One of the first big dividends from the U. S. space effort will occur in satellite communications. Of the many areas in which our national space program is engaged, none promises more in utility to mankind in general. Communications by satellite relay offer a great and quick hope for political, technological, and economic gain.
World interest in the international space race has been very high. While it has been conceded that the Russians are far ahead in the brute power of their rocket motors, it is also true that this country has shown a definite edge in space instrumentation. Thus far we have demonstrated this lead by a multitude of astronautic achievements, both useful scientific satellites and prototypes of advanced space systems. We still await a space accomplishment packing the practical punch that an operational system would provide. Satellite communications will do just that; for they promise not only enormous improvements in present communication modes, but they open the door to a new realm of communications techniques and applications. Moreover, communication by satellite appears to be economically rewarding. Witness the intense commercial rivalry which has arisen among the traditional common carrier industries, airframe manufacturers, and electronic equipment producers for a share in the future of the national program. Satellite communications is, in fact, the only area of operational space technology to which private industries are willing to pay the price of admission: their own huge development costs. We have, indeed, the rare example of an industry spending its own money to develop a complete communication satellite system and subcontracting services to the Government.
Communication Satellite Possibilities
Why are satellites so attractive in the realm of communications? To answer, it is first necessary to explore the present modes of long distance communications and their limitations.
Reliable, long-distance communications connecting the continents are limited at the present time to the use of submarine cables aided by radio transmissions in the range extending to what is known as high frequency (HF). The limit in system flexibility imposed by cables between fixed points is obvious. Cables are temporary, vulnerable, expensive, and limited by the rate at which they can carry data.
Radio waves, of course, are not bound to the fixed trunk line paths associated with cables, but their propagation characteristics offer some real limitations. Long haul HF radio waves depend upon reflection by the ionosphere, layers of charged particles blanketing the earth from some 40 to 250 miles above the surface. It is only because of this ionospheric reflection that radio signals in the high frequency range may be used for long distance communications. Multiple reflections from ionosphere to earth (back and forth) are used to carry HF signals over continents and oceans. But ionospheric height and density vary markedly. They are affected by solar flares and magnetic storms which radically disturb the propagation path causing “fades” and “blackouts” in reception. A reliability factor of 80 per cent in an HF path is considered very reasonable. The data rate of HF communications is likewise limited. Low speed data, Morse Code, voice and teletype may be transmitted but neither television nor any high speed digital data.
Higher frequencies extending into the very short wave length or “microwave” region, characteristically transmit the higher data rates needed for television but, as any fringe area viewer knows, are limited to line-of-sight transmissions. Microwave radio signals are not reflected by the ionosphere but pass through. Limited as it is to line-of-sight transmissions, our present continental television program links consist of chains of microwave towers every 30 miles or so across the nation.
Very simply then, communication satellites serve as radio relay stations in space. As such, they extend the useful line-of-sight links for high data capacity microwave frequencies from the tens of miles (now possible with antennae mounted on towers or mountain peaks) to the thousands of miles possible (with antennae mounted on orbiting satellites many hundreds of miles above the earth’s surface). Thus, we are led to an appreciation of the great advantages of satellite over conventional communications: freedom from the inflexibility of cable systems, and the unreliability inherent in ionospheric reflection communications, and very high data carrying capacity.
And what of cost? Every study publicized so far has been enthusiastic concerning the economic position of satellite communications relative to conventional means. In a recent summation of space plans, it was estimated that the cost to maintain a single voice channel in a satellite would be $8,500 to $10,000 per year, considering both satellite life span and chances of launch success. This figure is to be compared with the current cost of $240,000 per year to lease a single trans-Atlantic voice channel.
Imagine then the impact of world-wide live television. Imagine the effect of person-to- person telephone dialing throughout the world. Imagine, also, the possibilities inherent in real time digital data links between computers in different continents. Imagine finally that these new accomplishments may be had at a cost less than existing long distance communications lines, and you have a concept of the potential importance of satellite communications.
Richard S. Morse, former Assistant Secretary of the Army for Research and Development stated, “In my view, a communications satellite offers for the entire world the most interesting space applications for years to come. Such an enterprise offers more commercial possibilities than reaching the moon.” Even more enthusiastically, George J. Feldman, former director of the Select House Space Committee, wrote that, “Within the next few years, communication by satellite could easily become a multibillion-dollar business. Within 10 to 15 years ... a $100- billion-a-year business.”
