Saika Aida

Flight Dynamics Operations of the TanDEM-X Formation

Since end of 2010 the German TerraSAR-X and TanDEM-X satellites are routinely operated as the first configurable single-pass Synthetic Aperture Radar interferometer in space. The two 1340 kg satellites fly in a 514 km sun-synchronous orbit. In order to collect sufficient measurements for the generation of a global digital elevation model and to demonstrate new interferometric SAR techniques and applications, more than three years of formation flying are foreseen with flexible baselines ranging from 150 m to few kilometers. As a prerequisite for the close formation flight an extensive flight dynamics system was established at DLR/GSOC, which comprises of GPS-based absolute and relative navigation and impulsive orbit and formation control. Daily formation maintenance maneuvers are performed by TanDEM-X to counterbalance natural and artificial disturbances. The paper elaborates on the routine flight dynamics operations and its interactions with mission planning and ground-station network. The navigation and formation control concepts and the achieved control accuracy are briefly outlined. Furthermore, the paper addresses non-routine operations experienced during formation acquisition, frequent formation reconfiguration, formation maintenance problems and space debris collision avoidance, which is even more challenging than for single-satellite operations. In particular two close approaches of debris are presented, which were experienced in March 2011 and April 2012. Finally, a formation break-up procedure is discussed which could be executed in case of severe onboard failures.

Collision Avoidance Operations for LEO Satellites Controlled by GSOC

The ever increasing population of objects in the near Earth environment has created growing concerns among satellite owners and control centers about the safety of their missions. The GSOC (German Space Operations Center) is currently building up an operational proximity monitoring and mitigation concept. Contrary to locally operated satellites, high accurate orbital parameters are not available for the bulk of other space objects. Currently, the TLE (Two-Line Elements) catalogue maintained by the USSTRATCOM (US Strategic Command) constitutes the only publicly available and reasonably comprehensive orbit information. Despite evident deficiencies in the quality and timeliness of this information, it is currently a mandatory element for any operational proximity monitoring. The careful assessment of the TLE accuracy is therefore required to reveal the inherent modeling accuracy of the SGP4 analytical orbit model, as well as the orbit determination and orbit prediction accuracy for TLEs provided by USSTRATCOM. Even after a realistic error analysis, the exclusive use of TLE data is insufficient for a proper planning and implementation of collision avoidance maneuvers. The orbit information of a possible jeopardizing object has to be refined in due time before a predicted proximity, if a predefined threshold of collision probability or safety distance is violated. To this end, the use of radar tracking is foreseen. The orbit refinement using radar tracking is necessary for a consolidated decision and implementation of an evasive maneuver. Following a presentation of GSOC collision avoidance procedure for LEO (Low Earth Orbit) satellites, the paper will discuss the orbit accuracy as well as the improvement of the TLE orbit information by a radar tracking campaign. The orbit accuracy analysis is done by comparing corresponding orbit data with accurate orbit information from locally controlled space missions. The application to the collision risk monitoring system at GSOC is discussed hereafter, followed by the presentation of the monitoring system and the handling of close approaches.