Arthur C. Clarke Lectures

University of Moratuwa, Sri Lanka 1990

Space Communications

Harold Rosen
Senior Scientist, Hughes Aircraft Company

Many useful and enlightening programs have resulted from the blend of radio and rocket technologies that led to space communications. Communication satellites have linked the world into a global community, with profound political as well as economic effects. The planetary probes that provide us with rich details of the planets and moons of our solar system use very long distance space communication links to report their data to earth. And, if we are ever to communicate with extra terrestrial civilizations presumed to exist in other star systems, it will be by means of radio links or far greater range.

In this lecture, the origins of radio and microwave communications will be examined and their use in several interesting and challenging programs probed in order to learn how they helped to unlock the future. Hertz working in his laboratory at Karlsruhe, Germany in 1887 is the beginning of our story. He generated radio waves with a spark transmitter and detected them with a tuned loop antenna having a spark detector. By reflecting the signals from the far end of his laboratory, he was able to measure the length of the standing waves and from the calculated frequency determined that these electromagnetic waves traveled at the speed of light as Maxwell's theory had predicted.

Within fifty years of Hertz' historic experiments, microwave towers were introduced to relay wide-band communication signals. The earth's curvature limited each hop between relay stations, however, to about 30 miles. That is, limited so long as the relay towers were confined to the surface of the earth. Even before we entered the space age, however, imaginative scientists envisioned the use of spacecraft for communications. Arthur C. Clarke's historic paper, written in 1945, described a communication relay station in the geostationary orbit (now called the Clarke orbit). John Pierce, a pioneer in satellite communications and an earlier recipient of the Arthur C. Clarke Award, also wrote important papers predicting space communications.

In October 1957 the U.S.S.R. startled the world with the launch of Sputnik. That spectacular event was soon followed by the launch of Explorer by the United States, and the space race was on. With radio equipment no longer confined to the surface of the earth, radio communication over very great distances became possible. In addition to making the exploration of our solar system possible, the dawning of the space age made communication satellites a reality.

Despite the many advantages which would result from the commercial use of the Clarke orbit, it was at first deemed impractical. Many problems resulted from the size and complexity of the designs then being considered. The large size precluded the use of launch vehicles available at the time; but, even if they could be launched, these designs were too complex to have an economically attractive life expectancy.

While pondering these matters, it occurred to me that the satellite's size and complexity could be greatly reduced if a spinning configuration were adopted. This would provide stabilization of a toroidal antenna beam which continuously encompassed the earth, simplifying the attitude control, while permitting orbital period control by use of spin phased impulses, thus simplifying the orbit control system. Additional savings in weight could be achieved by the use of a solid state receiver for the communications receiver and a traveling wave tube amplifier for the transmitter, a device perfected by John Pierce and his colleagues at the Bell Telephone Laboratories.

These concepts provided the basis for a lightweight geostationary communications satellite design; and, starting in the fall of 1959, we created a small team at Hughes that was charged with perfecting the design and reducing it to practice. We proceeded to construct and test a prototype geostationary satellite during the year 1960, an effort that was actively supported by Hughes Aircraft Company. We, in fact, began a campaign to get it launched.

In 1961 the U.S. Space Agency, NASA, with cooperation from the Department of Defense, sponsored a program to test the concept - this became known as the SYNCOM program, for Synchronous Communications. Syncom III, launched three years later, established continuous communications over the Pacific basin and, among other things, carried television transmission of the Tokyo Olympics.

The first INTELSAT satellite, INTELSAT I (also known as Early Bird), was a derivative of Syncom. It led to the rapid growth of international and, later, domestic satellite communications. The most successful domestic satellite has been the HS 376 series, of which the 32 currently in orbit have provided many different countries with a total of 3200 transponder years of service. INTELSAT VI is the latest of the INTELSAT series. Since the other Arthur C. Clarke lectures presented in this volume describe in some detail the remarkable advances in capacity of communication satellites over the intervening years, it might be more interesting to examine some other key developments.

