Space communications

Space communications

Communications between a vehicle in outer space and Earth, using high-frequency electromagnetic radiation (radio waves). Provision for such communication is an essential requirement of any space mission. The total communication system ordinarily includes (1) command, the transmission of instructions to the spacecraft; (2) telemetry, the transmission of scientific and applications data from the spacecraft to Earth; and (3) tracking, the determination of the distance (range) from Earth to the spacecraft and its radial velocity (range-rate) toward or away from Earth by the measurement of the round-trip radio transmission time and Doppler frequency shift (magnitude and direction). A specialized but commercially important application, which is excluded from consideration here, is the communications satellite system in which the spacecraft serves solely as a relay station between remote points on Earth. See Telemetering

Certain characteristic constraints distinguish space communication systems from their terrestrial counterparts. Although only line-of-sight propagation is required, both the transmitter and the receiver are usually in motion. The movement of satellites relative to the rotating Earth, for example, requires geographically dispersed Earth stations to achieve adequate communication with the spacecraft on each orbit.

Because enormous distances are involved (over a billion miles to the planets beyond Jupiter), the signal received on Earth from deep-space probes is so small that local interference, both artificial and natural, has to be drastically reduced. For this purpose, the transmitted frequency has to be sufficiently high, in the gigahertz range, to reduce noise originating in the Milky Way Galaxy (galactic noise background). The receiver site must be remote from technologically advanced population centers to reduce artificial noise, and at a dry location to avoid precipitation attenuation of the radio signal as well as the higher antenna thermal noise associated with higher atmospheric absolute humidity and relatively warm cloud droplets. The receiving antennas must be steerable and large, typically 85 ft (26 m) or at times 210 ft (64 m) in diameter, to enhance the received signal strength relative to the galactic noise background. Special low-noise preamplifiers such as cooled masers are mounted on the Earth receiver antenna feed to reduce the receiver input thermal noise background. Sophisticated digital data processing is required, and the ground-receiver complex includes large high-speed computers and associated processing equipment. See Maser, Preamplifier

The spacecraft communications equipment is constrained by severe power, weight, and space limitations. Typical communications equipment mass ranges from 25 to 220 lb (12 to 100 kg). Another major challenge is reliability, since the equipment must operate for years, sometimes for decades, unattended, in the difficult radiation, vacuum, and thermal environment of space. Highly reliable components and equipment have been developed, and redundancy is employed to eliminate almost all single-point failures. For example, it is not unusual to have as many as three redundant command receivers operating continuously, because without at least one such receiver in operation no command can get through, including a command to switch from a failed command receiver to a backup radio. Power can be saved by putting some or all of the redundant radios on timers, and to switch to a backup receiver if no commands have been received through the primary receiver within a predetermined interval; but the saved power may come at the cost of a possible delay in emergency response initiation.

Spacecraft power is always at a premium, and other techniques must also be used to minimize its consumption by the communication system. The transmitter is a major power consumer, so its efficiency must be maximized. All aspects of data transmission must contribute to error-free (very low bit error rate) reproduction of the telemetry data using no more power or bandwidth than is absolutely essential. Pulse-code modulation is a common technique which helps meet this goal. In general terms, space communication systems are far less forgiving than terrestrial systems and must be designed, constructed, and tested to much higher standards. See Space technology

The Tracking and Data Relay Satellite System (TDRSS) consists of a series of geostationary spacecraft and an Earth terminal located at White Sands, New Mexico. The purpose of TDRSS is to provide telecommunication services between low-Earth-orbiting (LEO) user spacecraft and user control centers. A principal advantage of the system is the elimination of the need for many of the worldwide ground stations for tracking such spacecraft. The Tracking and Data Relay Satellite (TDRS) provides no processing of data; rather, it translates received signals in frequency and retransmits them. User orbits are calculated from range and range-rate data obtained through the TDRS by using transponders on the user spacecraft.