time-division multiplexing (TDM)
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TDM has now been replaced by a newer method known as packetizing.
COMMUNICATIONS AND TRACKING