Types of Communication Satellites
Communication satellites may be of two basic types, passive or active. Passive satellites serve as reflectors or re-radiators of electromagnetic energy and need contain no energized electronic circuitry of their own. Radio energy is reflected from a passive satellite in much the same manner as light from a mirror. The moon has been used as a passive communications satellite; the Echo balloon in the same capacity. In both cases radio energy as a communication link between two terrestrial points, is established by bouncing signals radiated from one station off the satellite and receiving them at the other. Instead of a solid surface, though, dispersed particles may be used as reflectors. If these particles are of the proper length, they may be resonant to a certain frequency, in which case they serve as excellent passive relays by effectively reradiating that frequency. This latter procedure, in fact, is to be attempted in the Air Force West Ford program. Passive satellites have the advantage of high reliability. Once in orbit, there is little, if anything, aloft to fail.
Active satellites on the other hand contain electronic circuitry—receivers, power sources, amplifiers, transmitters, which receive an incoming transmission from a ground station, amplify it, perhaps change its frequency or other characteristics, and retransmit it to another ground terminal. Active satellites really perform a function similar to micro- wave relay towers on the ground. Because they boost the energy level of the relayed signal, their associated ground stations transmit less power and need smaller antennae. Thus, these terminals may be smaller, an advantage to shipboard structure. Active satellites, of course, containing electronic circuitry, are less reliable than passive ones.
Active satellites divide conveniently into two groups: delayed repeaters and real-time repeaters. Delayed repeaters store and forward information. Data received by the satellite is stored in some memory device, such as a tape recorder, and transmitted later either on demand from the ground or automatically according to some planned sequence. Delayed repeaters are properly used at low altitudes where the satellite may not be in line of sight of the sending and receiving ground stations at the same time. Real-time repeaters, as the name implies, repeat instantaneously. They store no information but transmit what is received without delay. Real-time repeaters are properly used at high altitudes. Most communications satellites now in development are real-time devices.
Satellites may also be grouped in accordance with the frequency band employed, whether commercial or military, the orbit employed, and in other ways. All current U. S. communication satellite programs are tabulated in Table I, with a few identifying characteristics for reference.
Navy Communication Requirements
The Naval Communications Service has served the Fleet well for many years. The demands upon the Service have been great, imposed by many classes of users, in wide geographic dispersions, under infinitely varied conditions, and for all types of services and problems encountered. Because the intent of this article is to explain the role satellite communications may play in the Navy’s communications, by meeting the demands and solving the problems, it shall first be necessary to consider the requirements. Perhaps Navy communication requirements were most cogently summarized by Rear Admiral Frank Virden, former Director of Naval Communications, at the fifth Annual National Convention on Military Electronics, sponsored by the Institute of Radio Engineers, held recently in Washington, D. C. “Even today,” Admiral Virden stated, “the Naval Communications requirement is quite simple. The Commander must be able to communicate whenever he needs to, in any mode, between and among ships separated by varying distances, and from ships to and from selected shore stations, aircraft, and satellites. He must do it in an utterly reliable, rapid, and secure way despite various kinds of disruption that may occur. With this capability in hand, the Navy and national defense will be able to realize the fullest potential of seaborne mobile forces including the capability to command and control them under any and all hazards of war and conditions less than war.”
TABLE I UNITED STATES COMMUNICATION SATELLITE PROJECTS |
|
Passive Satellites |
|
CMR |
Navy, Moon Relay System, operational, connects Maryland and Hawaii, uses moon to reflect UHF signals. |
ECHO |
NASA inflatable sphere, developmental, one satellite launched. August 1960 employed 100-ft. diameter sphere used as passive reflector at about 1,000 miles, new 135-ft. sphere planned. |
REBOUND |
NASA, developmental, multiple 135-ft. diameter spheres launched together to be equally spaced in same orbit. |
WEST FORD |
Air Force, developmental band of fine copper dipoles resonant at microwave frequencies planned to act as re-radiators, altitude 2,000 miles. |
Active Satellites |
|
SCORE |
Delayed Repeaters Army, completed development, one satellite launched 4 October 1960, used as “store and forward” repeater on microwave frequencies, lasted 17 days, altitude 300 nautical miles. |
Very Low Frequency (VLF) Satellite |
Navy, proposed development, to be used as “store and forward” on VLF. |
ADVENT |
Real-Time Repeaters Army, developmental, multiple satellites in low orbits, wideband, microwave frequencies, altitude 1,000-3,000 nautical miles. |
TELESTAR |
AT&T, developmental, similar to RELAY for commercial applications microwave frequencies. |
SYNCOM |
NASA, developmental, small, narrow band satellite in inclined near-synchronous orbit, microwave frequencies, altitude 19,300 n. miles. |
Navy Satellite Communications Program
The Navy Advanced Communications System (NACS) is the Navy’s over-all communications development effort aimed at providing a solution to the problems posed by Admiral Virden. As a part of its over-all communications effort, the Navy is investigating just how extensive the role of satellite communications will be.
