Space Communications

the transmission of information between the earth and spacecraft, between two or more points on the earth via spacecraft or using artificial means located in space (a belt of needles, a cloud of ionized particles, and so on), and between two or more spacecraft. Space communications systems are widely used for various purposes: for the transmission of telemetry, telephone, telegraph, television, and other information; for the transmission of command and control signals to spacecraft; and for carrying out trajectory measurements. Radio communication is most widely used in space communications systems.

The main features of space communications systems that distinguish them from earth communications systems are the continuous, often very rapid change in the position of the spacecraft, the necessity of knowing the current coordinates of the spacecraft and of aiming the receiving and transmitting antennas of the earth communications station toward a given spacecraft, the continuous change in the frequency of the received signals resulting from the Doppler effect, the areas of mutual visibility of the spacecraft and a point on the earth (which are limited and vary with time), the limited power of radio transmitting apparatus on board the spacecraft, and the long communications ranges, which result in operation with very low-level received radio signals. These considerations resulted in the creation of special systems consisting of complex apparatus such as large guided antennas, receiving apparatus with a very low noise level, and highly efficient systems for detecting, discriminating, and recording radio signals. The need to know the current position of the spacecraft requires periodic measurements of its coordinates and calculations of the parameters of its trajectory. Thus, a space communications system is usually the combined operation of measuring apparatus (a trajectory-measuring system), a computer center, and a spacecraft control system. Various frequency ranges are used for the radio channels of a space communications system, depending on routing and purpose. Their assignment and sequence of use are determined by radio communications regulations.

Earth-to-spacecraft communications. Communications systems between a point on the earth and a spacecraft are designed to provide two-way transmission of all types of necessary information. Communications with distant spacecraft (unmanned interplanetary probes) are characterized by very low levels of received radio signals and long periods of mutual visibility, since the variation in the direction between the spacecraft and a point on the earth is determined primarily by the diurnal rotation of the earth. Communications with nearby spacecraft (artificial earth satellites, manned spacecraft, and unmanned orbital space stations) are characterized by rapid changes in the direction of communication, a short period of mutual visibility, relatively short range, and a correspondingly high level of radio signals.

The earth-to-spacecraft and spacecraft-to-earth links carry different information loads and have different power capabilities. An earth-to-spacecraft link provides for transmission of command signals and trajectory measurements to the spacecraft and for telephone, telegraph, and television communication with the cosmonauts in manned spacecraft. A spacecraft-to-earth link usually has a substantially lower power capability, because the spacecraft’s transmitter power is lower than that of the earth station (the power of spacecraft transmitters ranges from a few to several dozen watts, and that of transmitters at earth stations ranges from a few to dozens of kilowatts). However, the main information flow is along the spacecraft-to-earth link. This requires the use at ground stations of antennas with very large effective areas (dozens of square meters) to receive data from the spacecraft; when receiving data from interplanetary spacecraft, the effective area must be hundreds and thousands of square meters, since the power of the received signal decreases as the square of the distance. Effective areas of 2,000-5,000 sq m can be attained only in unique, expensive antenna systems, which can provide telephone communication at interplanetary distances.

Radio communication with a man in space began on Apr. 12, 1961, when cosmonaut Iu. A. Gagarin, in the spacecraft Vostok, became the first man in history to orbit the earth; he maintained steady two-way telephone-telegraph communication with the earth on meter and decameter wavelengths. In subsequent flights of Vostok and Voskhod manned spacecraft, radio communication with the earth was improved and was tested successfully between spacecraft in group flights. At the time of the flight of the Vostok 2 in August 1961 the first television image of cosmonaut G. S. Titov was transmitted from space to the earth. To transmit the television image the number of frames was reduced to 10 per second in order to narrow the frequency spectrum. The use of television systems with the usual standard became possible later.

The longest radio communication distance was achieved during flights of unmanned interplanetary probes. For example, in flights to Mars the communications range between the earth station and the probe reached 350 million km, and for the Jupiter flight it was 800 million to 900 million km. To provide for such long-range communications an antenna on the probe aimed at the earth is usually used.

Satellite communications. Long-distance communication is usually provided by line-of-sight microwave links, which consist of two terminal stations and a series of intermediate repeater stations separated by direct line-of-sight distances (50-70 km). If a repeater is placed on board an artificial earth satellite with a high orbit, communication can be achieved between two points thousands of kilometers apart. In this case the maximum range for direct communication is determined by the possibility of seeing the satellite simultaneously from both points. Such communications satellites can be used both in individual communications links and in networks of microwave links to transmit television programs, multichannel telephone and telegraph signals, and other forms of information. An example of a network with a large number of earth stations is the Orbita system, which has been operating in the Soviet Union since 1967. Satellites that are in different orbits and at different altitudes may be used for communication. The principal orbits for communications satellites are the circular stationary orbit, the extremely elongated elliptical synchronous orbit, the medium-high circular orbit, and the low circular orbit.

