Using Broadcast Time Offsets for Multi-Constellation Users in Harsh Environments

As the number of Global Navigation Satellite Systems (GNSS) is increasing, more and more users depend on multi-GNSS Position, Velocity and Timing (PVT) solutions. Each GNSS, however, operates at its own timescale. Therefore, inter-system time offsets (i.e. system X to system Y Timing Offset [XYTO]) are necessary to align the measurements to a common timescale (e.g. GPS-Time). This study analyses the impact of using the XYTO broadcasted by each system (e.g. Galileo to GPS Time Offset [GGTO]) on the PVT accuracy. This accuracy is compared to the PVT accuracy when this time offset is estimated by the receiver (at the expense of one additional satellite). Real world test data have shown that in good visibility conditions, it appears to be better in terms of PVT accuracy to estimate the XYTO by the receiver. However, for a broadcasted XYTO with an accuracy of a few nanoseconds, the position performance while using either the broadcasted XYTO or the estimated XYTO is very similar. In more challenging environments (e.g. urban canyons), a benefit is evident when the XYTO disseminated by the system is used. Due to the low number of available satellites in these types of environments, estimating XYTO by the receiver negatively impacts the effective PDOP, resulting in larger position errors. When two systems are used for which no direct XYTO is (currently) broadcasted by the system (e.g. Galileo and BeiDou), a XYTO can always be obtained by using UTC as a pivot. In this way the broadcasted time offsets of each system with respect to UTC are used to acquire XYTO. Real world test data have shown that for both the combination GPS and Galileo as well as the combination Galileo and BeiDou, using UTC as a pivot is a viable option. In open-sky conditions, using the XYTO computed using UTC as a pivot results in a very similar position accuracy compared to the position accuracy while using the XYTO estimated by the receiver.