Rapid calibration of HY-2A satellite-borne microwave radiometer using coastal GNSS observations

Satellite-borne microwave radiometers provide wet tropospheric correction (WTC) for altimetry observations, which is critical to high-accuracy sea surface height (SSH) measurements. However, it was reported that the Haiyang-2A (HY-2A) calibration microwave radiometer (CMR) experienced abrupt 18.7 GHz band failure. The global navigation satellite system (GNSS) can provide (near) real-time accurate WTC with high temporal resolutions. In this study, GNSS observations of 132 global coastal sites from January 2017 to February 2018 are used for CMR validation and calibration. In addition, the European Centre for Medium-Range Weather Forecasts (ECMWF) products during the same period are also used for comparison. After calibration with GNSS observations, the root-mean-square (RMS) of the WTC differences between CMR and ECMWF decreases from 2.53 to 1.66 cm. After calibration with ECMWF products, the RMS of the WTC differences between ECMWF-calibrated CMR and ECMWF is 1.54 cm. Moreover, different processing strategies of WTC are applied to SSH measurements. The mean sea level anomaly (SLA) values of SSH obtained with ECMWF-calibrated CMR are comparable to those obtained with GNSS-calibrated CMR WTC and with ECMWF WTC. In addition, compared to the SLAs of Jason-3, the SLAs of HY-2A obtained with ECMWF WTC show the smallest differences of 7.79 cm in RMS, which are 0.1 cm smaller than those obtained with ECMWF-calibrated CMR WTC and 0.23 cm smaller than those obtained with GNSS-calibrated CMR WTC. The SLAs obtained with original CMR WTC are reduced by approximately 0.8 cm in RMS after using GNSS-calibrated CMR WTC. Therefore, even with the known limitations of ground-based GNSS (non-collocated measurements over land instead of over ocean), it can indeed serve as a valuable WTC source for radiometer calibration due to the lower latency of GNSS observations.