The standard positioning service provided by the GNSS systems usually has an accuracy of meter-level. For precise applications, such as geodesy, surveying and mapping, as well as geo-hazard early warning, positions or position changes up to millimeter-level accuracy are required. In order to meet such requirement, the critical biases in satellite orbits, clocks and atmospheric delays must be precisely tackled according to their spatial and temporal characteristics. The group concentrates on the improvement of the performance of real-time GNSS precise positioning service in terms of accuracy, integrity, availability and continuity to meet requirements of various high-precision applications.
The main topics are:
The group has been working on the development of its own real-time precise positioning service since 2007 motivated by geohazard early warning system. At server end, with the EPOS-RT software platform we are providing multi-GNSS orbits, clocks and uncalibrated phase delays (UPDs) via INTERNET for real-time precise point positioning (PPP). At user end, PPP ambiguity-fixing and regional augmentation were invented and widely recognized for shortening the convergence time of traditional PPP and for instantaneous ambiguity-fixing, respectively. As one of the IGS real-time analysis center, we are providing a scalable positioning service with up to millimeter level accuracy using GPS, GLONASS, GALILEO and BeiDou signals for various applications, such as real-time GNSS seismology, GNSS meteorology, etc..
Recently, several dense LEO constellations of hundreds to thousands satellites have been planned for global communication and a few of them are ready to launch satellites beside the IRIDIUM system with its 66 satellites already in operation. With the similar navigation signals transmitted from the LEO satellites, the observing geometry of the user stations will be significantly increased not only because of more satellites but due to the fast movement of LEOs, 1 to 2 hours per revolution. Consequently, positions of cm-level could be achieved within few minutes. We have extended the current GNSS data processing platform to include all GNSS data in order to develop a prototype of LeGNSS and to demonstrate the impact of a LEO constellation on current GNSS RTS and finally to identify the best and pragmatic data processing strategy of LeGNSS and to realize the software platform.
Nowadays, GNSS is widely used for outdoor positioning and navigation. However, its performance depends sensitively on the quality of continuous signals directed to a receiver. Hence, it usually does not work well when satellite signals are severely blocked, for example, in city canyons and indoor conditions. Thus, active navigation systems or extra sensors are introduced into GNSS, for example, the most effective inertial navigation system (INS). Other sensors like the Odometer, magnetometer, Wi-Fi, ultra-wide-band (UWB), Vision etc. are also considered. Since GNSS/INS integration system can also provide precise attitudes that are very critical for a number of moving platforms in Earth Observing System (EOS), in the last few years the group has been working hard to develop its own software package for both post-mission and real-time multi-sensor navigation. Beside the theory and algorithms of each individual sensor, rigorous mathematic models and sophisticated quality control are major concerns in multi-sensor integration.
With the real-time precise positioning service, site displacements can be derived for geohazard monitoring, for example, earthquake, volcano, tsunami monitoring and early warning (Dept 2). Precise trajectories and attitudes can also be provided for airborne and ship-borne GNSS reflectometry and optical remote sensing (Section 1.4), gravity surveys (Section 1.2). It can also provide real-time ionospheric and tropospheric information from both static and mobile platforms for short-term weather events forecasting (Section 1.1). The deep level combination of GNSS precise positioning service with the current sensors in the relevant fields, such as accelerometers, seismic sensors, water vapor radiometers could improve the results dramatically because of their complementarity.
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 Ge M, Douša J, Li X, Ramatschi M, Nischan T, Wickert J (2012): A novel real-time precise positioning service system: global precise point positioning with regional augmentation. - Journal of Global Positioning Systems, 11(1):2-10. DOI: http://doi.org/10.5081/jgps.11.1.2
 Li X, Zhang X, Ge M (2011): Regional reference network augmented precise point positioning for instantaneous ambiguity resolution.Journal of Geodesy, 85(3):151-158. DOI: 10.1007/s00190-010-0424-0
 Li X, Ge M, Dai X, Ren X, Fritsche M, Wickert J, Schuh H. (2015): Accuracy and reliability of multi-GNSS real-time precise positioning: GPS, GLONASS, BeiDou, and Galileo. Journal of Geodesy, 89(6):607-635. DOI:10.1007/s00190-015-0802-8
 Li, B, Ge H, Ge M, Nie L, Shen Y, Schuh, H. LEO enhanced Global Navigation Satellite System (LeGNSS) for real-time precise positioning services, Journal of Geodesy, in revision
 Reid TG, Neish AM, Walter TF, Enge PK (2016): Leveraging Commercial Broadband LEO Constellations for Navigation. The 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland,
 Gao Z, Ge M., Shen W, Zhang, H., Niu, X. (2017): Ionospheric and receiver DCB-constrained multi-GNSS single-frequency PPP integrated with MEMS inertial measurements. Journal of Geodesy, 91(11):1351-1366. DOI:10.1007/s00190-017-1029-7.
 Gao Z, Zhang H., Ge M., Niu X., Shen W, Wickert J., Schuh, H. (2016): Tightly coupled integration of multi-GNSS PPP and MEMS inertial measurement unit data. GPS solutions, 2(21), 377-391.