GPS Receiver for Occultation, Reflectometry and Scatterometry (GORS)

JAVAD GeNeSiS-112 receiver board

Within the German Indonesian Tsunami Early Warning system project (GITEWS), the GeoForschungsZentrum (GFZ) Potsdam has set up a team with the German Aerospace Center (DLR) and JAVAD GNSS to adapt and extend JAVAD's new generation GNSS receivers for advanced space applications.

Signal simulator testing of the GORS receiver prototype

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 new GPS L2C 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 5 msec 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. 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.

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 capabilties under high signal dynamics. The tests were performed with a Spirent GSS7700 signal simulator capable of simulating dual-frequency (L1, L2) GPS signals for up to 12 visible satellites. For compatibility with earlier tests of spaceborne GPS receivers an established scenario for a polar satellite at 515 km altitude was used throughout all tests, which is representative of the TerraSAR-X satellite. The distribution of tracked satellites on the celestial sphere illustrates that the GeNeSiS receiver properly acquires and tracks all simulated satellites above the adopted elevaton mask of 5°. Likewise, the histogram of tracked satellites shows a smooth distribution up to the simulated maximum of 12 satellites. Following the start-up phase at least six satellites are permanently available for navigation and 10-11 satellites are simultaneously tracked on average. The achievable measurement quality and possible systematic errors have been assessed by forming double-differences between satellite pairs and between the receiver and the simulator-truth values. For the directly tracked C/A code an average noise of 17 cm has been obtained, while the semi-codeless P-code tracking yields a noise of roughly 50 cm. The associated carrier-phase measurements exhibit noise values of 0.7 mm and 1.6 mm, respectively. The initial signal simulator test 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. Further tests will be conducted to optimally tune the tracking-loop bandwidth in a trade-off between low noise measurements and robust tracking.

Correlation diagram for L2C signals

Courtesy A.Helm (GFZ)

Measurement of GPS signal reflections at Fahrenberg mountain on 16-20 June 2007

Courtesy A.Helm (GFZ)

A first reflectometry experiment using the GeNeSiS-112 receiver was conducted on 16-20 July, 2007, at mount Fahrenberg, in the Bavarian alps, 50 km south of Munich. A single conventional GPS patch antenna (JAVAD MarAnt+ antenna) was used and tilted by 45° from the zenith direction to enable parallel reception of direct and reflected GPS signals. 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 but steered with an additional delay in code space. Fig. 3 shows a sample set of correlation amplitudes for the L2C signal. A reflected signal with a delay of about 600 m may be recognized.

Further Reading

Helm A., Stosius R., Beyerle G., Montenbruck O., Rothacher M.;
Status of GNSS reflectometry related receiver developments and feasibility studies in the frame of the German Indonesian Tsunami Early Warning System;
IAGRSS-07; International Geoscience and Remote Sensing Symposium 2007; 23-27 July 2007, Barcelona (2007).

Helm A., Montenbruck O., Ashjaee J., Yudanov S., Beyerle G., Stosius R., Rothacher M.;
GORS - A GNSS Occultation, Reflectometry and Scatterometry Space Receiver;
ION-GNSS-2007 Conference; 26-28 Sept. 2007; Fort Worth, Texas (2007).

Helm A., Beyerle G., Stosius R., Montenbruck O., Yudanov S., Rothacher M.;
The GNSS Occultation, Reflectormetry and Scatterometry Space Receiver GORS: Current Status and Future Plans within GITEWS;
1st Colloquium Scientific and Fundamental Aspects of the Galileo Programme; 1-4 Oct 2007, Toulouse (2007).