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Ground Station Components
Satellite telemetry system .
on a special link received and then monitored. In the first method, time division multiplexing,
the measurements are either buffered or integrated into the data to be transmitted. In the second
method, frequency division multiplexing, the measurements are sent at particular frequencies. An
example of a satellite telemetry system can be seen in Fig. 11.1.
Packet format is flexible in the type of data to be sent. The data are sent in a burst and have an
address heading indicating where their destination is. This allows the use of different packet types
each time the data are sent. Figure 11.2 shows the possible flow of the data into the packet format.
Telemetry frames have more of a definite fixed structure and tend to be used when lots of data
will be sent with multiple measurements over time. In each frame, the measurements are usually
in the same place in the frame.
In general the payload would be the sensor(s), or the device that accumulates data, and the
ground station is where the data would be processed. The channel refers to the transmission
medium between where the data are acquired and where the data are delivered. There are two
main formats for telemetry data. The data are either put into packets (used especially for Internet
users) or put into telemetry frames. Examples of the packet telemetry are shown in Figs. 11.2 and
11.3, and examples of frame telemetry are shown in Figs. 11.4 and 11.5.
The internal ordering of packet or frame telemetry can change for different uses, but the outer
structure of each is the same no matter what the application.
Today, telemetry includes images and voice data as well as the housekeeping type of data.
Some of the telemetry used by the Air Force interleaves the pilot's voice data with the aircraft
monitoring data. Each military exercise that is performed has its own telemetry order. Hence every
telemetry data packet or frame is unique. This adds to the complications of compression. Each
test could require a different compression technique.