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time-division multiplexing (TDM)




time-division multiplexing
An older telecommunications technique in which a spacecraft's onboard computer samples one measurement at a time and transmits it. On Earth, the samples are demultiplexed, that is, assigned back to the measurements which they represent. In order to maintain synchronization between multiplexing and demultiplexing (mux and demux) the spacecraft introduces a known binary number many digits long, called the pseudo-noise code at the beginning of every round of sampling (telemetry frame), which can be searched for by the ground data system. Once recognized, it is used as a starting point, and the measurements can be demuxed since the order of muxing is known. TDM has now been replaced by a newer method known as packetizing.


Related category

   • SPACECRAFT COMMUNICATIONS AND TRACKING