Guidance and navigation
Maintenance of the orbit and attitude of the spacecraft is performed by the Attitude and Orbit Control and Measurement Subsystem (AOCMS). Spacecraft attitude and spin data are provided by an internally redundant star mapper and an internally redundant X-beam Sun sensor. The reconstitution of attitude data, such as inertial attitude, spin rate and spin phase, is performed on the ground. This information is essential for the interpretation of the payload science data. The necessary accuracies for these attitude data are comfortably met by the subsystem.
Orbit and attitude maintenance are performed by using control thrusters, both semi-radial and axial, together with the main engine, which are used to perform the large orbital change manoeuvres required to reach the polar mission orbit from geostationary transfer orbit.