The Tracking, Occultation and Ranging (TOR) instrument onboard TerraSAR-X and on TanDEM-X

TerraSAR-X, to be launched at the end of May, 2007, carries the tracking, occultation and ranging (TOR) Category A payload instrument package. The TOR consists of a high-precision dual-frequency GPS receiver, called integrated GPS occultation receiver (IGOR), for precise orbit determination and atmospheric sounding and a laser retro-reflector (LRR) serving as target for the global satellite laser ranging (SLR) ground station network. The TOR is supplied by the GeoForschungsZentrum Potsdam (GFZ) Germany, and the Center for Space Research (CSR), Austin, Texas. The objective of the German/US collaboration is twofold: provision of atmospheric profiles for use in numerical weather predictions and climate studies from the occultation data and precision SAR data processing based on precise orbits and atmospheric products. GFZ also supplies the TOR instrumentation for the follow-on mission TanDEM-X. For the scientific objectives of the TanDEM- X mission, i.e. bi-static SAR together with TerraSAR-X, the dual- frequency GPS receiver is of vital importance for the millimeter level determination of the baseline between the two spacecrafts. In the following, the TOR instrumentation is characterized by the features of and accuracies achievable with the GPS receiver and the LRR. The data flow is shown, as GFZ operates a high latitude data receiving station for fast data access, and runs a data system for preprocessing, archiving and retrieval. With this data system, higher level products are generated for TerraSAR- X, including near real-time orbits by GFZ and post-processed precise science orbits by GFZ and CSR. In the case of TanDEM- X, GFZ operationally determines the inter-satellite baseline on the millimeter level. CSR produces calibration orbits to support the GFZ operational activity. Orbit results in terms of latency and accuracy from current missions are given and achievable results for TerraSAR-X and TanDEM-X are discussed.