Within the Navy, the Bureau of Ships, as the lead bureau for communications, manages the Navy’s satellite communications program. This program encompasses the Navy’s participation in joint projects particularly where the Navy capability is unique, such as in the development of mobile shipboard terminals; and in special Navy projects where the Navy requirement is unique, such as in submarine communications.
The Communications Satellite Relay Program of the Bureau of Ships currently is composed of three parallel efforts to take full advantage of work being done by NASA and other military services and to exploit all modes of satellite communications to satisfy Navy requirements. These efforts are:
(1) Exploring usage of Passive Satellites— for high reliability.
(2) Participation on Project Advent and SYNCOM—for high capacity long range communications.
(3) Development of a VLF Satellite—for reliable communications with ships at sea.
Communications Moon Relay System
The Communications Moon Relay (CMR) System is a communication link which employs the earth’s natural passive satellite— the moon—as a reflector of radio energy for communications purposes.
CMR grew out of discoveries by the Naval Research Laboratory, where the feasibility of using moon reflection techniques for communication purposes was demonstrated in 1951. On 25 July 1954, NRL transmitted the first voice messages over the earth-moon- earth path. The Laboratory was also the first to accomplish transcontinental communication over the moon radio circuit when, in November 1955, it transmitted teletype messages from Washington, D. C., to San Diego, California. Another pioneer event occurred two months later when NRL conducted transoceanic communication between Washington and Hawaii via the moon.
On the basis of NRL experiments, the Chief of Naval Operations directed, in 1956, the establishment of a two-way teletypewriter and facsimile moon relay circuit between Washington and Hawaii. Several months later, the Bureau of Ships awarded a contract to the Development Engineering Corporation of Washington, D. C., to set up a CMR system at an approximate cost of $5,500,000. The Bureau of Yards and Docks constructed the buildings located at the sites. NRL performed a consulting role to provide scientific guidance to the project. In November 1959, the system was successfully used when solar disturbances in the ionosphere disrupted conventional high frequency circuits between Washington and Hawaii.
In January 1960, demonstration messages were exchanged via the moon between Admiral Arleigh Burke, Chief of Naval Operations, and Admiral Hopwood, Commander- in-Chief, Pacific Fleet. In addition to the messages, photographs were transmitted by facsimile via the moon relay circuit.
In the Washington area, the transmitter is located at the U. S. Naval Radio Station, Annapolis, Maryland, while the receiver facility is situated at Cheltenham, Maryland. The Hawaiian facilities are located at Opana and Wahiawa on the island of Oahu. Transmitted signals, reduced in strength by absorption and propagation losses over the 480,000-mile path, are received 2½ seconds after transmission.
The Washington and Hawaii terminals each utilize two 84-foot-diameter, dish-shaped antennae—one for transmitting, the other for receiving. The transmitter antennae are coupled to 100,000-watt transmitters producing a radiated signal equivalent to more than 400 million watts isotropic.
The Communications Moon Relay System offers reliable long distance communication, presently limited only by the availability of the moon, which has to be within sight of the apparatus, and by the number of radio terminals, which are too few in number to make CMR a widespread system. These limitations would be overcome by the use of man-made passive satellites in fixed orbits around the earth and by increasing the number of stations.
While most long distance communication systems utilize the medium and high frequency portions of the electromagnetic spectrum, CMR uses ultra-high frequency waves which penetrate the upper atmosphere and are directed to the moon by a high-powered transmitter. Bounced off the moon by a highly directional, narrow beam antenna on the earth, the radio waves are picked up by an antenna at the receiving terminal. Both the transmitting and the receiving antennae must be electronically visible from the moon at the same time. CMR is jam-resistant, since jamming requires the use of very high power and can be accomplished only by another station within sight of the moon at the same time as the receiving station.
