Galileo Open Service Time Performance
Amale Kanj, Jérôme Delporte, Norbert Suard, Bernard Bonhoure, Centre National d’Études Spatiales, CNES, France; Pascale Defraigne, Royal Observatory of Belgium
By the Galileo Initial Services Declaration on 15 December 2016, Galileo has officially started delivering services to users around the world. In addition to the positioning service, Galileo is able to disseminate an accurate time scale referring to UTC or providing to users interoperability with GPS and improving resilience.
Galileo OS users are thus able to obtain a UTC SIS realization by applying the UTC conversion parameters to Galileo System Time (GST), broadcast in Galileo OS navigation message additional parameters, according to the GST-UTC conversion algorithm as described in the GALILEO ICD .
GST is under responsibility of the Galileo Mission Segment (GMS). It is realized using the atomic clocks located at the Precise Timing Facility (PTF) and a steering on an average of realizations of UTC maintained in some European Time Laboratories.
Since GPS and Galileo are not based on the same reference time system, it is fundamental to monitor their offset in order to ensure interoperability between the two systems allowing users to benefit from a combined GPS/Galileo positioning using the Broadcast Galileo to GPS Time Offset (GGTO) parameters and algorithm rather than estimating it at the receiver level. This could be more efficient in some particular environments where the number of visible satellites is limited. This offset is computed afterwards on both GPS and Galileo sides independently but it is broadcasted for the moment in Galileo navigation message only. Since 29 June 2017, it is calculated by the GMS with measurements from a dual constellation receiver Galileo-GPS. The United States Naval Observatory (USNO) data are not used anymore by the GMS to calculate the broadcast GGTO.
In order to monitor Galileo OS Time performance, the French Space Agency (CNES) which is involved in the performance monitoring from the early steps of the program has developed specific independent means using calibrated GPS/Galileo stations. In this context, three Key Performance Indicators (KPIs) are monitored: the offset between the Galileo System Time and UTC, the OS dual-frequency UTC dissemination accuracy and the GGTO dissemination accuracy.
Since 2010, CNES performance monitoring means are managed under one operational environment called Navigation and Time Monitoring Facility (NTMF) dedicated to GNSS and Satellite Based Augmentation Systems (SBAS). For the activities concerning Galileo Time performance, CNES developed two independent Time monitoring chains to be integrated in the new updated version of this facility. Two different tools are used: SPRING software (CNES tool) and R2CGGTTS software (Royal Observatory of Belgium (ORB tool). SPRING is dedicated to advanced display, analysis and simulations around systems integrating GNSS positioning. R2CGGTTS is a software program dedicated to provide clock solutions for GNSS time transfer in the CGGTTS format (Common GPS GLONASS Time Transfer Standard). Both tools require RINEX observation and navigation files as input; as well as BIPM circular T files for monthly results.
To achieve this monitoring purpose, the GNSS stations used are the Royal Observatory of Belgium (ROB) station BRUX connected to UTC(ORB) on one hand and CNES station CS11 connected to UTC(CNES) on the other hand, both calibrated for Galileo and GPS. For a relevant monitoring, the KPIs are computed with the two independent time monitoring chains using each station. The results are also crosschecked monthly and compared to the corresponding system requirement to check the compliance.
This paper presents the developed monitoring means; explains the used approach for the KPIs computation and shows the results of one year of monitoring at CNES.
 GALILEO Open Service - Signal In Space: Interface Control Document, 1.3 Dec 2016