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.
The PRISMA in-orbit test-bed was launched in June 2010 to demonstrate strategies and technologies for formation flying and rendezvous. OHB Sweden is the prime contractor for the project which is funded by the Swedish National Space Board (SNSB) with support from DLR, CNES, and DTU. Mission operations are carried out from OHB Sweden's purpose built control-room in Solna, Sweden, using the company's own GNC and platform experts to conduct the mission. As an experimental technology demonstrator a large number of in-orbit experiments were initially planned, with desires exceeding the constraints of available funding. In an effort to extend the use of the satellites and enable more experiments DLR/GSOC offered to temporarily operate the satellites from their control center in Oberpfaffenhofen, Germany, for a period of five months. Control of the spacecraft was transferred to GSOC in March, 2011, after a training period of several months. A number of experiments were executed, including GSOC's own formation flying and autonomous orbit keeping, SSC ECAPS's green propulsion and several different OHB Sweden experiments. Handover back to OHB Sweden was then performed in August the same year, from where the mission continues to be run. Transferring control of a satellite project from one organization to another, including new operational personnel and a new control room, posed a great challenge to both parties. This paper describes the mission concept, the background for the transfer, implementation of a mirrored control room and the process of transferring knowledge from the design and operations team of OHB Sweden to the GSOC operations team.
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.
This paper describes the TerraSAR-X / TanDEM-X formation control concept, which bases on the relative eccentricity / inclination vector separation method, implemented within GSOC's flight dynamics system. The paper specifically elaborates on the results obtained during the first six month of the formation flying mission showing the safe and precise automated groundcontrol of the space-borne radar interferometer.
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