Publications

The Sumatra earthquake of December 2004 was the second largest earthquake ever recorded by instruments. The Federal Ministry of Education and Research commissioned the Helmholtz Association of National Research Centers directly after the disaster with developing a German Indonesian tsunami early warning system (GITEWS) for the Indian Ocean which can later be extended to the Mediterranean and the Atlantic Ocean. The system integrates terrestrial observation networks of seismology and geodesy with marine measuring processes and satellite observation. While the early warning system is being established, concept studies on enlargements are initiated. New technologies like space-based Global Navigation Satellite System (GNSS) reflectometry and scatterometry are developed which are to facilitate a future global cutting-edge system. The future system is intended to use all then available signal sources of opportunity of, e.g. the modernized GPS, the planned Galileo, the restored GLONASS and the upcoming Compass GNSS. Low Earth Orbit (LEO) satellite-based GNSS receivers for bistatic altimetry, reflectometry and radio-occultation measurements are considered to be of great interest as one component for a future enhanced tsunami and Earth observation system. The general idea is that with multi-frequency receivers, as add-on payload to independently planned Earth observation missions, densely spaced grids of sea surface heights with decimeter precision could be established fairly rapidly. In future a dedicated constellation of 10 to 20 small affordable LEO satellites is planned which can monitor the ocean with the required high resolution in space and time in order to detect a tsunami. The required performance of such a space-based monitoring system requires most advanced GNSS receivers with improved algorithms for the various possible applications and quasi real time data processing capabilities to satisfy as a minimum the needs of a future space-based Tsunami Early Warning System. The small market segment and high specialization of Spaceborne GNSS receivers as well as the associated test and qualification effort inevitably results in high unit cost ranging from roughly 100 k€ to 1 M€. Various companies and research institutes have therefore made efforts to come up with low cost solutions based on the use of commercial off-the-shelf (COTS) components. The feasibility of this approach is nicely illustrated by the GPS Orion receiver design of MITEL, which forms the basis of several independent one frequency GPS receivers like, e.g. the Space GPS Receiver SGR-20/10 on UK-DMC or the Phoenix GPS receiver. Promising investigations show that this approach can be extended to existing dual-frequency COTS receivers, e.g. the OEM4-G2L and PolaRx2 receiver have been demonstrated to cope with the signal dynamics and the environmental conditions of a LEO satellite. Aside from enhanced ionosphere corrections, a dual-frequency system can overcome the limitations that sea roughness imposes on carrier phase coherence which is a major issue in reflectometry. For use in reflectometry, scatterometry and radio-occultation measurements as well as high-precision navigation applications, specific adaptations of the 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 to adapt and extend their new generation GNSS receivers for advanced space applications. The GORS receiver prototype consists of a COTS JAVAD GeNeSiS-112 72-channel GNSS OEM receiver board with raw data and position solution output. The 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 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 increased sea roughness conditions. The paper presents the current status of terrestial receiver validation and verification studies as well as results from signal simulator and environmental testing of the receiver.

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Helm, A.; Beyerle, G.; Stosius, R.; Montenbruck, O.; Yudanov, S.; Rothacher, M. (2007): The GNSS Occultation, Reflectometry and Scatterometry Space Receiver Gors: Current Status and Future Plans Within GITEWS. 1st Colloquium Scientific and Fundamental Aspects of the Galileo Programme (Cité de l’Espace, Toulouse, France 2007).