A satellite in a stationary orbit is constantly situated (”suspended”) over a chosen point on the equator and provides round-the-clock communication between earth stations at latitudes less than 75° within a radius of 8,000 km from the point over which the satellite (for example, Intelsat) is positioned. Equidistant distribution of three such satellites around the equator provides communications for any earth stations within the indicated latitude limits. For regions located at latitudes above 70°-75°, extremely elongated elliptical synchronous orbits with an apogee over the center of the link being served and an orbital period of 12 or 24 hours are the most suitable. If the angle of inclination and the location of the orbital apogee are selected properly, the satellite will be within the line of sight of a specified region for a substantial portion of the day. Large antennas are used at the earth stations for operation with a satellite in a stationary or an elliptical synchronous orbit, since the distance between the satellite and the earth exceeds 30,000 km and the power of the received signal is low. Satellites in medium-high and low circular orbits, such as the Kur’er and Rele satellites, provide considerably higher power of the signals received. However, a reduction in the altitude of flight shortens the time of mutual visibility between the satellite and the earth communications station, necessitating a substantial increase in the number of satellites needed for continuous communication. In addition, the system of tracking and antenna guidance at the earth stations becomes complicated. For low altitudes of flight, direct communication is not possible between points that are very far apart, and it is necessary to use a system of radio links, with delayed relaying. However, in this case the levels of the received signals are adequate, and large and expensive antenna systems are not needed; as a result, communication with low-flying satellites is possible even using small, mobile earth stations.

A communications satellite for through transmission of signals can be equipped with an active repeater, which also provides amplification of the signals, or it may be a passive repeater (that is, a reflector). In addition to satellites in the form of reflectors, communications links using scattered reflectors in the form of a belt of needles or a cloud of ionized particles have been proposed and tested. A passive repeater can serve a radio network composed of a large number of links with different signal frequencies, since it reflects or scatters the energy of many incoming radio signals simultaneously, without interference (for example, the Echo satellite). On the other hand, an active repeater can only serve a communication network with a limited number of links; in addition, to avoid interference, frequency, timing, or coding separation of the signals must be used and the required level of the signals must be maintained without overloading the repeater. Nevertheless, active repeater systems, which provide simultaneous transmission of information over several (up to ten) television channels or several thousand telephone channels (for example, the Molniia, Intelsat, and Syncom satellites), are the most popular.

In the interests of economy, multichannel radio communication links are used, leading to the necessity of increasing the passband in a link. A wide passband is also required for relaying television signals. As the passband is increased, the risk of distortion of the messages by interference will increase. Consequently, the reception of messages with acceptable distortion is the most important problem, which may be solved by increasing the power of the radio signals, by selecting the communication frequencies, by reducing the noise level of the radio receivers, by using efficient coding, and by choosing the type of modulation, reception, and processing for radio signals with a low signal-to-noise ratio. For example, frequencies of 1 to 10 gigahertz are chosen for radio signals, since the interference from space noise is sharply higher at lower frequencies and that from atmospheric noise is higher at higher frequencies; the first stages of the earth stations’ receiver amplifiers use low-noise quantum-mechanical and parametric amplifiers cooled by liquid helium.

To provide the necessary level of the received signal in communication links with a passive repeater, the transmitter power and antenna size of the earth station, as well as the size of the repeater’s reflector are increased, or repeaters with directional scattering of energy toward the earth station are used, or the passband in the link is narrowed and the information transmission rate is reduced. Such measures have limitations because of the higher cost of the equipment for the communications link and for its operation.

Communication between spacecraft. Communication is carried out between spacecraft in order to exchange information between the crews of two or more spacecraft that are in space at the same time and between the crews of spacecraft and cosmonauts who are outside their craft. Communication may also be established between two unmanned spacecraft for the purpose of relaying signals, positional measurements and navigation, flight control, and rendezvous information. Some of the particular features of communication between spacecraft are as follows: (1) communication usually takes place between interacting spacecraft—that is, between artificial earth satellites at comparatively short ranges (the spacecraft Vostok 3 and Vostok 4 or Vostok 5 and Vostok 6); (2) because of difficulties in aiming the antennas of the spacecraft, nondirectional communication is preferable; (3) the absence of atmospheric effects—and, in higher orbits, ionospheric effects—permits a freer choice of frequency range and the use of optical communications facilities; (4) possible interference from powerful earth stations must be taken into account when choosing the frequency range and the communication arrangements between satellites; (5) space communications systems become complicated in the case of the landing of space expeditions on the moon (for example, the Apollo spacecraft) or other celestial bodies because of the necessity of maintaining communication between the expedition and the spacecraft, which remains in a circumplanetary orbit, and (through the spacecraft or directly) between the expedition and the earth (in this case all the features of communication between a satellite and an earth station and between distant spacecraft and the earth stations are combined).

The future development of systems for the transmission of television programs through stationary satellites directly to television receivers is possible; this would create the possibility of a complete telecommunications system that would provide for the transmission of central programs to any place on the earth. As lasers become more advanced, optical communication becomes promising, since messages may be transmitted over very great distances (up to dozens of light years) in the visible band because of the very high directivity of the beam (the divergence is no more than fractions of a second of arc), with relatively small radiators and acceptable power requirements. However, highly directional radiation and reception in the visible band requires careful stabilization of the equipment and accurate aiming of the spacecraft’s optical systems, and the establishment and maintenance of communication are difficult. Optical communications links between spacecraft are most satisfactory outside the earth’s atmosphere, because the atmosphere strongly absorbs and scatters optical waves.