Electromagnetic bias of 36‐GHZ radar altimeter measurements of MSL

Abstract

Electromagnetic (EM) bias is caused by the tendency for the troughs of ocean waves to be better reflectors than the crests. This results in the centroid of the returned power to a radar altimeter being shifted away from mean sea level (MSL) toward the troughs of the waves. Being able to model and predict the magnitude of the EM bias is critical to satellite altimeter missions such as TOPEX, where determination of MSL to a few centimeters is required. By measuring both return power and elevation to high accuracy, the 36‐GHz surface contour radar (SCR) can determine for various sea states how the backscattered power per unit area varies as a function of the displacement from MSL. The SCR uses its pencil beam (0.85° × 1.2°) to determine the spot on the surface to be interrogated. This allows independent histograms of the sea surface height distribution and the return power distribution to be developed from SCR data. The return power distribution (which is what an orbiting altimeter would measure) is shifted toward the troughs by about 1.1% of the significant wave height. The percentage shift does not vary with wave height, but does show trends with wind speed and the dominant wavelength of the sea surface waves. When the percentage shift was parameterized by using multivariable linear regressions, the best two‐variable fit was obtained by using the combination of dominant wavelength and the kurtosis of the surface height distribution.