Electromagnetic bias of 10‐GHZ radar altimeter measurements of MSL

Electromagnetic bias, the small difference that exists between the radar‐measured mean sea level and the geometric mean sea level, is an important issue in high‐precision satellite altimetry. Present‐day satellite altimetry has achieved, with SEASAT‐1, a precision of 5 cm root‐mean‐square (rms) in range measurement. Future altimeter designs are expected to improve range measurement precision to 2 cm rms. In order to exploit the capability of these precise radar altimeters for marine geodesy and oceanography, it is necessary to understand and account for all of the known biases in the range measurement. The electromagnetic (EM) bias, which has been attributed to the observed fact that ocean wave troughs tend to be better reflectors of nadir viewing microwave radar energy than ocean wave crests, can be observed with high‐resolution airborne radar. This report presents the result of EM bias measurements made by NRL, using an airborne radar altimeter operating at 10 GHz with a 1‐ns range resolution. Data were taken for various sea states and wind conditions. The experimental results are compared with current theories.