The TerraSAR-X/TanDEM-X Formation Acquisition–from Planning to Realization

Abstract

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.

The joint TerraSAR-X / TanDEM-X ground segment

“…The total TDX-1 velocity increment contained within 23 acquisition maneuvers performed during LEOP and in the drift phase was only 6.5 m/s out of a maximum budget of 18.5 m/s [5].…”

TanDEM-X: A radar interferometer with two formation-flying satellites

“…TanDEM-X was successfully launched on June 21, 2010. The initial separation between TDX and TSX was 15700 km and after one month of drifting a formation in pursuit monostatic configuration with an along-track distance of 20 km was reached [15]. This formation was maintained for 3 months to calibrate the TanDEM-X radar instruments and to perform first bistatic and interferometric experiments employing large baselines.…”

Operational aspects of the TanDEM-X Science Phase

“…The missions share a joint ground segment 1 , the German Space Operations Center (GSOC) serves as control center for the satellites. The TanDEM-X mission combines the payloads on both satellites to a SAR interferometer in space in order to retrieve data of the Earth's topography 2, 3 .The two spacecraft were maneuvered into a so-called close formation flight with typical inter-satellite distances of a few hundreds of meters in October 2010 4 . Since then, the Earth's land surface was scanned multiple times to generate a global Digital Elevation Model (DEM).…”