MEASUREMENTS OF ELECTROMAGNETIC BIAS AT KU-BANDS AND C-BANDS

The electromagnetic (EM) bias epsilon is an error present in radar alt imetry of the ocean surface due to nonuniform reflection with surface displacement. The electromagnetic bias is defined as the difference in height between the mean reflecting surface and the mean sea surface. A knowledge of the electromagnetic bias is required for reducing error s in mean sea level measurements by satellite radar altimeters, Direct measurements of the EM bias at 14 GHz (Ku band) and 5 GHz (C band) we re made from an oil production platform in the Gulf of Mexico over a 6 -month period during 1989 and 1990. A total of 1280 hours of usable da ta was collected, During the experiment the significant wave height (S WH) varied from 0.6 to 3.2 m; the wind speed at 25 m above the surface varied from 0.1 to 14.3 m s(-1): the Ku band bias varied from -1.0 to -13.8 cm, or from -1.6% to -5.3% of the SWH; and the C band bias vari ed from -0.4 to -19.9 cm, or from -0.6% to -6.3% of the SWH. The biase s had mean values of -3.7% and -3.6% of SWH with standard deviations o f the variability about the mean of 0.7% and 1.0% of SWH for Ku and C bands, respectively. We found a nonlinear relationship between dimensi onless bias (bias/SWH) and wind speed at both low and high wind speeds . For wind speeds less than 3-4 m s(-1), both biases were found to be approximately constant. For wind speeds greater than 3-4 m s(-1) but l ess than 10 m s(-1), both biases were found to increase linearly with wind speed. For wind speeds greater than 11-12 m s(-1), the C band bia s reaches a maximum. The Ku band bias reaches a maximum and then begin s to decrease for wind speeds greater than 9-10 m s(-1).