GPS-GALILEO Urban Interoperability Performance with the GPS-GALILEO Time Offset

Bernard Bonhoure, Inge Vanschoenbeek, Marco Boschetti, Jérôme Legenne

Abstract:	GPS solution in based on the synchronization of emitting satellites to the same time which is maintained in the GPS system. Glonass, Galileo… have their own time. So the differences between the different times of systems shall be known to synchronize all pseudo-ranges measurements in the receiver to a common time, and then to calculate a GNSS global position. Considering the GPS and Galileo possible combinations, there will be two main solutions to get the GPS to Galileo Time Offset (GGTO) : GGTO will be broadcast by both constellations, or it can be estimated in the receiver. In unconstrained environments, with typically 8 to 10 satellites from each constellation, those two solutions will have similar performances (4 or 5 unknowns versus nearly 20 measurements). The article focuses on constrained and in particular urban environnement performances for both solutions, where there are less satellites in visibility and multi-path effects. Simulations with a realistic 3D model of the city of Toulouse in France, have been done using a special software Ergospace® to evaluate the difference between the two solutions. A third solution using the “best” solution according to the number of satellites received and DOPs criteria has been also analysed. As shown from simulations, in the different constrained environments considered, positioning errors are mainly due to poor visibility and multi-path effects, and horizontal errors are typically between a few meters to nearly 50 meters. Main results are the following: • There are no big differences in simulated positioning variances using Broadcast GGTO or the receiverestimated GGTO, even in constrained environment. • Positioning performance sensitivity to GGTO accuracy is low, in particular in urban environment. As Broadcast GGTO error is likely to be a few nanoseconds, it can be directly used, in particular in urban environment, it could also be used for integrity checking. It shall be also noted that using a 5th unknown for GGTO will cope with GGTO residual error, inter-system biases, or SBAS corrections and will generally lower the residuals.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	971 - 981
Cite this article:	Bonhoure, Bernard, Vanschoenbeek, Inge, Boschetti, Marco, Legenne, Jérôme, "GPS-GALILEO Urban Interoperability Performance with the GPS-GALILEO Time Offset," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 971-981.
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