ELECTROMAGNETIC BIAS IN SEA-SURFACE RANGE MEASUREMENTS AT FREQUENCIES OF THE TOPEX POSEIDON SATELLITE

Range measurements made by satellite radar altimeters experience an electromagnetic (EM) bias toward the troughs of ocean waves. This EM bias was measured at the 13.6 GHz and 5.3 GHz operating frequencies of the NASA altimeter on the TOPEX/POSEIDON satellite in a series of 11 aircraft flights from January 17, 1991, through March 4, 1991, during the Surface WAve Dynamics Experiment (SWADE). The data are consistent with an earlier set of airborne measurements and indicate that EM bias is slightly higher at 5.3 GHz than at 13.6 GHz, and that the magnitudes of both biases increase with increasing wind speed, as does their difference. With some exceptions, EM bias shows little variation over a mesoscale region on a given day or within 1 or 2 h, but it can change significantly over a 6-h period. An examination of recent tower, airborne, and satellite measurements shows a consistency in the characteristics of the wind speed dependence but suggests that there may be a height dependence in the determinations, with the bias decreasing with increasing altitude. The airborne measurements appear to be the most reasonable basis for correcting the NASA altimeter range data from the TOPEX/POSEIDON satellite. In addition, a preliminary analysis of a very limited amount of the data acquired at 20.3 m s-1 in Southern Ocean Waves Experiment (SOWEX) has given confidence that the quadratic models for the prelaunch EM bias corrections are more appropriate for wind speed dependence than are the linear models initially considered.