Prototyping of Galileo URA Determination with TGVF and Extended Galileo Performance Characterisation for SoL Applications

Abstract

The Time and Geodetic Validation Facility (TGVF) has played an important role in the System integration verification and performance monitoring of Galileo during its deployment phase. Now Galileo has moved into its exploitation phase and the TGVF has become a key asset to support Galileo System improvements providing early prototyping and experimentation, as well as monitoring and performance assessment. This is for instance the case of the use of the TGVF environment as a proof of concept for the future computation of the User Range Accuracy (URA) in the Galileo System. Several URA computation algorithms have been prototyped and tested. TGVF is shown to be the ideal testbed for prototyping and testing the URA concept. The paper focuses on the TGVF capability to reproduce the most representative Galileo capabilities involved in the URA concept, namely Orbit and Clock Determination, historic URA computation using own and external orbit and clock references, and finally user performance assessment considering the broadcast URA. The prototype URA algorithm implemented in TGVF is used to determine URA values for GPS and Galileo; results for both constellations are provided in this paper. It is particularly interesting to compare for GPS the URA values obtained with the URA prototype implementation in TGVF against the actual URA values broadcast in the GPS Signal in Space. The TGVF offers also the capability to measure the impact of the new algorithms on the user, including the ARAIM user, checking that the actual Galileo Signal in Space Error (SISE) is sufficiently overbounded by the obtained URA, or the protection levels and expected availability computed in the ARAIM solution. This is done in near real-time, offering the capability of comparing how the proposed algorithms behave under the appearance of new events, but it can also be done in post-processing, using all the historical data within the platform, allowing to see the behavior of the algorithms during the occurrence of well know events. As a complement to the URA prototyping in TGVF this paper also addresses in detail the characterization of Galileo for ARAIM relevant parameters based on measurements obtained from the system in operation. This is one of the outcomes of the Integrity Failure Mode and Effect Analysis (IFMEA) that has been established for Galileo in the frame of the Galileo Open Service Safety of Life (GoSoL) workplan, which is conducted by ESA, EC and GSA together with Industry. Since the Galileo initial service declaration in December 2016, millions of real observations have been collected over more than 3.5 years reflecting the real Galileo performance and behavior. All this data has been used to characterize the SISE at Worst User Location (WUL) serving as data base for an a-posteriori URA computation. The results presented in this paper are an extension of a corresponding paper (see [Ref. 5.]) presented at ION GNSS+ 2019 and indeed confirm the good performance and the low number of relevant faults observed in Galileo. Results based on a-posteriori characterization for URA and Broadcast Group Delay (BGD) error overbound are presented in this paper.