Abstract
The tropospheric delays estimated from the Global Navigation Satellite System (GNSS) have been proven to be an efficient product for monitoring variations of water vapor, which plays an important role in meteorology applications. The operational GNSS water vapor monitoring system is currently based on the Global Positioning System (GPS) and GLObal NAvigation Satellite System(GLONASS) dual-frequency observations. The Galileo satellite navigation system has been evolving continuously, and on 11 February 2019, the constellation reached 22 active satellites, achieving a capability of standalone Precise Point Positioning (PPP) and tropospheric estimation that is global in scope. This enhancement shows a 37% improvement if the precision of the Galileo-only zenith tropospheric delay, while we may anticipate further benefits in terms of tropospheric gradients and slant delays in the future if an optimal multi-constellation and multi-frequency processing strategy is used. First, we analyze the performance of the multi-frequency troposphere estimates based on the PPP raw observation model by comparing it with the standard ionosphere-free model. The performance of the Galileo-only tropospheric solution is then validated with respect to GPS-only solution using 48 globally distributed Multi-GNSS Experiment (MGEX) stations. The averaged bias and standard deviations are −0.3 and 5.8 mm when only using GPS satellites, respectively, and 0.0 and 6.2 mm for Galileo, respectively, when compared to the International GNSS Service (IGS) final Zenith Troposphere Delay(ZTD) products. Using receiver antenna phase center corrections from the corresponding GPS dual-frequency observations does not affect the Galileo PPP ambiguity float troposphere solutions. These results demonstrate a comparable precision achieved for both Galileo-only and GPS-only ZTD solutions, however, horizontal tropospheric gradients, estimated from standalone GPS and Galileo solutions, still show larger discrepancies, mainly due to their being less Galileo satellites than GPS satellites. Including Galileo E1, E5a, E5b, and E5 signals, along with their proper observation weighting, show the benefit of multi-frequency observations, further improving the ZTD precision by 4% when compared to the dual-frequency raw observation model. Overall, the presented results demonstrate good prospects for the application of multi-frequency Galileo observations for the tropospheric parameter estimates.
Highlights
Tropospheric delay refers to the refraction of the microwave signal when it passes through the neutral atmosphere
Lu et al [12] further demonstrated that the initialization time and the precision of multi-Global Navigation Satellite Systems (GNSS) tropospheric delays could be improved by using the Precise Point Positioning (PPP) ambiguity-fixing technique, which can meet the requirements of the troposphere estimates for time-critical meteorological applications
We analyzed the performance of Galileo observations for ZTDs and tropospheric horizontal gradient estimation
Summary
Tropospheric delay refers to the refraction of the microwave signal when it passes through the neutral atmosphere. Lu et al [7] developed an NWM-constrained PPP processing system to improve multi-GNSS precise positioning and demonstrated the contribution of the NWM model. Lu et al [8] derived horizontal delay gradients from the NWM model to augment the BeiDou System (BDS) PPP, and more than 30% of the precision improvement in the height component was observed. Lu et al [12] further demonstrated that the initialization time and the precision of multi-GNSS tropospheric delays could be improved by using the PPP ambiguity-fixing technique, which can meet the requirements of the troposphere estimates for time-critical meteorological applications. Liu et al [18] demonstrated that the Galileo triple-frequency PPP, with ambiguity resolution, was helpful in reducing the convergence time and to improve the positioning accuracy.
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