Real-time linked common-view time transfer for monitoring GNSS system time differences

Abstract

This paper presents results obtained by conducting real-time linked common-view (CV) Global Navigation Satellite System (GNSS) time transfer between stations that are supposed to maintain various GNSS system times including those of GPS and Galileo. The distance between those stations is typically more than 5000 km and the conventional CV GPS time transfer may not be feasible for such long-distance time transfer. We used the real-time linked CV time transfer technique to monitor the GNSS system time differences, and the time links examined in this study are the USNO–PTB link for the GPS–Galileo system time difference and the USNO–NICT link for the GPS–QZSS system time difference. The performance of the developed time transfer method was assessed by comparing against other time transfer solutions such as TWSTFT. Finally, global closure was formed to examine the inherent noise level of the method in a self-sufficient way. Results showed that the real-time linked CV time transfer solution yields sub-nanosecond-level agreement with the reference solutions including TWSTFT, and its frequency stability is generally at the level of (3–4) × 10−15 in terms of the modified Allan deviation at the averaging time of one day, which seems to be sufficient to support civil interoperability between different GNSSs for sub-metre-level accuracy.