A Study on Analysis Setting Optimization of Ship-Based GNSS Measurements for Maritime Precipitable Water Vapor Monitoring

We performed kinematic precise point positioning (PPP) to determine the optimum analysis settings for precipitable water vapor (PWV) retrieval at sea using a ship-based Global Navigation Satellite System (GNSS). Three analysis parameters were varied: the SD of random walk process noise (RWPN) of Zenith Total Delay (ZTD) time variation, the analysis time width, and the time interval of update of the Kalman filter state vector. A comparison with the Meso-scale Analysis (MA) of the Japan Meteorological Agency revealed that, depending on the update interval and the time width, a strengthened RWPN constraint suppresses the unnatural time variation of GNSS-derived PWV, reduces negative bias against MA but decreases the regression coefficient.Based on the results of the comparison of GNSS-derived PWV with MA, a setting combination of 3 × 10−5 m s−1/2, 1.5 h, and 2 s for the RWPN, the time width, and the update interval, respectively, was selected to compare with other observations. Biases and root-mean-square differences between the ship-based GNSS-derived PWV and radiosonde observation, a nearby ground-fixed GNSS station, and a satellite-borne microwave radiometer were −0.48 and 1.75, 0.08–0.25 and 1.49–1.63, and 1.04–1.18 and 2.17–2.43 mm, respectively.The factors yielding the differences in the GNSS-derived PWV bias were discussed, especially the errors in the estimated GNSS antenna altitude. The error in the vertical coordinate in GNSS positioning was confirmed as negatively correlated with the error in the GNSS-derived PWV. We found that the kinematic PPP would overestimate the altitude with shorter update intervals and wider time widths. When the RWPN and the update interval were set to 3 × 10−5 m s−1/2 and 2 s, respectively, the bias of the analyzed altitudes by the kinematic PPP changed from negative to positive at approximately 1 h width. The results suggest that precise GNSS positioning is necessary for accurate GNSS-derived PWV analysis.