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The communication subsystem provides capabilities for telemetry with bit rates up to 8 kbits-1, ranging, telecommand and radio science. It operates in X-band (20 W, downlink) and S-band (5 W, up- and downlink). The subsystem includes two redundant transponders (each consisting of an X-band exciter, a modulator, an S-band receiver and an S-band power amplifier), two redundant 20 W X-band Travelling-Wave-Tube Amplifiers (TWTA), a TWTA Interface Unit and an S-band Radio-Frequency Distribution Unit. A considerble amount of cross-coupling capability exists within the subsystem.

The 1.65-m diameter parabolic High Gain Antenna (HGA), with both X-band (8.4 GHz) and S-band (2.3 GHz) capabilities, is the prime communications link. Telemetry is provided in X-band, with a 2° beamwidth (3 dB); downlink S-band is used for ranging, and radio- science investigations. S- or X-band ranging operations can be performed with or without telemetry transmission. Both transponders can be operated simultaneously, one in X-band and the other in S-band.

The HGA is kept pointing toward Earth throughout the mission. A special feature of the HGA is its ability to measure the offset of the spin axis from the direction of the ground station by the CONSCAN (conical scan) system. This is accomplished by a tilt of 1.8° between the S-band antenna pattern and the spin axis which results in a spin modulation in the uplink signal strength as the satellite rotates. Processing within the Attitude and Orbit Control Subsystem (AOCS) gives the offset magnitude and direction which is either transmitted for ground analysis or employed in a closed loop control system to minimise the offset. Attitude adjustments are made by operating hydrazine thrusters.

Last Update: 30 November 2006

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