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Guidance and navigation

The primary operational functions of the Attitude and Orbit Control Subsystem (AOCS) are to maintain the spacecraft spin axis Earth-pointing and control the spin rate (~5 rpm). Additional operational functions are dictated by trajectory control requirements, nutation damping, and by the measurement of the attitude for scientific reasons.

The spacecraft Earth-pointing attitude is measured and controlled by the CONSCAN system with spin-rate, spin-phase and solar-aspect-angle information determined from redundant Sun sensors. The sun-sensor outputs are processed in the data-handling subsystem to provide the spin reference pulse and the spin segment clock.

The signals and the Sun-sensor data are then used in the AOCS electronics to determine the spacecraft spin rate and solar aspect angle for the purpose of closed loop on-board control, failure detection and recovery. Eight hydrazine thrusters are actuated either by telecommand or autonomously. These are fed from a single tank, mounted at the launch centre of gravity, and arranged in two blocks of four thrusters each, providing complete redundancy.

Another AOCS operation is the periodic precession manoeuvring for correction of the apparent Earth drift with respect to the spin axis. These can be performed in closed loop on-board or in open loop via ground or time- tagged command.

A special manoeuvre strategy is required for conjunctions, since the proper spacecraft attitude depends on the operation of the Sun sensors with a safe operational limit of the solar aspect angle greater than 1.25°.

The AOCS also includes failure-mode-detection and protection functions which result in fail-safe operation and a reacquisition capability in both automatic and ground initiated recovery sequences. The AOCS also provides autonomous system capabilities for safe spacecraft reconfiguration. This is required during unexpected and/or predicted periods of nontracking and because of the long signal travel time between ground and spacecraft.

The preprogrammable functions include search-mode initiation to reacquire the Earth if no command is received after a preselectabe time of up to 30 days, switch-over to redundant units, and preprogrammed attitude manoeuvres at superior conjunctions.

Last Update: 30 November 2006

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