First Integrity Results in Galileo: GIOVE-A Performances Obtained with the Experimental Integrity Process Facility

A. M. Curiel , B. Martín, A. Juez, M. E. Ramírez, C. Igual, F. Amarillo, A. Ballereau, B. Schlarmann

Abstract:	Galileo is the European Global Navigation Satellite System, under civilian control, and consists on a constellation of medium Earth orbit satellites and its associated ground infrastructure. This Global Navigation Satellite System will provide to their users highly accurate global positioning services and associated integrity information. In the frame of Galileo, the element in charge of the computation of the integrity information within the Galileo Ground Mission Segment (GMS) is the Integrity Processing Facility (IPF). One of the main features of any satellite navigation system that provides integrity is the monitoring of the signals broadcast by the satellites by means of a network of ranging stations. These ranging stations should provide to the IPF, the observations necessary for the satellite monitoring, enabling the IPF to generate, when necessary, timely alerts to the users, preventing them from the use of signals from satellites not working within the specifications. As the alerts should arrive to the users in the order of a few seconds the integrity monitoring is performed in real-time. The IPF was designed to this effect. It uses raw measurements collected by a worldwide distributed set of Galileo receivers that are properly pre-processed and validated. These validated measurements are then passed to the station clock estimation algorithm that follows a two-step approach: a real time algorithm based on a standard common view technique, executed every second, steered by a second one that is executed every minute, processing a batch of pseudorange and carrier phase measurements. This second algorithm has the objective of solving for certain parameters such as the carrier phase ambiguities and the zenith tropospheric delays which are not estimated by the real time algorithm. This global station clock estimation algorithm provides, in real-time, an accurate estimate of the zenith tropospheric delay and of the sensor station clock offsets necessary for subsequent algorithms. Finally, the last algorithms, based on the above mentioned inputs estimate the integrity data, in real-time, on second by second basis. Galileo requirements for the IPF are very stringent, not only in terms of products accuracy and availability, but also regarding continuity and integrity. A SW prototype of the Galileo IPF algorithms (E-IPF) has been developed as part of the verification environment to demonstrate the compliance with the requirements. After finishing the development, an experimentation and performance test phase started during which very promising preliminary results were obtained, based on the processing of real GPS data from a set of IGS sensor stations worldwide distributed (synchronisation error was in the order of 0.4 nanoseconds, and zenith tropospheric delay error was around 2 cm). The objective was to perform a full characterization of the algorithmic performance, not only in terms of accuracy in nominal conditions but also in terms of robustness to feared events and extreme conditions. The experimentation activities carried out with the E-IPF were based on the analysis of a wide set of E-IPF output data gathered in both, fault-free and feared event scenarios, including the last either satellite or sensor station feared events. These scenarios are built from real GPS data, and from synthetic data emulating Galileo or Galileo plus GPS data. GIOVE-A was the first experimental Galileo satellite to be launched and constitutes the first element of the Galileo In Orbit Validation (IOV) phase. GIOVE has made possible a further validation/experimentation step for the IPF algorithms thanks to the capability of its prototype, named E-IPF, to process GPS+GIOVE-A real data. This paper will provide a detailed analysis of the outputs of this last validation and experimentation process. The scenario under analysis consists on a network of 11 sensor stations worldwide distributed and a satellite constellation, including GIOVE-A and a set of real GPS satellites. Some E-IPF internal barrier thresholds were slightly tuned in order to overcome the poor geometrical coverage of the scenario. Among other performances, the estimated residuals, the sensor station clock estimation errors and the tropospheric delay estimations will be analyzed and compared with the results obtained in a scenario with better geometrical coverage. At the end, the obtained SISMA results and the integrity information will be also shown, together with the availability performance. The results presented demonstrate the suitability of the IPF algorithms for Galileo real data, providing integrity messages every processed epoch, even with a single Galileo satellite complemented by GPS satellites and a reduced Galileo Sensor Station (GSS) network.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	2751 - 2763
Cite this article:	Curiel, A. M., Martín, B., Juez, A., Ramírez, M. E., Igual, C., Amarillo, F., Ballereau, A., Schlarmann, B., "First Integrity Results in Galileo: GIOVE-A Performances Obtained with the Experimental Integrity Process Facility," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 2751-2763.
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