The Definition and Implementation of Galileo System Time (GST)

ICG-4 WG-D on GNSS time scales

Jérôme Delporte CNES – French Space Agency [email protected] 1 Outline

• General Architecture • PTF design • Link to UTC • Interoperability

2 General Architecture

• GST = Galileo System Time = reference time for the whole system in particular for prediction of Galileo satellite clocks • GST is under responsibility of the Galileo Mission Segment (GMS) • GST is generated physically and autonomously by two Precise Timing Facilities (PTF) • GST is visible to Orbitography and Synchronization Facility (OSPF) through the Galileo receiver at PTF

3 General Architecture

• GMS shall also support : – metrological function : GST will be precisely measured vs. UTC and steered to it modulo 1 second with inputs from GTSP – GPS-Galileo interoperability : GPS-Galileo Time Offset (GGTO) will be precisely measured and broadcast to users in coordination with USNO

• GST start epoch is 00:00 UT on August 22 nd 1999 • At this epoch, GST shall be ahead of UTC by 13 s and will not apply leap seconds • GST is therefore “aligned” to GPST • As of today, GST is ahead of UTC by 15 s

4 [TAI - Time scale (i) ]

35 UTC GLONASS time 25 GPS time 15 GALILEO time /seconds 5 Time difference TAI

-5 1970 1980 1990 2000 2010 Year

5 General Architecture

Galileo GPS constellation constellation

GPS receiver Link to USNO

Link to Galileo TWSTFT the other receiver PTF Orbitography GST station and Synchronization Facility Precise Timing Facility (PTF) GST steering (OSPF) to UTC

Galileo Time Service Provider (GTSP) 6 PTF Design

GNSS receivers and TW station

GST

GST steering/distribution

Processing Measurements Clocks

Precise Timing Facility (PTF) • Both PTF comprise : – a master clock (AHM) steered to a local ensemble of Cs clocks + steering to UTC – a back-up clock (AHM) steered to the master clock – GNSS receivers and TWSTFT stations for PTF/PTF steering, GGTO determination and steering to UTC 7 PTF Design

• The 2 PTF are in master-slave configuration : – each one provides its own realization of GST – GST is the output of the master PTF – PTF-to-PTF offset is measured through internal OSPF products, TWSTFT and GNSS Common View – the difference between the 2 PTF shall be < 4 ns (95 %) – a transition law is implemented in case of switch to ensure a smooth transition

8 Link to UTC GEO TWSTFT with GNSS CV as back-up

TWSTFT TWSTFT

ata rison d compa Clock UTC(k) Cl ock com GST pariso n data GTSP GST-UTC offset and steering PTF ) C(k -UT (k ) GMS TC TC U T -U GS UTC-TAI BIPM | UTC – GST | ≤ 50 ns (95 %) IERS

9 Interoperability

• GGTO = GPS-Galileo Time Offset – GGTO is computed on both GPS and Galileo sides independently but its broadcast value will be coordinated between the systems – GGTO is determined by TWSTFT and/or GNSS Common View between the PTF and the USNO – GGTO accuracy shall be < 5 ns (95 %)

• Galileo will also broadcast GST – UTC and the leap second information • It is also foreseen that Galileo broadcast UTC – UT1