Session P5: GNSS Systems Timing Architectures and Capabilities

Prototyping the Future Time Monitoring Service for Galileo at ESA-ESTEC
J. Hahn, G. Galluzzo, B. Quaranta, S. Perea, G. Lopez, C. Plantard, P. Waller, ESA-ESTEC; J. P. Boyero, European Commission; F.J. Sobrero, C. Garcia, L. Dominguez, GMV Spain; E. Strazzer, N. Rana, A. Ascenzi, Thales Alenia Space
Location: Seaview A/B
Date/Time: Thursday, Jan. 30, 2:58 p.m.

The Galileo system is one of the most important space endeavours undertaken in Europe, with one of the largest European satellite constellation and ground segment. Through the recent satellites’ launch cadence acceleration, with the latest Launch (L)12 and L13 satellites already deployed in orbit, Galileo completes its constellation as designed, and it is ready to support full operational capabilities. The European Space Agency (ESA), entrusted as the Galileo system architect and design authority, has been supporting the development of the Galileo system since its early inception. 2025 will mark the 30th anniversary of the start of the very first European GNSS R&D program at ESA.
While the Galileo first generation (Galileo G1) is already providing services to billions of users worldwide, major development efforts are ongoing to build its second generation (Galileo G2). G2 will provide a portfolio of new services, that will have to meet the demand of a very dynamic market and user community. Location based services have become a pervasive technology, there is an increasing demand for both high accuracy and integrity. This demand is also extending to timing applications, from communication systems, financial and 4G/5G mobile networks to electrical power plants, multiple critical systems require a reliable source of time synchronization as part of their core business. Galileo G1 is already providing very accurate nanosecond level predictions of Galileo System Time (GST) and UTC, through its Open Service.
However, in order to support the new critical timing and synchronisation applications, the Galileo Programme has defined a dedicated Timing Service as part of Galileo G2 mission, consisting in a new Timing Service Message (TSM) informing the users in near real time of the level of trust in the disseminated GST and UTC predictions. As per TSM Operational Status Document [1], two sets of information will be broadcast, related to the accuracy performance of GST and GST-UTC disseminated by Galileo:
- GST Status, for each satellite realization
- GST-UTC Status, for the entire constellation
Different service levels are defined based on the maximum tolerable error guaranteed by the ground monitoring.
The Galileo Second Generation System Test Bed (G2STB) with its core infrastructure hosted and operated at European Space Research and Technology Centre (ESTEC), is the main experimentation and prototyping platform used by ESA to support the development and demonstration of new services and system capabilities. The G2STB industrial consortium, with the support of the ESTEC UTC laboratory, has implemented a first prototype of TSM running in near real time. G2STB also inherits the monitoring functionalities of previous platforms, allowing the comparison of the new real time TSM results with standard processing techniques for timing monitoring based on a non real-time approach. This monitoring includes the final Galileo timing performance, the satellite and station clocks stability analysis and the GST generation.
The timing monitoring service results included in this work are based on a real-time monitoring of Galileo timing performance done by a network of stations located in UTC(k) labs. The choice of having more stations is related to the need of isolate any possible issue in the reference timescales themselves: an anomaly detection algorithm is included in the data pre-processing to exclude the station having a timing anomaly or a lack of data. A nominal number of 3 stations is used in the current prototype of TSM; they are located at ESA-ESTEC (Netherlands), INRIM (Italy) and PTB (Germany).
A key prerequisite to perform this monitoring is to have calibrated stations in the frequencies of interest, that are Galileo E1, E5a and E5b. The three stations have been calibrated performing relative calibration for E1, E5a (other UTC(k) stations calibrated with BIPM G1G2 travelling stations were used as reference) and absolute calibration for E5b; in particular, the latter was executed thanks to a well-established absolute calibration procedure in ESTEC UTC laboratory. These multi-GNSS receiver calibration activities performed at ESTEC will be briefly presented.
The structure of TSM algorithm is composed initially of a preprocessing of 3 continuous stream of GNSS receiver binary data from the different UTC(k) labs: multiple steps of averaging and filtering are executed in order to reduce the noise and remove outliers from the data. Then an assessment of timing performance level is obtained for each data stream; finally, the UTC status is obtained by cross-checking the results of the three different data streams. The GST status is obtained by a dedicated SISE monitoring module that is able to determine the clock error for each satellite.
The preliminary results of the TSM prototyping and experimentation activities will be presented, including testing of anomaly detectors in case of UTC lab anomalies (both phase and frequency jumps), testing of GST status degradation for a single satellite, and UTC status degradation for the entire constellation.
[1] Galileo Timing Service Message Operational Status Definition (TSM OSD), Issue 1.0, April 2024