Galileo Navigation and Integrity Algorithms

Álvaro Mozo García, Carlos Hernández Medel and Miguel M. Romay Merino

Abstract:	Navigation algorithms play a key role in the provision of the Galileo Mission, since they are responsible for computing the essential information the users need to calculate their position: the satellite ephemeris and clock offsets. Such information is generated in the Galileo Ground Mission Segment (GMS) and broadcast by the satellites within the navigation signal. The algorithm definition process is driven by the demanding requirements placed on the navigation data. Accurate orbit and clock predictions are needed to satisfy the contribution to the range error allocated to the navigation message, and its validity time. While the orbit predictability is mainly driven by the performances of the Orbit Determination function (and hence by the quality of the observables and the accuracy of the models), the clock predictability depends heavily on the stability of the actual Galileo oscillators. We have been involved in the design and prototyping of the Galileo navigation algorithms since the very beginning of the project, and now are responsible for their operational implementation. The first part of this paper describes the process followed to come up with the current baseline, including the trade-offs between different strategies and observables, the performance assessment with GPS, the development of prototypes, the forthcoming experimentation with proto-flight models and the constraints imposed by the final implementation in a safety-critical operational environment. The performances reached at the different stages are presented, as well as the open points, critical issues and solutions adopted in each case. Galileo will also provide its users with an Integrity Service, which will broadcast information allowing the users to trust the system performances. This information is composed of parameters allowing the user to decide whether the system performances suit his needs (basically, a bound of the Signal-In-Space errors), and alerts warning the users when the system (or certain satellites) shall not be used because it is not possible to compute the error bounds or these are not considered reliable enough. The definition of the Integrity Service, that is, the means to achieve the required integrity requirements (probability that the system is correctly broadcasting the integrity information –error bounds and timely alerts-) has been subject of deep studies and has suffered significant evolutions (in comparison with the navigation algorithms), associated also to evolution of the system design and services definition. It should be noted that the provision of the Integrity Service is one of the major drivers of the Galileo system architecture definition. We are responsible for the operational implementation of the Galileo integrity algorithms, as well as for the last prototyping cycles. We have proposed innovative solutions and improvements to the algorithms derived in the previous phases of the project. The second part of this paper is devoted to describing the Galileo Integrity concept and the solutions proposed for its implementation, as well as the results obtained in our performance assessment experimentation.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	1315 - 1326
Cite this article:	García, Álvaro Mozo, Medel, Carlos Hernández, Merino, Miguel M. Romay, "Galileo Navigation and Integrity Algorithms," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 1315-1326.
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