In the form of satellite ephemerides and clock parameters, the space datum and system time information of one global navigation satellite system (GNSS) is transferred to users. With the continuous updating in the satellite payload such as the high-precision atomic clock, the monitoring and tracking technique such as the inter-satellite link, and in the data processing technique, the accuracy and real-time performance of the satellite ephemeris and clock error products are steadily improved. Starting from December 27th, 2018, the BeiDou Navigation System 3, or BDS-3, has provided the accurate and reliable basic positioning, navigation, and timing (PNT) service for the users in the countries within the “one belt and one road”. This paper has summarized the faced challenges of the precise orbit determination and time synchronization from the regional BDS-2 system to the BDS-3 global system, and the specific solutions at the control segment. In addition, this paper has compared the BDS with other GNSS systems in terms of technical characteristics. Finally, aiming at a higher accuracy and more reliable PNT service, the road map of precise orbit determination and time synchronization technique for the next generation navigation systems is discussed, which will provide a reference for developing the global navigation satellite systems with an even higher accuracy. 相似文献
The results presented show how glacier velocities can be measured and calculated from Earth Resources Satellite (ERS) tandem interferometric synthetic aperture radar (InSAR) data by the use of geographical information systems (GIS) and non-interferometric software. A semi-automatic algorithm using existing and newly implemented modules in ESRI's GIS software Arc has been developed to unwrap existing height differentiated interferometric SAR (DInSAR) data. The algorithm is applied to a data set covering glaciers from the arctic archipelago Svalbard. Estimated DInSAR velocities are decomposed into the glacier surface flow direction using a digital elevation model (DEM). Velocity fields for the glaciers Isachsenfonna, Akademikerbreen and Nordbreen on Svalbard are presented. 相似文献