GPS-Based Measurements of Ocean Surface Roughness

Since 1996 researchers at NASA's Langley Research Center have been investigating the characteristics of GPS signals reflected from the ocean surface. The highly coherent GPS signal is modified at the air-ocean inteface by surface roughness. The effect is manifested by a distribution of time delays in the reflected signals which are recorded in the receiver. Recent calibration of the relationship between the distribution of time delays and ocean surface winds has made possible successful wind speed retrievals from within tropical cyclones, most recently, those from the 2005 hurricane season. A GPS instrument was flown on one of the NOAA-AOC Hurricane Hunters during that time period. Three other methods are commonly used for the wind speed measurement: dropsondes, aircraft-drift, and step frequency microwave radiometer (SFMR). The SFMR operates by detecting the change in thermal emission from the surface induced by an increasing foam-bubble layer.

Comparisons of wind speed retrievals have shown general agreement among the competing methods as far as maximum winds. Nevertheless, there are areas along the aircraft storm track in which significant differences occur. Theoretical considerations show that the GPS technique works on the basis of a direct measurement of surface roughess. Thus, use of the GPS bi-static configuration may offer important insights into air-sea interaction in tropical cyclones at hurricane force winds.

This paper will present a description of the fundamentals of the GPS technique, results from several recent hurricanes, a comparison with wind speed retrievals from other methods and an identification of spatial distribution of discrepencies with the storm core.