Satellite Communications Glossary

C band.  The C band is a band of frequencies used for terrestrial radio relay and satellite transmissions. The band is commonly called 6/4 GHz to illustrate that the uplink frequencies are around 6 GHz and the downlink frequencies are around 4 GHz. The band of frequencies are between 5.925-6.425 GHz for uplinks and 3.7-4.2 GHz for downlinks.

Cassegrain.  A Cassegrain antenna has a parabolic reflector that focuses the incoming radiation in satellite communications onto a hyperbolic sub-reflector and thence into the waveguide.

Channel coding.  The numbers of errors at the output of the receiver can be reduced by channel coding, which involves adding additional information to the message to increase the receiver’s decision-making ability. This additional, redundant information increases the overhead that is included in the transmission.

Code-division multiple access (CDMA).  Code-division multiple access (CDMA) is a multiple access technique in satellite communications in which each user of a satellite channel is given access to the entire channel all the time, with users separated by giving each a unique spreading code.

Cold Welding.  In a vacuum, surfaces in direct contact do not have a layer of gas separating them and a process called cold welding causes the surfaces to diffuse into one another and to bind solidly.

Communications Moon Relay I (CMR).  The Communications Moon Relay (CMR) was established between 1960 and 1962 to provide reliable long-distance satellite communications between Washington DC and Hawaii.

Communications Subsystem.  The fundamental function of the communications satellite is to provide the Communications Subsystem, which comprises a number of clusters of receivers, frequency translators and transmitters called transponders. The total bandwidth of the satellite is provided by a number of transponders, each of which has a typical bandwidth of 36 MHz.

Comsat.  In 1962, the US Congress passed the Communications Satellite Act that authorised the establishment of the Communications Satellite Corporation (COMSAT).

Convolutional code.  A convolutional code extends the concept of a block code to allow memory from block to block. Each encoded symbol is therefore a linear combination of information symbols in the current block and a selected number of preceding blocks. Satellite communications systems commonly use a convolutional code to protect all data carried on the link.

Courier.  On 4 October 1960, the US Department of Defense launched Courier into an elliptical orbit between 956 km and 1 240 km, with a period of 107 minutes and an inclination of 28.3º to the Equator.

Cyclic code.  Cyclic codes are an important sub-class of linear block codes for error detection, where a new codeword in the code can be formed by shifting the elements along one place and taking one off the end and putting it on to the beginning. Satellite communications systems commonly use cyclic codes.

Cyclic redundancy check (CRC).  A cyclic redundancy check (CRC) is a very efficient error detection algorithm that is commonly used in satellite communications.

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