TDX-TSX - On-board autonomy and FDIR of whispering brothers

摘要

TerraSAR-X and TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) are German interferometry SAR missions realized as public private partnership programs with Astrium GmbH, as prime contractor for the space segment comprising spacecraft including the SAR instrument， and DLR， as mission lead and operations provider for scientific as well as for commercial. The TanDEM-X satellite represents an almost identical rebuild of the TerraSAR-X satellite. The two satellites orbit in a closely controlled formation and form a single pass SAR interferometer with adjustable baselines (typically below 250 meters) in across- and in along-track. The satellites operate individually for TerraSAR-X mission product generation as well as jointly for Digital Elevation Model product generation, thus forming an integrated system of systems. The paper presents the overall FDIR of the constellation, which is decomposed into ground rules and constraints for planning and on-board FDIR. Special focus is put on the on-board FDIR and autonomy features of the individual satellites as well as on the integrated overall FDIR of the space segment w.r.t. constellation and satellite interactions. The individual satellite on-board FDIR is based on a distributed, hierarchical, multiple level concept, in which each level performs its dedicated failure detection, and (if possible) failure isolation, and (if possible and desired) failure recovery. If a failure cannot be recovered on the respective level, a corresponding event reporting will be performed and the next higher level will take care of the predefined actions. On application SW level the concept is mainly realized by standardized configurable services supported by FDIR preprocessing within on-board algorithms. A special characteristic of each individual satellite is the extremely high power consumption and dissipation of the SAR Instrument during operation (i.e. Data Take). For economic reasons a design to cost and budget approach had to be chosen, with the consequence that e.g. the energy resources and the thermal control system of the satellite are not sized to support a continuous operation of the SAR Instrument. Consequently an improper operation of the instrument, violating the given resource limits, can endanger the satellite. Therefore onboard FDIR must not only cover hardware failures as identified in the FMECA, but also functional operational errors and transient failures e.g. caused by SEU-induced bit-flips in memories or EMI-induced corruption of data during transmission, because all these cases could lead to an exceeding of resource limits. The special characteristic of the TanDEM-x mission is the close formation flying constellation of TanDEM-X and TerraSAR-X satellites. The paper addresses the integrated FDIR of the overall system taking utmost benefit of the distributed hierarchical FDIR and autonomy system design, while utilizing only minimum interaction between the two satellites.