Promising Results on Detection Capability of Galileo OSPF and IPF Integrity Barriers

A.B. Martín-Peiró, M.E. Ramírez, C. Igual-Bets, A.M. Curiel, A. Juez-Muñoz, F. Amarillo-Fernández, A. Ballereau, B. Schlarmann

Abstract:	The Orbitography and Synchronization Processing Facility (OSPF) and the Integrity Processing facility (IPF) are the two critical elements included in the Galileo Ground Segment Facility (GMS) in charge of providing the navigation and integrity products in Galileo, to be disseminated to the Galileo users through the Galileo Signal-In-Space. Both elements use raw measurements collected by a worldwide distributed set of Galileo receivers to estimate the parameters that allow them to properly compute the navigation and integrity data, respectively. Then, OSPF provides the user with navigation messages which integrity is checked by IPF. Galileo requirements for these two elements are very stringent, not only in terms of products accuracy and availability, but also regarding continuity and integrity. In addition, performance must be guaranteed not only in the nominal, fault-free case, but also when some feared events are present in the input data, induced either by satellites, or by other ground segment elements (e.g. sensor stations), or by the environment. As a consequence, a trade-off needed to be done in order to optimise performances in nominal performances without loosing robustness for less favourable conditions. The Navigation Processing implemented in the OSPF includes different modules, aimed at computing the ephemeris, clocks, Signal In Space Accuracy (SISA), ionospheric model and differential group delay parameters. The Integrity processing implemented in the IPF also includes different modules, with the objective of computing an Integrity Flag qualified by the Signal In Space Monitoring Accuracy (SISMA). An important effort was devoted to reduce the time synchronization error to the sub-nanosecond level, leading to significantly reduce the SISMA upperbound and to reduce the Signal In Space Monitoring Error (SISME) In order to be able to mitigate the effect on the outputs of eventual problems in the input data, it has been necessary to include complex algorithm barriers for both elements, to ensure that potential degradation in the input data would be successfully palliated before affecting the output navigation and integrity messages. The barriers included in the design are aimed at detecting problems not only in the input data, but also in intermediate data that is exchanged from one algorithm to the others. For this purpose, each barrier is based in the comparison of a barrier-specific observable against a threshold; being excluded the input data from further processing when it is exceeded, thus avoiding the corruption of the output data. Different types of algorithm barriers have been defined, some of them leading to reject isolated measurements, and some others to reject a whole satellite or station from the estimation process. In order to anticipate the expected performances of OSPF and IPF and to support the navigation and integrity algorithms definition, two SW platforms prototypes named Galileo E-OSPF and E-IPF were developed. In addition, the barriers were preliminary designed using adhoc implementations in these prototypes, before including them in the operational element design. Furthermore, some design decisions were taken before the operational element development started, based on the analyses carried out at early stages of the programme. The E-OSPF and E-IPF have been of paramount importance in order to evaluate the suitability of the designed barriers, as they have made possible to test all algorithms in an integrated mode, using different input datasets. The experimentation activities carried out with E-OSPF and E-IPF include the evaluation of the navigation and integrity determination algorithm performances on fault-free and feared event scenarios, using real GPS data, GIOVE-A data, and simulated Galileo observables. The validation activities carried out with the prototypes in a wide set of data allowed to conclude that the stringent performances requested for OSPF and IPF can be met, even with margin under the defined input conditions. The evaluation of the navigation and integrity performances in fault-free scenarios has been complemented with the assessment of the behavior against the input feared events. In particular, it has been assessed that both elements are able to detect and isolate different problems simulated in the input data, including satellite unplanned manoeuvres or excessive measurement noise, for example. In general, if the problem is located in a satellite, we have checked that this is correctly flagged as “NOK”, so that the user is warned and it does not use it. On the other hand, when the problem is located in a receiver, both elements are able to isolate it to remove the faulty measurements from the process, without impact on the rest of the outputs. This allows producing valid navigation and integrity messages for all satellites, even in the cases with erroneous input data. This paper will present the most important algorithm barriers included in Galileo OSPF and IPF. Some details on the algorithm definition will be provided, together with the complex strategy followed to validate them in the prototype environment. Finally, promising results will be presented, showing the detection capability of the implemented barriers and the behavior of the Galileo navigation and integrity facilities.
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:	2764 - 2774
Cite this article:	Martín-Peiró, A.B., Ramírez, M.E., Igual-Bets, C., Curiel, A.M., Juez-Muñoz, A., Amarillo-Fernández, F., Ballereau, A., Schlarmann, B., "Promising Results on Detection Capability of Galileo OSPF and IPF Integrity Barriers," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 2764-2774.
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