The Cold Gas System on TDX - Accurate in-Orbit Evaluation

The TanDEM-X mission comprises two nearly identical satellites – TSX and TDX – that fly at an altitude of ~515 km in a close formation at distances down to 120 m. The standard orbit corrections are made with hydrazine propulsion on both spacecraft, but the relative geometry is maintained by a dedicated cold gas system on TDX only, which initially comprised 36 kg of nitrogen. The routine formation-keeping requires at least one pair of cold gas maneuvers daily. Formation flight started in late 2010 and continued ever since in several configurations. This means that by now of the order of 2500 cold gas maneuvers have been performed and the remaining amount of fuel will allow for another 1500 to be made. Maneuvers were done mainly with the four 40 milli-Newton thrusters of branch-A, but about 25 % of the time branch-B was used. The construction of a digital elevation model of the Earth requires very precise orbit information, which is delivered by the flight dynamics department at GSOC. This also yields an accuracy of <0.1 mm/s in the a posteriori reconstruction of maneuvers. The large number of maneuvers and the extremely accurate orbit information allow a unique in-orbit evaluation of the cold gas system. Thruster performance is reconstructed from the precise orbit determination combined with 1Hz telemetry during the burn and a priori laboratory measurements. Among others comparisons are made between the two branches and between maneuvers in flight- and anti-flight direction. The influence of the maneuver duration and the tank pressure will also be presented