lites have body mounted arrays of solar cells, One disadvantage of deployed arrays is the limited amount of typically producing about 10 watts per kilogram of solar array mass. power available before the array can be deployed, that is, while the Body stabilised satellites using extendable arrays, which can be satellite is still in its transfer orbit. rotated SO that they always face the Sun, deliver up to 23W/kg. Current research is aimed at increasing the dimensions of de- 51.3.6 Telemetry, tracking and command ployable solar panels and reducing their mass. Values are reaching 50 to 60 watts per kilogram and a few tens of kilowatts per panel. Telemetry, tracking and command (TT&C) facilities are needed, in High efficiency solar cells now being developed could cut substan- the launch phase and in normal operation of communication satel- tially the mass and size of arrays. lites, for a variety of purposes. Telemetry is used to monitor and 51/10 Communication satellites and systems evaluate the performance and behaviour of the satellite and to satellite transponder. A carrier transmitted from one earth station provide the ground control station with data for the diagnosis of may be received at many earth stations. There may be several or fault conditions that may arise. Tracking facilities, used in conjunc- many other pairs of earth stations passing carriers through the same tion with the command system, enable the location, velocity and satellite transponder simultaneously or in sequence, which is called orbit of the satellite to be determined, enabling the orbit and the multiple access. There may be other transponders in the satellite, nominal location in orbit to be set up initially, then maintained isolated from one another by frequency separation, satellite antenna throughout the satellite's operating life. Command facilities are directivity or polarisation discrimination and relaying different required for the initiation of all the manoeuvres which cannot be groups of carriers. The techniques which allow sate