Simulation-based concept for low cost attitude operations of Equator-S

Kurzfassung

Equator-S is a scientific satellite for the investigation of the magnetosphere of the earth under the responsibility of the Max-Planck-Institute für extraterrestrische Physik (MPE) in Garching, Germany. Mission Operations are performed by DLR's German Space Operations Center. The spacecraft has been launched on Dec. 4, 1997 into a final highly eccentric 500 x 65000 km orbit where it was spin-stabilised with magnetic torquers to control both spin rate and spin axis direction. The torquing maneuvers took place within 2,5 Earth radii altitude around perigee. After separation from the Ariane-4 launcher the main attitude maneuvers were the changes of the spin axis attitude from nearly parallel to the ecliptic up to 90°. There is a constraint for the sun-aspect-angle of the solar panels mounted on the surfaces of the cylindrical spacecraft. Therefore it is urgent to control not only the final spin axis orientation but also the trajectory from the initial to the final attitude. For this process the often used Shigehara algorithm had been implemented which uses the actual attitude, a selectable target attitude, orbit information and a model of the earth's magnetic field. By reducing the error in the angular momentum using an asymptotic stability criteria a switching function for the magnetic coils for periods around perigee is derived. Originally this strategy requires attitude determination and commanding capability once per orbit (every 22 h for Equator-S) to feed the Shigehara algorithm. In order to reduce ground station time and operator workload a different approach was implemented by using dedicated flight dynamics simulation and attitude determination algorithms. As described in earlier papers an object oriented approach for the flight dynamics simulation had been developed for mission analysis and preparation. The capabilities of this system turned out to be the key for low cost attitude operations. Its performance had been validated to be precise enough to predict the attitude dynamics for 4 to 6 days. I.e. the crucial torquer and antenna switching commands could be prepared in advance for usually 4 days. The simulation results were cross-checked and the system parameters validated using attitude determination results despite downlink intervals less frequent than planned. In this paper the results of this more offline-oriented low cost operations strategy are presented and it is shown how precise the attitude control requirements could be met. It is demonstrated how operational resources can be saved by efforts in mission preparations and sophisticated flight dynamics systems even and especially for small satellite missions.