GPS Signal Scattering from Sea Surface: Wind Speed Retrieval Using Experimental Data and Theoretical Model Attila Komjathy,* Valery U. Zavorotny, † Penina Axelrad,* George H. Born,* and James L. Garrison ‡ Global Positioning System (GPS) signals reflected from applications. Recently, the sensitivity of this signal to the ocean surface have potential use for various remote propagation effects was found to be useful for various sensing purposes. Some possibilities are measurements of environmental remote sensing techniques. For example, surface roughness characteristics from which wave ionospheric and tropospheric delays of GPS signals height, wind speed, and direction could be determined. caused by variations of atmospheric refraction are known For this paper, GPS-reflected signal measurements col- to be a common error source affecting the use of the lected at aircraft altitudes of 2 km to 5 km with a delay- GPS for positioning and navigation. This phenomenon is Doppler mapping GPS receiver are used to explore the being successfully used for atmospheric remote sensing possibility of determining wind speed. To interpret the purposes (Ware et al., 1996). Surface multipath is an- GPS data, a theoretical model has been developed that other error source affecting the GPS signal. However, it describes the power of the reflected GPS signals for dif- has only recently been recognized that multipath from ferent time delays and Doppler frequencies as a function the GPS signals reflecting off the sea surface could be of geometrical and environmental parameters. The results utilized as a new tool in oceanographic remote sensing indicate a good agreement between the measured and the (Martı´n-Neira, 1993; Katzberg and Garrison, 1996, Gar- modeled normalized signal power waveforms during rison et al., 1997; Garrison et al., 1998). In both cases, changing surface wind conditions. The estimated wind a phenomenon that is usually regarded as an error source speed using surface-reflected GPS data, obtained by com- to navigation was recognized to contain useful scientific paring actual and modeled waveforms, shows good data. Because the ocean surface roughness affects GPS agreement (within 2 m/s) with data obtained from a signal reflection, researchers can use the resulting nearby buoy and independent wind speed measurements multipath signal to determine factors such as wave derived from the TOPEX/Poseidon altimetric satellite. height, wind speed, and wind direction. The strength of Elsevier Science Inc., 2000 the reflected signal is also a discriminator between wet and dry ground areas, and therefore could be applied to coastal and wetland mapping. INTRODUCTION Conventional monostatic radar remote sensing of the The versatility and availability of signals from the Global oceans requires dedicated transmitters and receivers with Positioning System (GPS) gave birth to many new GPS large directional antennae to achieve high resolution. GPS is, in fact, a bistatic radar system, which requires * CCAR/University of Colorado at Boulder only a receiver, not a transmitter, because the GPS satel- † CIRES/University of Colorado at Boulder/NOAA/ETL lites provide illumination for free. This bistatic geometry ‡ NASA Goddard Space Flight Center and the special structure of GPS signals can provide Address correspondence to Attila Komjathy, Colorado Center for Astrodynamics Research (CCAR), University of Colorado at Boulder, complementary information about ocean surface charac- Campus Box 431, Boulder, CO 80309-0431, USA. E-mail: komjathy@ teristics from a small patch of ocean surface without the GPS.colorado.edu Accepted 18 January 2000. need for a directional antenna. All of this makes the re- REMOTE SENS. ENVIRON. 73:162–174 (2000) Elsevier Science Inc., 2000 0034-4257/00/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(00)00091-2