For almost two years now, CMR has been employed as a communications link making it the first (and at this writing, the only) space telecommunications system which is used to pass bona fide messages. The ground stations are manned by Navy personnel from four to eight hours daily (i.e., from moonrise in Hawaii to moonset in Maryland). Many times over the past months during severe ionospheric disturbances when normal MF /HF traffic was completely closed down, CMR served as the only radio link between the areas served carrying much military priority traffic for periods of several hours each time.
Although the use of CMR to carry actual traffic is certainly noteworthy, this system has also served in an even more valuable scientific capacity. As the grandfather of all satellite communications projects, CMR has generated a vast amount of basic scientific data on the nature of propagation of radio energy through space and the ionosphere and has provided an ideal tool for studies on modulation techniques, antenna characteristics, and terminal instrumentation. Information generated from studies at CMR has been used as the basis for formulating design criteria and system parameters for all follow-on communication satellite projects—both passive and active. Because there is no launching cost involved in this natural satellite and because the moon is so reliable, CMR has served as a most inexpensive source of space communications data and as a laboratory for equipment development. Modifications will be made to the CMR terminals and experiments undertaken with new terminal equipments and techniques to provide even more information in the newly important microwave frequency region and on more sophisticated modulation techniques.
The Advent Satellite Communications Ship
Project Advent is a Department of Defense Research and Development effort in active communication satellites. The objective of Advent is to demonstrate the feasibility of using real-time relay through active satellites in 24-hour equatorial synchronous orbits for long distance, reliable military communications purposes.
Advent, hovering over a fixed point on the equator because of its altitude of 19,300 nautical miles, will be available 24 hours a day as a real-time communication link between any two points on earth in its radio line of sight. A single such satellite can provide a full-time, all-weather communication circuit between points on earth separated by some 9,000 nautical miles. Three such satellites could provide overlapping coverage of the entire earth save for the polar regions. As a potential military system, Advent is projected at providing a multi-channel, high capacity, radio link between fixed and mobile stations characterized by both high-reliability and optimum security.
Over-all management and direction of Project Advent has been assigned to the Department of the Army, and development agencies of all three military services are co-operating. The Navy Bureau of Ships will design and develop a shipboard terminal complementary to the Army stations at Fort Dix and Camp Roberts. USNS Kingsport Victory (AG-164), a former MSTS cargo ship, is now being fitted with communications equipment providing two-way, wide-band, high data rate, microwave communications with either or both of the shore stations. In addition to her communication capability, the ship will have a tracking, telemetry and command capability. It can track the satellite using either beacon tracking, a computer-programmed mode, or a console-controlled mode. It will receive and record telemetry and doppler measurement of the satellite and will have a command control capability. The shipboard terminal will greatly increase the flexibility of the Advent research and development program. The ship not only will supplement the fixed communications and tracking stations as a third station, but its mobility will provide many unique contributions.
The choice of a ship platform as the mobile terminal for the Advent program is a most logical one, since a ship may traverse a large portion of the expected area of coverage of the Advent satellite. Approximately 72 per cent of the earth’s surface is covered by navigable water. It is easy for a ship to provide the weight, size, and power required for an adequate terminal, accommodations for personnel, and true mobility of operations.
The mobile shipboard terminal will provide an effective test of the limits of reliable coverage of the Advent communications system since the full extent of system capability can be probed only through a determination of the effects of surface location and geographically related atmospheric conditions on propagation. Propagation may be affected by refraction, by absorption, by interference, and by ground path-related noise. All of these effects may be investigated as a function of both time and geographic location through the use of the mobile shipboard terminal.
A most important contribution to the study of the Advent communication system will be the study of limits of coverage of the satellite. A theoretical prediction of coverage based upon line-of-sight considerations alone and neglecting all atmospheric and interference effects, would result in a circle of radius approximately 4,900 miles centered at the subsatellite point. Actual reliable coverage however, as opposed to theoretical predictions, may be determined only by a test program involving fringe area data collection. Mobile testing will enable an exact determination of these fringe areas of restricted bandwidth with great impact upon system parameters for an operational system.
Lastly, for a full consideration of the contribution to be made by Navy participation in the Advent program, the nature of Advent as a Research and Development program leading toward a military communications system must be emphasized. Strategic and tactical communications dictate mobility. The ship terminal, as a truly mobile communications and TT&C center, will facilitate the derivation of the design criteria and system parameters necessary for the development of an effective military satellite communications system.
