GPS Scatterometry and Reflectometry are seen as valuable new techniques in the field of altimetry, oceanography and glaciography. The high reflectivity of GPS signals in the frequency range of L-Band (1,2 and 1,6 GHz) on water as well as iced and snow covered surfaces partly compensates for the low signal intensity and allows the detection of reflected signal components.
In the past, experiences with special Delay Mapping GPS Receivers in balloons and planes have demonstrated, that measurements of the sea level can be achieved with an accuracy of up to 5 cm. Quite recently, the extraction of altimetric height information of occultation events of the CHAMP mission could be proven with a sensitivity in the decimeter range.
Thus, GFZ suggests to develop a GPS Occultation, Reflectometry and Scatterometry (GORS) Receiver Technology based on COTS in the context of a low flying mini-satellite constellation as quintessential instrument for a future tsunami detection system. The scientific and operation specific demands on GPS altimetry measurements for satellite missions as well as an architecture specific design concept are to be developed. Moreover, the impacts on satellite missions, their operation and design, as well as on GPS Receiver and antennas must be evaluated to be able to design corresponding measures.
Reflection point tracks along the Indian Ocean within a space receiver scenario at a CHAMP-like LEO orbit. The panels show the receiver ground track (blue) and reflection point distributions for GPS (red), GLONASS (yellow) and GALILEO (orange). The points farthest apart from the receiver location have lowest elevations.
Within GITEWS a feasibility study is performed on tsunami detection and warning from space using GNSS reflectometry (GNSS-R). GNSS-R takes Global Navigation Satellite System signals, e.g., GPS, reflected from the sea surface as a measure of height of the reflecting surface. These reflections can be used for scatterometry aswell. GNSS-R receivers on a satellite constellation may be used as multistatic altimeters that observe the sea surface at many reflection points simultaneously searching for tsunami wave signatures. A main topic of this study is the mission design for a GNSS-R receiver constellation at a low earth orbit (LEO). Therefore studies on swath width and spatial resolution with respect to elevation, height and transmitter system (GPS, GLONASS, GALILEO) have been carried out. They show, that the spatial coverage increases when signals of different transmitter systems can be received simultaneously and that the swath width is highest for reflections at low elevations. Another main topic is the altimetric accuracy that can be acchieved. Therefore simulations of the reflection process will be carried out using a scattering model.
The JAVAD GeNeSiS-112 receiver board has a physical dimension of 112 x 100 x 14 mm and a weight of 110 g.
Within the GITEWS project, the GeoForschungsZentrum Potsdam 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 GORS receiver prototype consists of a JAVAD GeNeSiS-112 72-channel GNSS OEM receiver board with raw data and position solution output. The receiver can process all presently available today GNSS radio signals, including the new GPS L2C signals. The specially adapted receiver firmware allows for the first time civil dualfrequency phase measurements of reflected GPS signals.
GeNeSiS-112 receiver with external low noise amplifier in the signal simulator test bed at DLR.
As part of the ongoing adaptation and space qualification of the GeNeSiS-112 receiver, initial tests in a GPS signal simulator test bed have been conducted to assess the tracking and navigation capabilities under high signal dynamics. The initial signal simulator tests demonstrate the capability of the GeNeSiS receiver to provide proper GPS measurements for orbit determination and scientific applications under the signal dynamics of a user satellite in low Earth orbit.
Evolving phase which is calculated from L1 C/A I,Q data (blue) and L2C I,Q data (magenta), respectively (left panel). Difference in path length between direct and reflected L1 C/A (blue) and L2C (magenta) signal (right panel).
The experiment was conducted on July 17–19, 2007, 50 km south of Munich, Germany, in the Bavarian alpine upland at the mountain top of Fahrenberg (11.32°E, 47.61°N, 1625 m above sea level) with unobstructed view to Lake Kochel and Lake Walchen. The lakes are situated 1026 m and 824 m below the receiver position, respectively. A single conventional GPS patch antenna was mounted on a tripod and tilted by 45° from zenith direction to allow for direct and reflected GPS signal reception in parallel. Depending on the predicted reflection event, the tilted antenna was oriented toward Lake Kochel and toward Lake Walchen, respectively. Depending on the visible GPS signals, a reflection event of one GPS satellite is triggered. During a reflection event the master channel continues to track the direct signal. A second, so-called slave correlator channel is set to the same GPS signal. The slave channel is steered with the estimated delay of the reflected signal with respect to the master channel. The receiver records I and Q data of the target GPS satellite with a data rate of 200 Hz. From the recorded L1 C/A I,Q and L2C I,Q data the difference in path length between direct and reflected signal can be calculated for both carrier frequencies. From the changing path length difference the altimetric height of the reflector can be derived in a subsequent processing step.
At the GITEWS Workshop New Earth Observation Techniques for Tsunami Detection and Geohazards Monitoring, organized by LAPAN and DLR in Jakarta in November, 26-28, 2007, first height profiles could be presented. The derived height profiles show good agreement with in-situ tide gauge recordings within cm-accuracy. Thus, for the first time civil dual-frequency phase measurements of reflected signals are possible.
Lake Walchen height profil at July 18, 2007 12:54:51, calculated from reflected GPS L1 C/A (blue) and L2C signals (magenta) of GPS satellite with PRN No. 17.
A. Helm, O. Montenbruck, J. Ashjaee, S. Yudanov, G. Beyerle, R. Stosius, and M. Rothacher: GORS - A GNSS Occultation, Reflectometry and Scatterometry Space Receiver, in: Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation ION GNSS 2007,Fort Worth, Texas, Sept. 25–28, 2007, pp. 2011-2021
A. Helm, R. Stosius, O. Montenbruck, J. Asjaee, S. Yudanov,G. Beyerle, and M. Rothacher: The GITEWS GNSS Occultation, Reflectometry and Scatterometry Space Receiver GORS, presentation held at the GITEWS Workshop New Earth Observation Techniques for Tsunami Detection and Geohazards Monitoring, Jakarta, Nov. 26-28, 2007