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

Abstract

In 2007 the TerraSAR-X satellite was successfully launched. Its payload consists of an earth observing Synthetic Aperture Radar, which supplies high resolution radar images. The yet to be launched TanDEM-X mission extends the TerraSAR-X mission with a second satellite of very similar capabilities. The two satellites will be flying in close formation (250m - 500m), such that the radar instruments can be synchronized (bistatic mode). This bistatic mode allows the generation of three dimensional pictures and thus a high resolution elevation model of the observed area (according to HRTI-3 specifications), which in the end should cover the whole earth. TanDEM-X is the first project worldwide to fly a close formation with a minimum distance of only a few hundred meters. During the development and integration of the ground segment, it became obvious that the amount of security measures have to be significantly increased to ensure safe operations. One of the main influences is the satellite orbit and attitude control system that is based on active control via hydrazine thrusters. The activation of the thrusters for attitude control possibly also changes the orbit, which leads to a change of the relative positions. Consequently two problems can appear, either mutual radar illumination which might damage electronic equipment or if the impulse is large enough, a collision. The paper gives an overview of how the various elements of the TanDEM-X ground segment, with main emphasis on the mission operations segment, had to be designed to provide a maximum degree of safety. This includes all aspects of operations, from ground station network and flight procedures to extended on-call services and specifically developed tools. It is shown how the operational design is influenced by the satellite design and how a different satellite design would have reduced the operational effort. It was originally planned to present the first real experiences with both satellite flying in close formation, but due to a launch delay to end of May 2010 this is unfortunately not possible.