Formation Flight in Low-Earth-Orbit at 150 m Distance - AOCS In-Orbit Experience

AOCS operations for two satellites flying at ~514 km altitude with unprecedented small separation of 150 m upwards pose various new challenges and require creative solutions. Several on-ground safety measures were installed, but due to the short reaction times emphasis always lies with the on-board handling of problems. This is achieved by implementation of a data link (one-way) between the satellites, by the design of a new AOCS safe mode that has no effect on the orbit, by the complete re-work of the TerraSAR-X FDIR concept (Fault Detection, Isolation and Recovery) and by autonomous formation control with a dedicated cold gas system on the second satellite TanDEM-X. This paper describes AOCS operations for the TanDEM-X mission in close formation. Most of the tasks are, thanks to extended safety measures and automated control mechanisms, routine. This will be described in the first section, where a short summary of all AOCS related operations will be given. The second section describes some special activities in connection with the cold gas system and the star trackers on TDX. The investigations and the fine tuning of the GPS receivers on both satellites are also described in this section. Although there was never a threat to the formation, close monitoring is required all the time and some mitigation measures were called for. Finally, the influence of the approaching solar maximum in 2013 will also be described in this section. The last part presents the conclusions and contains some suggestions for future missions. Nomenclature Δv = increment in velocity e = eccentricity i = inclination