TanDEM-X Close Formation Flight AOCS Safety Measures and Operations

Kurzfassung

The TanDEM-X project is implemented by a “Public-Private Partnership” between the German Aerospace Centre (DLR) and Astrium GmbH. The primary goal of the TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) mission is to generate a global digital elevation model. To achieve this, two satellites – TerraSAR-X (TSX) and TanDEM-X (TDX), satellites of almost identical construction in orbit since June 2007 and June 2010 respectively – form the first configurable SAR (Synthetic Aperture Radar) interferometer in space with a separation of only a few hundred metres. A powerful ground segment, which is interlaced with that of TSX, completes the TanDEM-X system. The satellites fly in close formation since December 2010 and will be operated in parallel for three years to cover the entire surface of the Earth. DLR is responsible for the scientific exploitation of the TSX/TDX data, as well as for planning and implementing the mission, for controlling the two satellites and for generating the digital elevation model. Astrium has built the satellites and shares in the costs of development and exploitation. The responsibility for marketing the data lies in the hands of Infoterra GmbH, a subsidiary of Astrium GmbH. AOCS operations for two spacecraft flying at low altitude with such unprecedented small separations pose several challenges and required creative solutions, mostly stipulated by the fact that the original mission was not designed for formation flight. Several on-ground safety measures were installed, but due to the short reaction times emphasis lies with the on-board handling of problems. This is achieved by implementation of a data link (one-way) between the satellites, by the regular interchange of a “health” signal (two-way), by the design of a new AOCS safe mode that has no effect on the orbit, by the complete re-work of the FDIR concept (Fault Detection, Isolation and Recovery on-board) and by autonomous formation control. Emphasis in this paper is on the implemented safety measures, specifically the FDIR concept which will be presented in Section 1.5.2. and on the routine AOCS operations during close formation flight (Section 2). No major contingencies have occurred in the formation so far – something we don’t want to change in the future either – but some minor operational issues did occur, which will be presented in Section 3. Section 4 finally gives some conclusions and an outlook on the way forward.