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Welcome to the new webpage of GPS Altimetry and Reflections Processing at the University of Texas at Austin Center for Space Research (CSR). This research is conducted under the supervision of Dr. E. Glenn Lightsey.

We hope you find this page interesting and informative. 
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Reflection Geometry

Glistening Zone

 Bistatic GPS Reflections

The satellites in the GPS constellation are constantly bombarding the earth with radio signals. These signals are detected by GPS receivers on Earth to be used for everything from navigation, to missile guidance, to plate tectonic studies. However, part of the signal is reflected from the earth's surface back into space. The reflected signal component is very weak.

A spacecraft placed into low earth orbit could simultaneously measure direct and reflected GPS signals, and the data could be used to deduce information about the reflecting surface (i.e., the Earth's surface and oceans).

The signal reflection footprint on the surface of the earth is defined by the intersection of equi-range and equi-Doppler contours in what is called the glistening zone, which is centered on the specular reflection point..  A delay-doppler mapping receiver (DDMR) would be used to take measurements across the range of delay and Doppler offsets.  Measurements taken at a specific Doppler and delay offset would correspond to specific regions within the glistening zone.
Anatomy of a GPS Reflection Measurement
Delay of Reflected Signal (Compared to Direct Signal)
The delay of the reflected signal with respect to the direct signal is analogous to a radar altimetry measurement. The delay provides information on the differential path between direct and reflected signal. Together with information on the receiving antenna position and the medium, the delay measurement can be used to determine sea surface height.

Signal Distribution
The distribution of the reflected signal is analogous to a scatterometer observation. The shape of the reflected signal can be associated with the surface properties of the reflecting surface. Comparison of the distribution with model predictions or empirical data provides estimates of ocean surface winds.

Signal Power
The magnitude of the reflected signal as compared to the direct provides a radiometric measurement. Radiometric measurements are sensitive to the dielectric properties of the reflecting surface. Careful calibration of the instrument and correction for atmospheric factors would enable the extraction of surface dielectric properties, such as soil moisture and ocean salinity.

GPS reflection measurement for entire glistening zone.


Direct and reflected GPS signal.
Student Reflected GPS Experiment (SuRGE)

The Student Reflected GPS Experiment (SuRGE) was proposed as a project to measure GPS signals that are reflected from the Earth's surface and oceans. SuRGE was a proof-of-concept experiment to demonstrate bistatic GPS as a viable technology for satellite remote sensing. It would allow us to compare and validate GPS derived estimates against other sources. Measurements would include ocean surface winds, sea surface height, and soil moisture. GPS reflection satellites will be an important component of future satellite constellations designed to observe ocean winds and sea surface height. The smaller, lower cost and lower power instrumentation using reflected GPS signals would allow a more global distribution of remotely sensed wind speed measurements at higher temporal resolution.

SuRGE would use a modified JPL  blackjack GPS receiver that would function as a Delay-Doppler Mapping receiver.  It would incorporate nadar-pointing antennas to measure the reflected signals, and zenith-pointing antennas for the direct signals.
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The purpose of this project is to train a student on the operation, algorithms, and software of JPL's Blackjack GPS receiver that is used for altimetry measurements on the ICESat and GRACE projects which are both managed by the Center for Space Research.  The Gravity Recovery and Climate Experiment (GRACE) is due to launch in November 2001, and the Ice, Cloud, and Land Elevation Satellite (ICESat) is due to launch in June 2002.  The student will:


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JPL Blackjack GPS receiver and block diagram

 JPL's Blackjack GPS receiver is a high-precision space-rated GPS receiver with dual-frequency tracking capability.  The Blackjack is an unclassified receiver, and uses a patented codeless processing technique that allows it to utilize the P-code signal without knowledge of the encryption code.  The Blackjack is controlled through flexible and versatile software implementations of various receiver functions. This environment is conducive to adding new capabilities, based on the mission requirements.

BlackJack GPS flight receivers are being used on the following space missions: SRTM (2000), SAC-C (2000), CHAMP (2000), JASON-1 (2000/01), VCL (2000), FEDSat (2001), ICESat (2001), and GRACE (2001).  ICESat and GRACE are both CSR-managed missions.

In the Fall, CSR will acquire a Blackjack from JPL to be used in research and mission support for ICESat and GRACE.

More information: JPL Press Release

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University of Texas> Center for Space Research> Lightsey GPS Research Group

GRACE - Gravity Recovery and Climate Experiment

ICESat - Ice, Cloud, and Land Elevation Satellite

SuRGE - Student Reflected GPS Experiment

Global Position System Overview

JPL GPS Systems Group
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Webpage by Jacob Williams  
 Last Updated 9/5/01