Performance Assessment of GPS-Sensed Precipitable Water Vapor using IGS Ultra-Rapid Orbits: A Preliminary Study in Thailand

Precipitable Water Vapor (PWV) is a significant variable used for climate change studies. Currently PWV can be derived from the Global Positioning System (GPS) observation in addition to the specific instruments such as Radiosondes (RS), Microwave Radiometers (MWR) and Meteorological Satellites. To accurately derive PWV from GPS data, long periods of observation time in conjunction with final orbit data have to be applied in the data processing steps. This final orbit data can be acquired from the International GNSS Service (IGS) with 13 days latency, which is not practical in climate change studies or meteorological forecasting. Alternatively, real-time ultra-rapid orbits are more suitable for this application but with lower orbit accuracy. It is therefore interesting to evaluate the impact of using different orbits in the estimation of PWV. In this study, data from permanent GPS base stations in Thailand were processed using Bernese 5.0 software to derive near real-time PWV values. Ultra-rapid orbit data have been introduced in the data processing step with different time windows and compared to that using final orbit data with the 24-hr time window. The results have shown that 1.0 mm and 2.9 mm biases can be achieved using 24-hr and 12-hr time windows, respectively. These results therefore address the potential use of ultra-rapid orbits for a near real-time estimation of PWV.