TOPEX/Poseidon microwave radiometer (TMR): 2. Antenna pattern correction and brightness temperature algorithm

The calibrated antenna temperatures measured by the TOPEX Microwave Radiometer are used to derive radiometric brightness temperatures in the vicinity of the altimeter footprint. The basis for the procedure devised to do this-the antenna pattern correction and brightness temperature algorithm-is described in this paper, along with its associated uncertainties. The algorithm is based on knowledge of the antenna pattern, the ground-based measurements of which are presented along with their analyses. Using the results of these measurements, we perform an error analysis that yields the net uncertainties in the derived TMR footprint brightness temperatures. The net brightness temperature uncertainties range from 0.79 to 0.88 degree K for the three TMR frequencies, and include the radiometer calibration uncertainties which range from 0.54 to 0.57 degree K. We also derive an estimate of the uncertainty incurred by using brightness temperatures measured in the similar to 40 km TMR footprint to estimate path delay in the similar to 3 km altimeter footprint. The RMS difference in path delay averaged over the largest TMR footprint relative to that in the altimeter footprint is estimated to be about 0.3 cm. Finally, we discuss the error associated with using unequal beams at the three TMR frequencies to derive path delays, and describe an approach using along-track averaging of the algorithm brightness temperatures to reduce this error.