The telemetry/telecommand link (TM/TC) is the vital communication channel connecting SPOT 5 to the CMP operations control centre.
The TM/TC maintains dialogue between the satellite and ground controllers. Most of the time the satellite is left to operate unassisted. SPOT 5 can only communicate with ground control for 5 per cent of each orbital revolution, when it is within view of a station in the 2-GHz network.

The TM/TC is a two-way or "duplex" link.

Telemetry

The telemetry downlink enables ground controllers to keep a check on the satellite's health and analyze anomalies detected by its onboard computer. Telemetry also communicates various readings to the CMP-temperatures, voltages, currents and other readings concerning every subsystem-for long-term monitoring of the satellite.

 

Telecommand

The telecommand uplink serves to send routine programming instructions to fulfil commercial imaging requests from Spot Image and to command the satellite to carry out specific actions, for example to perform orbital manoeuvres, run equipment tests and handle anomalies and malfunctions.

 

Location

The TM/TC link also transmits location measurements for precise determination of the satellite's orbit. These include measurements of the signal return trip time to calculate the satellite's range, i.e., the distance to the ground station, and Doppler measurements of its velocity relative to the station.

 

Subsystem architecture

Two antennas-S1 on the Earth-facing side of the satellite and S2 on the opposite side-receive telecommands and send telemetry. These antennas provide omnidirectional coverage to maintain the link with the satellite whatever its attitude-geocentric in nominal mode, random during the launch phase and Earth-pointing in safe mode.

Two transponder assemblies are connected to the two antennas via a hybrid coupler that switches uplink and downlink signals. Two receivers are always operational, one providing hot redundancy, as the satellite must be able to receive telecommands at all times. If the nominal receiver fails, the second one takes over.