Independent Assessment of On-Board Microwave Radiometer Measurements in Coastal Zones Using Tropospheric Delays From GNSS

Zenith tropospheric delays (ZTDs) computed at a network of 60 global navigation satellite system (GNSS) stations have been used to assess microwave radiometer (MWR) measurements from eight altimeter missions in coastal zones, where some of these observations become invalid. Results show that ZTDs are determined with an accuracy of a few millimeters; however, jumps are detected in some stations in standard products. The comparison between the MWR-derived wet tropospheric correction (WTC) and the GNSS-derived WTC at the nearby coastal stations illustrates the effect of land contamination in the MWR measurements and yields the distance from coast at which this contamination appears. This distance is different for the analyzed altimetric missions, due to their different footprint sizes and different MWR retrieval algorithms, varying from 10 to 30 km. The root mean square of the differences between GNSS and MWR-derived WTC, at the closest distance at which no land contamination occurs, is in the range of 1.6-1.9 cm for all missions. This coastal assessment also shows the ability of the GNSS-derived path delay plus algorithm to remove this land contamination and to improve the WTC retrieval. Aiming at inspecting the long-term stability of the MWR measurements, the comparisons with GNSS show nonsignificant differences and drifts less than 0.3 mm/year. Therefore, the GNSS-derived WTC is a useful independent source to inspect the land effects on MWR observations and to monitor the stability of these instruments, thus contributing to the retrieval of precise water surface heights from satellite altimetry.