Attitude and Orbit Control System For a High Resolution Cartosat-2 Spacecraft

摘要

Cartosat-2 is a very high resolution earth imaging satellite providing panchromatic imaging capability. The spacecraft bus is responsible for providing all on-orbit support required for the camera to obtain imagery as required and for pointing the payload transmission antenna to the prescribed ground station. An Attitude and Orbit Control System (AOCS) has been successfully designed and flight proven. This paper describes the salient features of the Cartosat-2 AOCS, and many innovative concepts evolved and implemented. The AOCS is configured to provide precise attitude and orbit computation while achieving fine pointing control as well as high speed slew capability to achieve rapid target acquisition. The Cartosat-2 spacecraft is a nominally sun pointing spacecraft. The spacecraft is pointed towards the earth only during imaging operations. The spacecraft is maneuvered to the imaging geometry and made to follow the imaging attitude profiles, and target-to-target slew profiles. Upon imaging completion, the spacecraft is slewed back to the sun-pointing attitude. The AOCS employed many innovative concepts like generalized reference quaternion generation, parallel computation of quaternion w.r.t various frames of reference like sun pointing frame, earth pointing frame, ECI frame for usage to switchover between various modes with ease at any instant; rate-to-rate maneuver algorithms to achieve prescribed attitude and attitude rate at precise prescribed times; usage of gyro stellar reference in various star sensor updating profiles; provisions for correcting various sensor and actuator misalignments; many safety features during imaging maneuvers; momentum dumping using three torquers; automated back and forth rotations for orbit maneuvers and various sensor calibration maneuvers; capability for autonomous reconfiguration of star sensors and reaction wheels; various types of safe modes; autonomous attitude acquisition modes etc., This paper also summarizes the onboard performance achieved. The AOCS pointing and platform stability performance have been achieved according to the specifications. The AOCS control accuracy has been achieved better than 36 arc sec during the imaging operations. The platform stability of 3.0e-4 deg/sec around the reference body rates has been achieved during imaging operations. The AOCS for Cartosat-2 has been designed using many innovative concepts and is functioning very well.