In particular, it might be useful to explore three applications that are of great current interest to me. Each one of these applications is characterized by the use of small earth terminals for individual users. They are the use of satellites for land mobile communications, for the distribution of television, and for private business networks.

Satellites are presently employed for providing voice and data communications to maritime mobile earth stations, and new systems will provide such communication service to land mobile stations as well. The provision of communication links to land mobile users is one of the most challenging tasks in satellite communications. The difficulty stems from the low gain available in the mobile antenna and the scarcity of the bandwidth allocated for this service. The low mobile antenna gain can be overcome by use of high antenna gain in the satellite, and the narrow bands allocated for the service can eventually be increased by reusing the allocated spectrum. The second generation Australian domestic satellite currently under construction will have a land-mobile transponder in addition to its fixed service payloads. The mobile service antenna occupies the entire earth facing side of the satellite and creates a beam which covers all of Australia. The use of this large antenna for the mobile service permits the antennas on the land vehicles to be relatively small and inexpensive.

In a current development for North American service, more communication traffic will be accommodated in the land mobile service by making the spacecraft antenna still larger, which will permit more concentration of the beams, thus increasing the number of communication channels which can be used by the small mobile terminals.

The distribution of television by satellite has a long history. In July 1962, well before the Tokyo Olympics, the Telstar satellite had demonstrated the first transoceanic television, relaying signals between an earth station in northeastern United States and large terminals in England and France. Because of the low power available in the early satellites, the earth stations receiving their signals required very large apertures, about 30 meters in diameter, and very sensitive receivers. As time went on, more powerful boosters became available, making possible more powerful and more sophisticated satellites. By the early 1970's, domestic communication satellites, distinguished by their ability to concentrate their radiated signals into single countries, were introduced. Their high effective radiated power, which is a measure of both the transmitted power and the antenna beam concentration, permitted a much lower cost earth terminal to be used than for the INTELSAT satellites whose beams were widely spread.

This also permitted, among other applications, the economical distribution of television signals nationwide to cable systems, which carried them to individual households. Soon individual homeowners, in areas not served by cable systems, began erecting their own television receiving systems, and there are presently several million such terminals being used in the United States, whose average cost is about one ten-thousandth that of the terminals used for the first INTELSAT satellites. Still further reductions in the costs of receiving terminals are made possible by the class of satellite called direct broadcast. Direct broadcast satellites (DBS) systems are currently operating in Japan and in Europe, and satellites now under construction will provide service in Australia and in the United States. An emerging technology, the compression of video signals, will permit the channel capacity of new DBS systems to be greatly increased. One of the interesting capabilities of a direct broadcast satellite system is the transmission of signals of a new format, high definition television (HDTV), which has a strikingly improved picture quality and lends itself to large screen displays.

The first satellite system designed for private business services was SBS, whose first launch occurred in 1980. The SBSS concept involved linking together the geographically dispersed units of major corporations via high data rate satellite links. The associated earth terminals were located on premise, so that no terrestrial connections were required. However, because the particular design of the communication system caused the earth terminals to be very expensive, on the order of one-million U.S. dollars each, there were few customers and this pioneering system was not a commercial success.

Within a few years new systems with lower data rates were being designed that involved much lower cost terminals. The first user of t his system was a major oil field services company that used the satellite links to report drill hold instrumentation data back to a central data processing facility. As improved performance and lower cost transmitters and receivers became available, the costs of these terminals were driven down, and they are presently available for less than five thousand U.S. dollars, or less than one percent of the cost of the terminals designed for the Satellite Business System (SBS). But or even greater significance to the present popularity of such systems has been the development of software for the hub terminal which supervises the business network and interfaces with the data processing center of the business in a way that makes the satellite based system indistinguishable from the more costly terrestrial service that it sometimes replaces. In addition to data services, the satellite system can also supply voice circuits and television broadcast capability to those customers desiring these services. There are presently installed in the United States more than 30,000 customer premise terminals which are also called VSATs (Very Small Aperture Terminals), and the number is growing rapidly. It is estimated that there will be several hundred thousand such terminals in operation by the year 1001. Many types of companies, large and small, will be conducting their internal communications by satellite.

