Characteristic Analysis of Offshore Zenith Tropospheric Delay Based on GPS/BDS/GLONASS PPP

In order to study the characteristics of zenith tropospheric delay (ZTD) at sea, the Precise Point Positioning (PPP) was used to process the data from 7 International GNSS Service (IGS) reference stations distributed in South China Sea. These stations can process GPS, BDS, and GLONASS data simultaneously. The offshore ZTD was estimated by random walk model. In order to evaluate the accuracy of the estimated ZTD, the ZTD data from three of the stations were used to compare with ZTD products of 300s sampling from IGS. The results show that the ZTD estimation results from GPS/BDS/GLONSS-based PPP is consistent with the trend of the ZTD products provided by IGS, and the position error in the convergence state is less than 10 mm. By calculating the ZTD time series of day of year (DOY) 14-154,2016, the mean diurnal ZTD values at each of local time (LT = 1, 2, ..., 24) were obtained. The results shows that the mean diurnal ZTD time series have two peak values, which are around noon and midnight local time, moreover, the maximum peak is around midnight. The ZTD estimation results of offshore and inland are compared and analyzed. The results show that ZTD of offshore stations is about 6-19 cm larger than that of inland stations. The maximum ZTD value can be observed at equatorial locations. Moreover, the change of ZTD from offshore stations is more intense, its peak-to-peak value of mean diurnal ZTD time series is larger than the inland by approximately 6 mm.