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In the frame of the German Indonesian Tsunami Early Warning System (GITEWS) project a multi-frequency Global Navigation Satellite System (GNSS) Occultation & Reflectometry & Scatterometry (GORS) space receiver is developed. It is based on commercial off-the-shelf (COTS) GNSS receiver technology, as the core instrument for a future tsunami detection constellation of small low Earth orbit (LEO) satellites. For use in reflectometry, scatterometry and radio-occultation measurements as well as high-precision navigation applications, specific adaptations of the GNSS receiver firmware are desirable, which require a close interaction between scientists and the receiver manufacturer. Within the GITEWS project GFZ has set up a team consisting of GFZ, DLR and JAVAD GNSS (JAVAD) to adapt and extend their new generation GNSS receivers for advanced scientific space applications. Specific adaptations address the improvement of the cold start time-to-first-fix, the selection of optimal tracking loop parameters and channel slaving for monitoring of reflected signals. Besides pseudorange, phase and signal-to-noise measurements, the modified receiver allows output of in-phase (I) and quadrature-phase (Q) accumulations at 5 msec intervals (200 Hz). As a major step forward compared to current space receivers, the new receiver supports tracking of the civil L2C signal of the GPS constellation. An overview of the current status is given and first results are discussed. Within GITEWS the feasibility of a tsunami detection mission is studied, including the constellation mission design, the options for operating the system and the ways to develop an end-to-end system for the quick response to tsunami events. In parallel simulation studies of the GNSS signals reflected to a LEO satellite are carried out. This will be realised by a Zavorotny and Voronovich scattering model with a two-scale model approach using an Elfouhaily sea wave spectrum. An overview of the current activities is given and first results are discussed.

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.