The GNSS Occultation, Reflectormetry and Scatterometry Space Receiver GORS: Current Status and Future Plans within GITEWS

Kurzfassung

Within the German Indonesian tsunami early warning system project, the eoForschungsZentrum Potsdam has set up a team consisting of the GeoForschungsZentrum Potsdam, the German Aerospace Center and JAVAD GNSS to adapt and extend their new generation GNSS receivers for advanced space applications. The occultation, reflectometry and scatterometry GORS Space receiver prototype consists of a commercial off-the-shelf JAVAD GNSS GeNeSiS-112 72-channel receiver board with raw data and position solution output. The GORS receiver can process all presently available GNSS radio signals, including the latest GPS L2C, GPS L5, GLONASS C/A L2, and GALILEO GIOVE-A signals. 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 and quadrature-phase accumulations at 5msec intervals (200Hz). As major step forward compared to current space receivers, the new receiver supports tracking of the civil L2C signal of the GPS constellation. This will enable loss-less dual-frequency tracking of occultation events down to very low altitudes. Channel slaving can be performed for GPS L1 C/A and L2C in parallel. Hence, carrier phase observations of coherent reflected signals are possible with two frequencies. By combining both observations and therefore enlarging the measuring wavelength, coherent carrier phase observations of reflected signals are expected to be recovered even at higher sea roughness conditions. The paper presents first results of a ground-based reflectometry experiment and first tests with a signal simulator. The experiment was conducted on 16-20 July, 2007 at the mountain top of Fahrenberg (11.32°E, 47.61°N, 1625m above sea level) with unobstructed view to Lake Kochel and Lake Walchen which are situated 1026m and 824m below the receiver position, respectively. A single right-hand circular polarized GPS L1/L2 patch antenna was used and tilted by 45° from zenith direction to allow direct and reflected GPS signal reception in parallel. Carrier phase observations of coherent reflected signals could be recorded successfully for both GPS L1 C/A and L2C signals. First tests with a signal simulator show that the GORS receiver prototype is able to acquire and track 7 to 12 GPS signals successfully in a simulated polar and sun synchronous low Earth orbit.