Modern satellite and television technology have accomplished a great deal and one of the very important results has been the improvement of the quality of primary and secondary education. The lure of entertainment television can be harnessed to capture the attention of students, and to increase their rate of learning and their retention. However, educational television as used in classrooms has so far, with few exceptions, been characterized by unimaginative preparation of the material and low quality displays, offering little improvement or incentives to learn over traditional methods. Much larger investments should be made in the development of educational programs, and the beautiful display capability of large screen high definition television, whose format includes digital stereophonic sound, should be used in the classroom to present the material in the best way possible.

Use of satellites to distribute the television signals would also make it easy to accommodate VSAT terminals at each school. The use of VSAT terminals would permit rapid and low-cost interaction with centrally located experts, who could answer teachers' questions and elaborate on the television presentations. This use of modern technology in the school environment could easily reverse the present discouraging trend in the quality of public school education, while lowering the total cost.

SPACE PROBES:

The exploration of our solar system by means of space probes began soon after Sputnik with missions first by the USSR and then by the United States to photograph the moon. The radio communication distances achieved over the years from Hertz' experiments to the moon landings in the 1960s represents a phenomenal rate of progress. The next phase involved probes to reach the nearer planets, Venus and Mars and then beyond. The Pioneer Venus orbiter launched twelve years ago used a low-resolution radar for remote sensing of the surface beneath the permanent cloud cover. The Magellan spacecraft, launched by NASA's space shuttle last year, uses a high resolution imaging radar to send back amazingly detailed views of Venus' surface. The entire surface of Venus should be mapped within a year.

The Voyager II spacecraft has had the most productive mission of any exploratory spacecraft. Launched thirteen years ago, it has returned spectacular optical images of the outer planets, Jupiter, Saturn, Uranus, and Neptune. Voyager II and its sister Probe Voyager I are expected to return valuable data until the year 2020, when their power sources will fade out and end their lives.

The challenge of deep space communications is prodigious. The propagation loss from Neptune to Earth is 102 db. (or16 billion times) greater than the loss from the geostationary orbit, which not too long ago was considered a difficult communication path. Communication over this distance was achieved by the use of an extremely sensitive network of ground antennas, many of which were arrayed together for the Neptune encounter. A 70-meter diameter antenna of the type used in Goldstone, California and the twenty-seven antenna array of the National Radio Astronomy Observatory in New Mexico is indicative of the types of extremely high gain antenna systems needed to cope with such large propagation losses. All of humanity should be proud of the accomplishments of the Voyager program in exploration and communication.

The exploration of the universe much beyond the solar system by means of space probes is impractical, because of the extremely long travel times that would be involved. Even an advanced spacecraft using the most effective propulsion technology now known - that is, ion engines - would take over twelve thousand years to reach the nearest star. So communication with other star systems, if it is ever to happen, will have to make use of distant transmitters, already in place, presumably operated by extraterrestrial civilizations. There is a small but very exciting program whose object is to use the technology of space communications to search for these civilizations presumed to exist throughout our universe. Called SETI, it will explore the microwave spectrum for the faint signals transmitted from planets of distant stars. The search involves several space centers in the United States, and uses existing radio astronomy telescopes and newly developed multi-channel spectrum analyzers. The world's largest radio astronomy telescope in Arecibo, Puerto Rico, will be used in the search. The sensitivity of the equipment used in the SETI search is expected to limit its range to the stars of our galaxy, the Milky Way. The participants in the program hope to achieve contact before the end of the century.

Looking farther into the future, it may someday be possible to receive signals from our nearest major galactic neighbor Andromeda transmitted two million years ago by a very powerful source among its 400-billion stars and possibly one-trillion planets. It would probably require an extremely large array of Arecibo sized antennas located n the far side of the moon, to shield the system from radio noise generated on the earth or by its radio emitting satellites.

The past quarter century has seen many benefits to humanity result from improved communications. We can hope that the advances to be achieved in the next quarter century will be as significant, and that Arthur and I are here to report on them.

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