COMBINED MICROWAVE RADIOMETER AND ALTIMETER RETRIEVAL OF WET PATH DELAY FOR THE GEOSAT FOLLOW-ON

The GEOSAT follow-on (GFO) satellite includes a two-frequency water va por radiometer (WVR) operating at 22.2 and 37.0 GHz to correct for the path delay of its altimeter signal through the wet troposphere, Path delay retrievals based on two-frequency radiometer measurements alone include a residual bias which is correlated with either cloud liquid w ater or surface wind speed, depending on the choice of frequencies, Th e GFO WVR frequencies were chosen in part to correlate the bias with w ind speed, This wind speed is then estimated and the bias corrected by using the radar cross-section data available from the altimeter. The specific algorithm by which this correction is implemented, together w ith estimates of its accuracy and sensitivity to measurement error, ar e presented, A final path delay retrieval accuracy of 1.18 cm rms is e xpected on orbit. This assumes absolute calibration of the WVR brightn ess temperatures to +/-1.0 K and of the altimeter radar cross section to +/-0.5 dB, The performance of a modified version of the path delay retrieval algorithm (to operate at 21 and 37 GHz) is also tested using a two frequency subset of the TOPEX/Poseidon microwave radiometer (TM R) flight data over ground calibration sites.