Telemetry

Telemetry refers to data collected by sensors onboard a satellite to monitor the status and performance of its subsystems, including parameters such as pressure, temperature, voltage, and current. This housekeeping and health-monitoring information is transmitted to the ground control station, where it is used to assess spacecraft condition, isolate faults, switch to redundant units, and support ongoing operations. Alarm functions are commonly implemented to alert operators when critical parameters move outside predefined limits.

Telemetry data from individual sensors is typically digitized and multiplexed using time-division multiplexing (TDM) into a composite data stream. This stream is modulated, commonly using phase-shift keying (PSK), onto a low-power, low-data-rate telemetry carrier. Typical telemetry data rates range from about 10 bps to 200 kbps, with target bit error rates on the order of 10⁻⁶. A single TDM frame may contain thousands of bits and require several seconds to transmit. The low data rate allows the ground receiver to operate with a narrow bandwidth, yielding a high carrier-to-noise ratio and robust reception.

In military satellite systems, telemetry—often as part of the TT&C function—is commonly encrypted to prevent unauthorized monitoring or interference.