AOCS for TanDEM-X formation flight at 200m separation in low-earth orbit

Abstract

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 - TanDEM-X (TDX) and TerraSAR-X (TSX), a satellite of almost identical construction which is in orbit since June 2007 - will 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 will fly in formation and operate 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. As is the case already with TSX data, 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 new challenges and require creative solutions, partly implemented on-board and partly on ground. Due to the extremely 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 sending 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 TSX FDIR concept (on-board Fault Detection, Isolation and Recovery) and by autonomous formation control. This paper addresses some of the more interesting on-board implementations in particular the on-board AOCS surveillance and the newly developed and implemented AOCS safe mode with the magnetic torquers as the sole actuators. It also will touch briefly on some ground aspects such as procedures, planning and execution of orbit correction manoeuvres, personnel and on-call strategy, as well as off-line analysis and tools. © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.