TOPEX Microwave Radiometer performance evaluation, 1992-1998

The stability and accuracy of the TOPEX Microwave Radiometer (TMR) measurem ent of the atmospheric path delay due to water vapor is assessed over the i nterval from launch (August 1992) through June 1998, Detailed global compar isons are made with path delays derived from the special sensor microwave i mager (SSM/I) instruments and a network of 15 island radiosondes. The resul ts provide consistent evidence that the TMR path delay measurements include d an instrument-related downward drift of 1.0-1.5 mm/yr between October 199 2 and December of 1996, The four-year drift correlates with an upward drift seen in the coldest TMR 18-GHz brightness temperature time series and is f urther supported by independent comparisons of TMR with ERS-1 and 2, GPS, a nd the Harvest Platform water vapor radiometer measurements. From January 1 997 through June 1998, no significant relative path delay drift between TMR and SSM/I is seen in the comparison data, although anomalies do appear in early 1998, In terms of accuracy, both the SSM/I and radiosonde comparisons indicate no significant (>2%) scale error in the TMR path delay. An overal l bias of <10 mm may be present, but the comparisons are not consistent in this determination.