Early PVT in Urban Environment with Operational and New Constellations

Bernard Bonhoure, T. Chapuis, T. Junique, F.X. Marmet, S. Rougerie, F. Lacoste, D. Lapeyre, P. Noirat

Abstract:	With 30 operational satellites and even more, the GPS system is an excellent mean to determine one’s position or time. GPS has become quite strategic today and similar systems will emerge in the coming years, like GALILEO for Europe or BEIDOU for China. The GLONASS constellation has been completed and improved, and is increasingly used today mainly as a complement to GPS. Best smartphones now include GPS and GLONASS for positioning, and a good improvement for availability and time to first fix is seen. Augmentation systems like WAAS and EGNOS may also provide a good ionospheric model and even usable range measurements for a multi-constellation PVT. The future is likely to be full GNSS, i.e. the majority of receivers will receive and combine signals from available operational systems. This will allow a further improvement of performances, in particular in constrained environments, and will also offer operational back-up or independence in case of failure or unavailability of one system or even more. An important agreement was signed in 2007 between USA and Europe for GPS/GALILEO compatibility and interoperability, and actually, measurements from all constellations can be combined. With the successful launch of the four In Orbit Validation GALILEO satellites in 2011 and 2012, it is possible to combine GALILEO with GPS and GLONASS measurements for positioning. BEIDOU signals from MEO, IGSO, or GEO satellites can also be used with the three GPS, GLONASS, and GALILEO ones. New satellites from both constellations GALILEO and BEIDOU should be launched in 2015. Mixed GPS/GALILEO PVTs have been successfully processed over the last years by CNES, for static and dynamic receivers. Results were presented and published in last ION GNSS workshops, and pseudo-range achieved accuracy are of the same order for GPS and GALILEO. The paper will now address a full GNSS PVT in urban constrained environment i.e. with GPS, GLONASS and new constellations GALILEO and BEIDOU. An analysis of measurements, navigation messages, and PVT results will be done, in the signal, system and service domains. This will include Key Parameter Indicators (KPI) like 95% accuracies or pseudo-range residuals RMS. The theoretical approach for multi-constellation PVT will be presented. Fundamental equations will be given for the different system time scale resolutions, using the Broadcast GGTOs or not. Global Navigation Satellite Systems positioning is based on the synchronization of emitting satellites to a common reference time. With at least 4 synchronized satellites, the receiver can calculate the four usual unknowns: the 3D position and the bias between receiver and system time. So, in order to combine measurements from different satellite systems like GPS, GLONASS, GALILEO, and BEIDOU, the biases between the different system times shall be broadcasted, or determined at the user level as additional unknowns. A powerful 3D tool called SPRING is used. This tool is developed for CNES, the French Space Agency, by THALES Services, France. This tool can easily calculate a PVT from receiver real data for quite any combination of GNSS signals, in dual or single-frequency using the different ionospheric models available, even from SBAS. The Broadcast ephemerides are available from the four constellations even for GALILEO and MEO BEIDOU satellites, and are normally used. For some experiments, a precise reference trajectory is determined in urban environment from the Novatel SPAN INS/RTK system and Waypoint software. A trajectory with an accuracy of a few tens of centimeters can be obtained with post-processing with the high-grade inertial system used. In mid-urban conditions, a trajectory can also be generated from RTK or PPP processing. The precise reference trajectory is used by the SPRING tool to calculate precise positioning errors. The SPRING tool can also model the 3D environment for building masking and multi-path estimate at local level, using a ray-tracing technique and an estimate of diffraction, reflections, and diffusion effects. A precise urban 3D Model from French IGN cartography institute is used. The positioning estimated errors from simulation are then compared to real performances. First PVTs from combined GPS, GLONASS, and GALILEO constellations have been already obtained in urban environments. Four-constellation PVT with BEIDOU are already available in static mode, and dynamic experiments will be extended to BEIDOU. It can be shown that a PVT can be calculated for each stand-alone constellation, or for any combination of the constellations. Pseudo-range have typically an accuracy of a few meters or better, but quite large outliers of tens of meters can also be seen in urban environment due to multi-path or masking. Positioning accuracy greatly depends on the number of constellations used in PVT post-processing. First results show a good consistency between the real results and the 3D simulation at GPS single frequency level. Those results will be consolidated over the coming months, synthesized, and presented in the paper.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1978 - 1991
Cite this article:	Bonhoure, Bernard, Chapuis, T., Junique, T., Marmet, F.X., Rougerie, S., Lacoste, F., Lapeyre, D., Noirat, P., "Early PVT in Urban Environment with Operational and New Constellations," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1978-1991.
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