Characterization of the Navigation Performances in Galileo: E-OSPF SW Experimentation Results

The Orbitography and Synchronization Processing Facility (OSPF) is the part of the Galileo system in charge of the computation of the navigation data products to be disseminated to the Galileo users through the Galileo Signal- In-Space. In order to anticipate the expected performances of this critical element and to support the navigation algorithms definition, a SW platform named Galileo E-OSPF has been developed as a prototype of the operational SW. The algorithms implemented in this prototype are an evolution of previous orbit determination and time synchronization products produced in the early phases of the Galileo Program, which were found to provide satisfactory performances with real GPS data, such as the Galileo System Test Bed Version 1 Experimental OSPF (GSTB-v1 E-OSPF, [RD.6]). The implemented evolutions include algorithm adaptations to OSPF real-time requirements. The GSTB-v1 project was launched in preparation for the development of the Galileo system, as an experimentation platform aimed at carrying out tests of Galileo orbit determination, integrity and time synchronization algorithms. For this purpose, several experimental facilities were used, namely: a ground segment consisting of a worldwide network of sensor stations collecting GPS observables, a Precision Timing Station providing the reference time scale steered to UTC/TAI and a Processing Centre located at the European Space Agency (ESA-ESTEC) in the Netherlands. The GSTB-v1 E-OSPF was integrated in the GSTB-v1 Processing Centre, and the experience gained in that development has been of paramount importance for the Galileo E-OSPF. Moreover, the GSTB-v1 E-OSPF has been extensively used for validation purposes. In parallel to the development of the Galileo system, and in preparation for that, two experimental satellites called GIOVE-A and GIOVE-B will be used. The launching of the two satellites is part of the Galileo System Test Bed Version 2 program (GSTB-v2), together with the associated GSTBv2 Mission Experimentation tasks [RD.7] and [RD.8]. The experimentation test cases are performed using GSTB-v2 EOSPF, which is basically an evolution of its counterpart in GSTB-v1 aimed at obtaining the best performances with GIOVE. In this context, in addition to the processing of GIOVE data, the GSTB-v2 E-OSPF SW is endowed with the capability of processing additional measurement sources, such as Satellite Laser Ranging (SLR) and GPS. The Galileo E-OSPF platform has been designed so as to fulfill both the performance requirements, for both Full Operational Capability (FOC) and In-Orbit Validation (IOV) phases, in terms of accuracy, integrity and availability, and the stringent operational constraints that will be present in the operational SW, such as the processing time and memory limitations of the operational processing boards. Due to these constraints, some differences can be found between both Galileo E-OSPF and GSTB-v2 E-OSPF SW architectures, although the core orbit determination and time synchronization algorithms implemented in the two EOSPFs are basically the same. Thus the lessons learnt in the GIOVE data processing with GSTB-v2 E-OSPF SW are directly applicable for the Galileo E-OSPF SW. In particular, the behavior of the on-board satellite clocks observed in GIOVE data has been injected in the Galileo E-OSPF SW design so as to improve the final performances of the Galileo navigation message. Due to the fact that there are a number of key elements of the Galileo System are not yet available (e.g. the Galileo satellites), the validation of the E-OSPF becomes a challenging task. In order to gain progressive confidence on the final product, the performance validation is carried out in several steps, including, for example: • Performance assessment of the Galileo E-OSPF using real GPS data, taking advantage of Galileo and GPS commonalities. • Complement the previous results using GIOVEA and GIOVE-B data. The GIOVE-A test campaign will allow the validation of some performance-critical satellite models implemented in the OSPF such as the solar radiation pressure (SRP) and the onboard clock models.