Recently a decision has been made to incorporate in the satellite communications ship the capability to operate with NASA’s SYNCOM satellite as well as Advent. SYNCOM is a smaller, simpler civilian version of Advent on a shorter development schedule. Kingsport Victory will have both a communications and a tracking capability for this satellite.
VLF Satellites
The Navy’s continuing need for reliable communications to ships at sea has led to the desirability of developing a very low frequency (VLF) Satellite. The Navy has long pioneered in VLF communications and has a great fund of technical knowledge and a pool of scientific talent competent in VLF techniques. A VLF satellite transmitting down through the ionosphere promises to be very attractive as a communication link. Since passive satellites are not feasible in this frequency range, the use of active satellites which receive messages at microwave frequencies and store them for later rebroadcast at VLF is considered to be the ultimate goal.
One of the major development problems in approaching an operational system was the almost total lack of knowledge which previously existed concerning the mode of propagation of low frequencies through the ionosphere. Therefore, under the sponsorship of the Bureau of Ships, the Naval Research Laboratory originated the Low Frequency Transionospheric (LOFTI) experiments.
The first satellite, LOFTI I, was launched atop a Navy Transit III-B satellite from Cape Canaveral on 21 February 1961. The two satellites did not separate, and on their 562nd orbit they are assumed to have reentered the atmosphere and burned up shortly after 7:47 a.m. (EST) on 31 March 1961.
LOFTI I was instrumented to receive 18- kilocycle signals from the powerful 300-kilo- watt NBA Naval Radio Station located in the Canal Zone and retransmit the signals via very high frequency (VHF) to observing stations on the ground. Three specially-equipped 24-foot-long Naval Research Laboratory trailers were located in Texas, California, and Canal Zone to receive telemetry signals from the satellite and ground signals from NBA. Seven NASA Minitrack stations were also used to receive telemetry data.
The orbit of the satellite—with a 110- statute-mile perigee and a 613-mile apogee— gave a good sampling of the effects of the ionosphere on very low frequency radio wave propagation as the satellite passed through the outer ionosphere and Fi layer, a heavily ionized region approximately 100 miles above the earth.
The eccentric orbit resulted in excellent altitude sampling. As the recorded data is processed, it will be possible to ascertain in still greater detail the variations in VLF signal strength as a function of altitude.
Signals received at NRL’s station at Stump Neck, Maryland, indicated that very strong signals from Navy Station NBA were received by the satellite when it was 1,000 miles to the north of the NBA station. In observations made at the station at San Diego, California, signals were received and retransmitted by the satellite when it was 3,700 miles west of the Canal Zone transmitter.
These results provide new information of the role played by the ionosphere in radio transmission. While the ionosphere does reflect VLF radio waves, the LOFTI I satellite has proved that VLF signals do penetrate through the ionosphere. Knowledge gained from this experiment will provide a better understanding and utilization of our existing communication systems, and aid in the design of new and improved communication systems.
From the data telemetered back to earth from the satellite, NRL scientists have been able to confirm that the ionosphere is not nearly as opaque at these frequencies as generally assumed and that VLF radio waves do pass through the ionosphere into the exosphere with relatively little attenuation. Thus, while the ionosphere reflects VLF radio waves back to earth to a large degree, it also permits very substantial penetration of the unreflected waves to outer space.
The information derived from LOFTI I certainly points to the possibility of an effective VLF communication satellite system.
The data received from the satellite also indicate that, unlike higher frequency radio waves which travel through the ionosphere at essentially the speed of light in free space, the velocity of the VLF waves is dependent upon the geometry of the signal path and local ionospheric conditions and is a fraction of the speed of light.
An attempt was made to launch a second experiment in this series, LOFTI II, into a high inclination orbit on 24 January 1962 as one of five satellites in the Navy Composite I launch. Unfortunately, the Thor-Able-Star booster vehicle failed and the satellites did not attain orbit. However, additional experiments in the LOFTI series will study propagation at other frequencies and at higher latitudes. The reciprocity of propagation will be investigated using a transmitting VLF Satellite. Finally the LOFTI experiments will lead to the system parameters needed to specify an operational communication system.
Satellite communications have a tremendous potential for all military and commercial users, but particularly for the U. S. Navy whose operations are very dependent upon rapid, reliable, and secure communications. Three promising areas are now under investigation in determining the extent of the role satellites of various types will play in Navy communication—passive satellites, synchronous real-time active satellites, and very low frequency delayed-time active satellites. Each of these types holds promising dividends for the Navy, and the exploitation of the techniques involved will be a most interesting development over the next few years.