Real time PPP using IGS products for quality monitoring of GNSS reference stations

GNSS continuous monitoring networks provide observations to the users for computing coordinates, among other applications. Such observations are also used for orbit and satellite clocks corrections computations, which are performed by the analysis centers of IGS, allowing real-time GNSS positioning using PPP method. It was developed a real-time monitoring tool for monitoring active GNSS stations quality, using PPP method and open source interactive map tools (Open Street Map - OSM). For testing the concept, we used five GNSS stations at State of São Paulo, Brazil, which have high accuracy coordinates estimated in the context of SIRGAS. BKG NTRIP Client (BNC) software is used to estimate the real-time coordinates in a kinematic mode. The station data are provided by UNESP caster server. The streams of orbits and satellite clock errors are obtained from the IGS product caster server. Dual frequency observations L1 / L2 (carrier wave phase and pseudorange) are considered only for the GPS and GLONASS constellations. Constraints of 0.020 m and 0.050 m are imposed to the a priori coordinates of the reference stations, for the horizontal and vertical components respectively and maintained for a period of 2 minutes. Noise level of 100 m per epoch is assigned for coordinate variation after the initial 2 minutes. The quality was analyzed in the position domain, comparing the estimated coordinates with those provided by SIRGAS more recently solution for the epoch of the observation. The Velocity Model for SIRGAS (VEMOS 2017) was used as needed. A web interface based on Leaflet JavaScript libraries is used to show the results. Some features have been incorporated into OSM maps, such as the viewer of stations according to the status, as well as the coordinates and estimated real-time accuracies. These features indicate the real-time status of the stations. Finally, we used an open source tools to decode, process, compare, and visualize the coordinate discrepancies, whose values are an indirect indicative of the quality of the data generated by the stations. It allows to the "managers" and users of the networks to track the integrity of each station in real-time and to take any necessary decision. In the future, additional information may be included, such as the S4 index for ionospheric scintillation alert, in case of unexpected results.