GPS SIGNAL SCATTERING FROM SEA SURFACE: COMPARISON BETWEEN EXPERIMENTAL DATA AND THEORETICAL MODEL *

NASA Langley Research Center, Hampton, VAABSTRACTGlobal Positioning System (GPS) signals reflected from the ocean surface havepotential use for various remote sensing purposes. Some possibilities are measurements ofsurface roughness characteristics from which wave height, wind speed and direction couldbe determined. For this paper, GPS reflected signal measurements taken from aircraft areused to explore the possibility of determining wind speed. To interpret the GPS data, atheoretical model has been developed which describes the power of the reflected GPSsignals for different time delays as a function of geometrical and environmentalparameters. The results indicate a good agreement between the measured and the modelednormalized signal power waveforms even during rapidly changing surface wind conditions. The inferred wind speed, obtained by comparing actual and modeled waveforms, showsgood agreement (within 2 m/s) with data obtained from a nearby buoy.1. INTRODUCTIONThe concept of bistatic radar sensing of the ocean surface using two satellites has been demonstratedby Rubashkin et al., (1993) using a transmitter in low Earth orbit and a receiver in geosynchronous orbit.Martin-Neira (1993) first proposed and described a bistatic ocean altimetry system utilizing the signal of theGlobal Positioning System (GPS). The reception of surface reflected GPS signals from aircraft was firstreported by Auber et al., (1994). Recently, a number of applications have been proposed which use GPSsignals reflected from the ocean surface (Katzberg and Garrison, 1996). One of the proposals is to use thereflected power to infer information on wind speed and wave height. The viability of the proposedapplications depends upon the ability to acquire and code track the reflected signal for an extended periodof time over a variety of sea states. These difficulties were overcome by development of the first GPS delay-mapping receiver (Garrison et al., 1998). Qualitative assessment of characteristics of the signal using such areceiver has been presented by Garrison et al., (1997). The dependence of a received signal structure ontransmitter-receiver configuration and surface wind conditions has been studied theoretically by Zavorotnyand Voronovich (1998a). The modeling of wind speed effect on the received signal at aircraft altitudes ispresented in subsequent sections along with experimental data collected by the above mentioned delay-mapping receiver. Quantitative comparison of data with the model provides us with evidence that thistechnique can be used for inferring ocean surface winds and wave heights.