Bistatic Radar Cross Section Measurements of Ocean Scattered GPS Signals from Low Earth Orbit

Signals from the Global Positioning System are constantly being scattered off the entire Earth's surface. These signals can be detected from airplanes, surface platforms and in Low Earth orbit and are known to contain information on the oceans. Recently, a new opportunity to study GPS reflections in space has been realized with the launch in October 2003 of the United Kingdom's Disaster Monitoring Constellation (UK-DMC) satellite. The UK-DMC carries a dedicated GPS reflections receiver and a custom designed downward facing antenna. Using this GPS experiment it is possible to process surface scattered signals and estimate the ocean wind and waves. This paper will present a method for calculating a bistatic radar cross section (BRCS) for ocean scattered GPS signals. This method will then be applied over a range of signals detected by the UK-DMC experiment. For purposes of comparison, numerous data collections have been collocated with National Data Buoy Center (NDBC) buoys. These buoys provide an independent measurement of the ocean wind, waves and often a wave frequency spectrum from which the surface mean square slopes, an indication of ocean roughness, can be determined. BRCS values have been estimated over a range of ocean conditions (from less than 2 m/s winds to greater than 14 m/s winds) and vary generally as expected when compared to buoy measurements of the ocean wind and waves. An empirical relationship between the estimated BRCS and the ocean wind speeds and surface roughness will be presented for a limited set of data gathered by the UK-DMC experiment.