That is what this book is about. The task of recording "what happened next" will fall to future historians.
Eloquent histories of NASA's planetary explorers have been written by others, and it was not my intent to revisit those magnificent enterprises. To them belonged the high drama associated with spacecraft encounters and landings on distant planets, spectacular launches, startling new science, and stunning color images from distant corners of the solar system.
As a key element in all of those dramatic spacecraft events, the DSN shared their excitement, but saw a quiet, unreported drama of its own. There were occasions when the success or failure of a multimillion dollar spacecraft, the reputation of NASA, or the recovery of critical science data from a far planet, lay in the hands of the DSN and, not infrequently, those of a crew member at a distant tracking station. Such events appear throughout the narrative. More often though, the determined, sometimes heroic, efforts of engineers and technicians in laboratories at JPL, and in control rooms and antennas at remote tracking stations, provided drama enough for those of us who were aware of it. Struggling to meet the seemingly insatiable demands of the planetary space missions for more, bigger, or better capability, and all of it sooner, those individuals bore the brunt of the burden of change that characterized progress in the DSN from its beginning. These events, too, are identified in the narrative.