AOCS for Tandem-X Formation Flight at 150m Separation in Low-Earth Orbit

Abstract

HE 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 2007will 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 (twoway), by the design of a new AOCS safe mode that has no effect on the orbit, by the complete rework 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.

The TanDEM-X Mission Operations Segment: Close Formation Flight, Preparation and First Experiences

“…Consequently, one of the first actions was to discuss with the satellite manufacturer, if a different safe mode without changing the orbit could be established. The solution was the introduction of an additional safe mode based upon the magnetic torque rods (ASM-MTQ) instead of the propulsion system (for details see 1 ). Since the torque rods only influence the attitude, this concept allows the satellite to survive in safe mode until the ground has time to identify the problem and recover the satellite without having to fear a potential collision.…”

“…There will be several steps including on-board computer re-boots (to cope for possible problems due to S/W "hang-ups") and the activation of redundant equipments, before the satellite changes back to the ASM-RCS mode, The details are described in Ref. 1 . The availability of the ground stations is an important point concerning the timing aspects of the FDIR concept and is discussed in the next section.…”

“…Initially, the TanDEM-X Acquisition Planner (TAP) specifies the satellite-specific exclusion zone geometry (Exclusion Zone Angles) based on the current helix formation. These contain the forbidden areas/angles in which all radar images are "excluded" to prevent possible damage (see also 1 ). The TAP also provides an Exclusion Zone Product for satellite upload to assure sufficient reliability (will be explained later).…”

Formation Flight in Low-Earth-Orbit at 150 m Distance - AOCS In-Orbit Experience

“…If there is still no convergence or the power/thermal limits are yet violated, the spacecraft will be sent by FDIR to ASM-RCS once more, but this time without limitations to the thruster on-time. A more detailed description of the FDIR design implemented on TSX and TDX is found at [5].…”

“…Images can be made in several instrument modes and spacecraft formations with ground resolution ranging from ~0.5 m to 16 m and an area covered from ~50km2 to 104km2. A more detailed description of the several options can be found in [5] and [9]. Both spacecraft are equipped with a hydrazine propulsion system with four redundant 1N thrusters mounted at the rear side of the satellite (Fig.…”