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 nonroutine 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.
On June 21, 2010 the TanDEM-X satellite (TDX) was injected into orbit at 15,700 km distance from its twin satellite TerraSAR-X (TSX), which has been in orbit since 2007. Already one month later TDX acquired a formation with TSX in order to build up the first single-pass radar interferometer in space. Within three years of close formation flying with flexible baselines ranging from 150 m to a few kilometers the twin satellites will collect interferometric radar measurements for the generation of a global digital elevation model with unprecedented accuracy. This paper elaborates on the TDX pre-launch analysis performed in the fields of collision assessment during orbit injection and target formation acquisition. To avoid a critical close approach shortly after TDX separation, the risk of collision between the already flying TSX satellite and the newly injected elements (DNEPR upper-stage, gas dynamic shield, and TDX satellite) had to be carefully analyzed. Further, the paper discusses a fuel-saving formation acquisition strategy, for which the maneuver budget is analyzed as a function of launch day and launch injection accuracy. Finally, flight results are presented to illustrate the successful formation acquisition realized in July 2010 and the formation reconfiguration process from the 20 km wide formation into the 300-400 m close formation performed in October 2010. This reconfiguration marked the start of the bi-static TDX/TSX instrument operation.
The German Space Operations Center (GSOC) is monitoring close approaches of the operational satellites against the tracked space objects. Contrary to the controlled satellites, precise orbit information is not available for a massive 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, which has been used for such a monitoring. In addition to TLEs, warnings from the Joint Space Operations Center (JSpOC) can be recently used as another source for the proximity prediction. Although JSpOC provides orbit information including covariance information with a relatively higher accuracy, its availability is limited and the accuracy is still not enough for a maneuver decision and also for a proper planning of an avoidance maneuver. An orbit refinement using a radar tracking is therefore foreseen in case of a critical close approach. This paper describes the operational collision avoidance system, followed by the discussion of the orbit prediction accuracy of the TLE propagation as well as the numerical propagation from the operational point of view. The radar tracking accuracy is additionally presented for a comparison. A recent close approach of TerraSAR-X is presented as an example of the event handling together with the radar tracking results performed for the debris, followed by the operational experiences for the last 1.5 years.
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