Galileo Orbitography and Synchronisation Processing Facility (OSPF): Preliminary Design

J.R. Martín, I. Castrillo, A.B. Martín, S. Cilla, E. Mora

Abstract:	The essential mission of the OSPF within the Galileo Ground Segment is the determination of the navigation data products to be disseminated to the Galileo users through the Galileo Signal-In-Space. The navigation data products computed by the OSPF include: - Satellites ephemeris in Galileo Terrestrial Reference Frame (GTRF). Eight ephemeris datasets are provided for a total period of 24+1 hours, each individual dataset covering 3+1 hours with 1 hour overlapping between consecutive datasets. Each individual dataset contains the coefficients of a 16-parameter ephemeris model similar to that currently being broadcast by GPS. - Satellites clock correction parameters in Galileo System Time (GTS) for dual frequency users operating with the L1-C / E5a-Q (Open Service –OS), L1-C / E5b-Q (Safety-of-Life - SoL) and L1A /E6A (Public Regulated Service – PRS) ranging signals. The clock correction parameters for a given satellite include a phase offset af0, frequency offset af1, and frequency drift af2 corrections w.r.t GST plusa clock reference time. - Satellites Signal-In-Space A ccuracy (SISA) indicators. The SISA parameter is one of the cornerstones of the Galileo Integrity Concept, being an statistical bound of the a-priori expected ranging errors experience by dual-frequency users employing the ephemeris and clock correction parameters computed by the OSPF for a particular service (OS, SoL, PRS). Different SISA indicators are provided for OS, SoL and PRS. The SISA parameter is always meaningful as a ranging accuracy indicator for all services but it provides an integrity guarantee for SoL and PRS only during the initial 100 minutes of validity of the associated ephemeris and clock correction dataset. The OSPF SISA is processed by the user together with the integrity flags and the Signal-In-Space Monitoring Accuracy (SISMA) parameters computed by the Integrity Processing Facility (IPF, [RD.9]) to determine the integrity of its current positioning solution. - Ionospheric correction parameters for single frequency users (SFIONO). These are the coefficients ai0, ai1 and ai2 of the second order polynomial thatbetter fits the effective ionization level “Az” of the NeQuick model [RD.3] as a function of the geomagnetic latitude. - Ionospheric Disturbance Flag (IDF). This flag is used to warn the users when the ionospheric correction model in a given region of the Globe is deviating excessively from the actual behavior of the ionosphere. The OSPF provides capacity to compute IDFs for up to 5 different regions. - Broadcast Group Delay (BGD) correction parameters. The BGD parameter contains the correction due to satellite HW delays that single-frequency users have to apply to employ dual-frequency clock corrections parameters. The BGD is similar in purpose to the TGD parameter currently broadcast by GPS. Since it is linked to the clock correction model being applied, three BGD corrections are computed, one for each clock correction model (L1-C/E5a-Q, L1-C/E5b-Q, L1A/E6A). The OSPF implements all the algorithmic processes needed to compute the above products in a single facility, including the necessary monitoring and control capabilities to allow its operation by ground operators. The OSPF has been designed to operate in a near autonomous manner with minimum supervision from the operators. Four redundant OSPF facilities are deployed in the Galileo Ground Segment for service availability reasons. Two OSPF facilities are deployed in each Galileo Control Center (GCC). The different OSPF performance requirements include in particular some integrity requirements aimed to limit the risk of generating misleading information at the output of the OSPF. The Galileo Mission Segment (GMS) safety analyses have found that this OSPF Output Feared Event is of Major severity, and hence, OSPF Operational SW has to be developed for a development assurance level (DAL) “C” as defined in the Galileo SW Standards [RD.4], which is rather equivalent to RTCA DO-178B [RD.5]. This paper provides an overview of the most relevant requirements, design drivers, architecture and performance features of the OSPF facility.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	575 - 583
Cite this article:	Martín, J.R., Castrillo, I., Martín, A.B., Cilla, S., Mora, E., "Galileo Orbitography and Synchronisation Processing Facility (OSPF): Preliminary Design," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 575-583.
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