ALBERT

Description: Within the German Indonesian tsunami early warning system (GITEWS) project, the GeoForschungsZentrum Potsdam (GFZ) has set up a team consisting of GFZ, the German Aerospace Center (DLR) and JAVAD GNSS to adapt and extend their new generation GNSS receivers for advanced space applications. The GNSS 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 quadraturephase 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 dualfrequency 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. This paper presents first results of a ground-based reflectometry experiment and first tests with a signal simulator. The experiment was conducted on July 17-19, 2007 at the mountain top of Fahrenberg with unobstructed view to Lake Kochel and Lake Walchen which are situated 1026 m and 824 m 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 6 to 12 GPS signals successfully in a simulated polar and sun synchronous low Earth orbit.

Description: Within the development of the German Indonesian Tsunami Early Warning System (GITEWS) project, concept studies for space-based warning systems were initiated. Related activities to develop new and appropriate remote sensing technologies like space-based GNSS reflectometry and scatterometry were started in 2006. The intention behind these studies is to apply multi-frequency GNSS receivers aboard of independently planned Earth observation or dedicated multi-satellite missions to monitor sea surface heights with decimeter precision for tsunami detection. Simulation studies within GITEWS indicated that a large number of LEO satellites is required to monitor the ocean with the appropriate resolution in space and time in order to detect a tsunami wave signature. For example, a GNSS reflectometry/scatterometry LEO constellation of 48 satellites would have needed 10 minutes to detect the Sumatra tsunami with 96% probability. Such a constellation could be realized by a set of small and affordable satellites which are equipped with appropriate GNSS instrumentation based on commercial off-the-shelf (COTS) receivers rather than dedicated expensive space receivers. GFZ has set up and leads a team complemented by the German Aerospace Center (DLR) and JAVAD GNSS to adapt and extend the new generation JAVAD GNSS receivers for advanced scientific space applications on small LEO satellites. The GNSS occultation, reflectometry and scatterometry (GORS) space receiver prototype consists of a COTS JAVAD GNSS GeNeSiS-112 72 channel receiver board. As major step forward compared to current space receivers the new GORS receiver prototype supports tracking of the civil L2C signal emitted from modernized GPS satellites (GPS-M). Signal simulator tests show that this prototype provides proper GPS measurements for orbit determination and scientific applications under the signal dynamics of a simulated LEO satellite. The receiver firmware is modified to allow for two-frequency 200 Hz in-phase and quad-phase data output and open-loop tracking of reflected GPS signals. Ground-based water level observations at Lake Walchen were conducted in the Bavarian Alps on July 17-19, 2007. On Mount Fahren (11.315°E, 47.607°N, 1673.5m) a single right-hand circular polarized GPS L1/L2 patch antenna was positioned and tilted by 45° from zenith direction to allow direct and reflected GPS signal reception in parallel from Lake Walchen water surface 824.6m below. Carrier phase observations of coherent reflected signals could be recorded successfully and height profiles of the lake surface could be obtained from both GPS L1 C/A and L2C signal observations within cm-level accuracy. These results show good agreement with water level observations from a conventional tide gauge sensor. Furthermore, it is possible to record the reflected signal waveform in order to derive surface roughness, wave height and wind speed information. The current activities are focused on the next generation GORS2 receiver prototype which is based on the JAVAD GNSS TRIUMPH COTS receiver platform. The TRIUMPH chip has 216 channels for tracking all types of GNSS signals including GPS, GLONASS, Galileo, QZSS, WAAS, EGNOS, and Compass/Beidou. Beside of the high number of available channels and GNSS signals the TRIUMPH receiver family can provide 1-4 RF antenna frontends and external frequency in/output. Within GITEWS signal simulation, performance analyses and environmental tests will be conducted with a set of 1-RF TRIUMPH TR-G3T boards. Additionally, further receiver software modifications will be made together with the manufacturer and ground-based and airborne campaigns are planned with a multi-frontend GORS2 prototype.

Description: Within the German Indonesian Tsunami Early Warning System (GITEWS) project concept studies and new technology developments using Global Navigation Satellite System (GNSS) reflectometry and scatterometry have been initiated. These techniques use ocean reflected GNSS signals for tsunami detection from space. With modified multi-frequency GNSS receivers onboard a satellite densely spaced grids of sea surface heights with decimeter accuracy could be established fairly rapidly. Simulation studies analyze various scenarios of low earth orbit (LEO) satellite constellations with respect to tsunami detection time and show that only a large number of LEO satellites can monitor the sea surface with sufficient high resolution in space and time. The studies assume optimum signal reception. Such a LEO constellation could be realized by small and affordable satellites, equipped with GNSS instrumentation based on the use of Commercial Off-The-Shelf (COTS) rather than dedicated expensive space receivers. Within GITEWS the new generation JAVAD GNSS receivers are adapted and extended for such advanced space applications. The GNSS Occultation, Reflectometry and Scatterometry (GORS) space receiver prototype uses a modified firmware that allows for open-loop tracking of reflected GPS signals. As major step forward compared to current space receivers GORS supports tracking of the civil GPS L2C signal. Signal simulator tests show that the prototype provides proper GPS measurements for orbit determination and scientific applications under LEO signal dynamics. A ground-based experiment evaluates the extended functionality of the GORS receiver and illustrates the proof-of-concept of a GPS reflectometry measurement for two frequencies. Water level observations of Lake Walchen could be remotely obtained by carrier-phase delay with cm-level accuracy. Furthermore, the receiver is able to record reflected GPS signal waveforms.

Description: In the frame of the German Indonesian Tsunami Early Warning System () 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 5msec intervals (200Hz). 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.