Much has already been accomplished in space communications; for example:
We may, of course, expect certain practical differences between the conditions of space-flight communications and those of terrestrial communications. Among these are the following:
Communications engineers have already investigated in considerable detail the basic requirements in terms of radiated power, antenna performance, and so forth, for a variety of space-communications tasks.
1 Easton, R. I., U. S. Naval Research Laboratory Report No. 5035, Project Vanguard Report No. 21: Minitrack Report No. 2, The Mark II Minitrack System, September 1957.
2 Richter, H. L., W. F. Sampson and R. Stevens, Microlock: A Minimum Weight Radio Instrumentation System For A Satellite. Vistas in Astronautics (proceedings of the first annual Air Force Office of Scientific Research Astronautics Symposium), Pergamon Press, 1958.
The factors of primary importance in communications are: radiated signal power; area and directivity of transmitting and receiving antennas; communication frequency; receiver sensitivity; external interference; communication range; and channel bandwidth. Others of importance are termed "loss factors," such as absorption losses, polarization losses, losses due to inefficient conversion of consumed power to radiated power, and losses due to inefficient types of modulation.
The feasibility of communication across lunar distances (about 240,000 miles) with present components is well established. With components that can be made available in a relatively few years, communication will be possible over distances as great as 50 million miles (about the distance to Mars or Venus when these planets are relatively close to the Earth) although the communication bandwidth for such a distance would probably be small. For communication to Jupiter (about 500 million miles) or farther, additional technical advances would be required.
Following are some areas of research and development which are important for communications tasks associated with space flight:
Electrical energy and power sources. Clearly, space communications will be dependent upon the availability of electrical energy and power sources. This factor may in many cases be a limiting one in the useful lifetime of a space vehicle, as in the case of the initial Earth satellites.
Radio-frequency power sources. Research is necessary to improve the efficiency of conversion of energy from various sources to electromagnetic energy of the desired frequency.
Data storage and data encoding. Many situations will arise in space flight applications in which it is necessary to store data for retransmission at some later time, or at a slower rate than they are received.
Also, greater communication efficiency, resulting in power savings, can be achieved by improving methods of signal encoding or modulation.
Receiver sensitivity. In communication engineering, interference with a received signal is referred to as "noise." There are two kinds: receiver noise, which arises in the receiver itself, and external noise. Major sources of external noise are man-made interference, and solar and cosmic radiations. At the present time, receiver noise, in the majority of cases, is the most important limiting factor in sensitivity of signal reception. However, certain ultrasensitive types of receivers, such as cooled detectors and masers,3 4 are well along in development, and these may reduce receiver noise to 10 percent or as little as 1 percent as that in present-day receivers. With such receivers, external noise would become the dominant type of interference in space communications, and because of it, overall interference levels might be reduced only 80 or 90 percent, even if receiver noise were reduced a hundredfold. Since power requirements are proportional to the overall interference level, however, even an 80-percent reduction is worth striving for.
3 Culver, W. H. The Maser: A Molecular Amplifier for Microwave Radiation, Science vol. 126, No. 3278, October 1957.
4 Higa, W. H., Maser Engineering, External Publication No. 381, Jet Propulsion Laboratory, California Institute of Technology, April 25, 1957.
Directive vehicle antennas. Antennas may be either omnidirectional-that is, radiating energy roughly equally in all directions or receiving energy equally from all directions-or directive, radiating to or receiving from a preferred direction. The power saving from using directive antennas is very large-from ten to a thousand times depending upon the directivity. However, the use of directive antennas on space vehicles would require a certain amount of attitude stabilization of the antenna to insure that the energy is radiated to or received from the right direction in space. This-could be achieved most easily by attitude stabilization of the vehicle itself. The better the attitude stabilization, the more highly directive all the antennas and the greater the resultant power savings. Attitude control of antenna beams on space vehicles, therefore, offers one of the most promising avenues for research and development.
Very large surface antennas. Radio astronomy, the air-defense surveillance net, and current space-flight tracking activities are building up a backlog of experience in the use of very large steerable ground antennas, which will be needed for many space communication missions.
Circuit components. Work must be continued on improved miniaturization and packaging of components; greatly improved reliability for unattended operation up to perhaps several years; and investigation of, and protection from, damage caused by meteoric impact or radiation in space.
Research on cosmic, solar, and other external noise sources. As noted above solar and cosmic noise may become the main source of signal interference with the advent of ultrasensitive receivers. Radio astronomers5 have already conducted much detailed investigation of the intensity of solar and cosmic noise as a function of direction in space, frequency, and solar activity. Continued research in this field is necessary with a view to compiling the most complete maps possible of noise intensity as related to these various factors.
Research on physics of the solar system. The fact that the Earth's gaseous atmosphere and ionosphere crucially affect present-day communication is well known. Similarly, the atmospheres and ionospheres of other bodies in the solar system will affect communication to or from the surface of these bodies. In the vicinity of the Earth's moon, which has negligible gaseous atmosphere, electron densities may reach the value of 1 million or 10 million per cubic centimeter, which would certainly affect communication in this region.6 Even in the space between Earth and Moon, electron densities may be as much as 1,000 per cubic centimeter. This affects, among other things, the velocity of light in Earth-Moon space. More precise measurement of electron densities in these regions constitutes an immediate possibility for useful research.
5 Proceedings of the Institute of Radio Engineers, radio astronomy issue, vol. 46, No. 1. January 1948.
6 Chapman, S., Notes on the Solar Corona and the Terrestrial Ionosphere, Smithsonian Contributions to Astrophysics, vol. 2, pp, 1-14,1957.