Satellites can be used as components of a communication system to relay signals from one point on or near the Earth's surface to another. 1 2
Of singular interest for this purpose is the so-called "24-hour satellite" which completes 1 orbital revolution in 24 hours; so that if its motion is in the same direction as the Earth's rotation, and if other features of its orbit are properly selected, it will remain within line of sight of a fixed region on the Earth's surface.
Actually, the subsatellite point could remain truly fixed only if this point were on the Equator, and if the satellite orbit were perfectly circular, perfectly in the equatorial plane, and with precisely the right altitude (approximately 22,000 miles above the Earth's surface.) Even then, natural perturbations would cause a slight motion of the subsatellite point. Although these conditions cannot be fulfilled with perfect precision, reasonable precision in establishing the orbit will result in the subsatellite point moving relatively slowly over a preselected area on the Earth's surface, and occasional adjustments with small on-board propulsion units can compensate for such defects indefinitely.
The advantages of a 24-hour orbit for a satellite as a communication relay are twofold:
1 Adams, C. C., Space Flight, McGraw-Hill Book Co., Inc., New York, 1958, p. 278.
2 Haviland, R. P.. The (Communication Satellite, presented at the Eighth International Astronautical Congress, Barcelona, October 6-12, 1957.
3 Clarke, A. C., Extraterrestrial Relays, Wireless World, October 1945.
The considerable disadvantage of lower satellites is that they would be, at any given time, within line of sight of a smaller area on the earth's surface. For example, a 4,000-mile satellite would be within line of sight of only about 25 percent of the Earth's surface at any one time. Thus, at least four such satellites would be needed to provide worldwide coverage. Actually, taking into account the details of the orbital motion, it turns out that the number necessary to insure worldwide coverage at all times would be greater than four, the exact number depending on the precision with which the orbits can be established. Low-altitude satellites may use data storage devices, such as tape recorders, to retain data received over one point for retransmission later over the intended point of receipt.
Possible communication satellites may be active or passive. An active satellite is one which has transmitting equipment aboard, such as a transponder, a device which receives a signal from Earth, amplifies it, and retransmits the same signal back to Earth (either immediately or after a delay). A passive satellite merely reflects or scatters incident radiation from the Earth, a portion of the radiation being reflected or scattered back in the direction of the Earth.
Passive satellite relays would require surface transmitters of much greater power than would active relays (unless the passive reflectors are extremely large ); however, active satellite relays must carry aboard receiving and transmitting equipment and the necessary power sources, thus decreasing reliability and longevity.
One may provide all active satellite with omnidirectional transmitting antennas (radiating roughly equally in all directions) or directive antennas (radiating most of the energy toward the Earth). Directive antennas would require much less transmitted power, thus saving weight in that part of the payload devoted to transmission equipment and power supply, but would also require antenna stabilization so as to direct the radiated energy toward the Earth, thus increasing the payload weight devoted to attitude stabilization and its power supply.
A passive satellite relay could consist of an omnidirectional scatterer such as a spherical body, like a balloon satellite, or a directive scatterer such as a corner reflector. A corner reflector has the advantage that it tends to reflect radiation in the approximate direction from which the radiation came.
Giving numerical examples can be misleading, since numerical results depend sensitively on the specific values assigned to such quantities as transmitted power, antenna sizes and directivities, operating frequencies, orbital characteristics, and other things. However, if reasonable values of these factors are assumed, it turns out that an active relay could reasonably be expected to require about 0.10 watt of transmitted power (from the satellite) per kilocycle of channel bandwidth, assuming an omnidirectional satellite antenna. (A voice channel requires 5 to 10 kilocycles of band width.) On the other hand, for a passive relay to have approximately the same communication capacity as an active relay which radiates, say, 10 watts, would require a spherical scatterer of the order of a mile in diameter, or a corner reflector of the order of a few hundred feet in diameter. Such sizes do not preclude the use of passive relays, however, since it might be possible to construct such objects, or their equivalent in scattering power, with surprising little weight.
Perhaps the most important question to consider is that of the utility of satellites as components of communication systems, both civil and military. This question is generally one of economics. The eventual feasibility of establishing satellites as communication relays cannot be doubted. On the other hand, alternative methods are available for relaying signals long distances, or around the world, over either land or water. Among these alternative methods are high-frequency radio; very high or ultrahigh frequency wireless communication using land-based, shipborne, or airborne relay stations; transmission lines and cables (including submarine cables); and tropospheric, ionospheric, or meteor-burst scatter propagation. 4 Establishing the usefulness of satellite relays would involve a thorough analysis of the comparative cost of using satellites as opposed to (but, of course, not necessarily to the exclusion of) other available alternatives, for specified levels of capacity and reliability. 5
In assessing the military usefulness of satellites as communication relays, the same sort of cost comparison must be made, taking into account possibly different required levels of capacity and reliability as well as additional factors such as security and vulnerability.
Even in cases where satellites may not be economical in direct dollar cost, they may be very valuable because they can be set up quickly. Extensive long-distance communication systems in some forms take years to complete-a satellite can be launched in a very short time.
Satellites can provide radio broadcasting coverage of wide areas with use of a multiplicity of interconnected ground transmitters.
Satellites can be used to pick up messages transmitted straight up and rebroadcast them straight down at the intended point of receipt even if that point is moving, as on a ship. Low power levels may be used throughout, and interference among neighboring stations on the same frequency eliminated. This is a point of considerable importance, since it would greatly increase the number of stations that could use a given region of the radio-frequency spectrum.
4 Chu, Ta-Shing, Ionospheric Scatter Propagation at Large Scatter Angles, the antenna laboratory department of electrical engineering. the Ohio State University Research Foundation, July 1, 1957.
5 Pierce, J. R., Orbital Radio Relays, Jet Propulsion, vol. 25, No. 4, 1955, p. 153.