GNSS-based synchronization and monitoring of LEO-PNT onboard time

For best interoperability, navigation signals transmitted by a positioning, navigation, and timing system in low Earth orbit (LEO-PNT) should be synchronized to the system time scale of an existing GNSS, such as GPS, Galileo, and BeiDou or the Coordinated Universal Time (UTC). Even though an end-to-end assessment of inter-system time offsets can, in principle, be performed based on the analysis of LEO-PNT and GNSS ranging measurements from a joint receiver on ground, this concept lumps multiple error sources and provides only limited insight into the individual contributions. Subject to availability of a GNSS receiver onboard the LEO-PNT satellites, the principle of GNSS time transfer may be used as an alternative to directly determine the difference between the onboard time and a reference GNSS or UTC time scale. The technical requirements for this form of time synchronization are outlined and the individual analysis steps including precise orbit determination (POD) of the LEO satellite and precise point positioning (PPP) processing of GNSS measurements from a timing laboratory are described. For illustration, GNSS measurements from the Sentinel-6A satellite are processed offline in a reference implementation of a real-time navigation system. The resulting “onboard” time scale is then compared to UTC and Galileo System time. Using two fully independent POD/PPP implementations a better than 0.05ns consistency is demonstrated for the space-to-ground time transfer, which characterizes the achievable precision of the LEO-PNT onboard time monitoring. Making use of Galileo and GPS broadcast ephemerides, the sample Sentinel-6A navigation system is shown to provide an onboard time synchronized to Galileo System Time with RMS errors of 0.75ns (23cm) over a continuous 10-day test period. For comparison, 3D RMS position errors of about 9cm are obtained relative to a precise reference trajectory. The different performance of the onboard time and position determination is mainly attributed to the correlation of clock offsets and carrier phase ambiguities in the real-time navigation filter and appears as a limiting factor for the achievable onboard time synchronization accuracy using GNSS without complementary augmentation data.