Timing Calibration of a GPS/Galileo Combined Receiver

摘要

Navigation users will soon benefit from multiple GNSS satellite constellations, potentially doubling or tripling the number of usable GNSS satellites. The improved satellite visibility, and reduced dilution of precision (DOP), will be particularly useful in urban canyon environments where sky visibility is challenged. The independent GNSS navigation time scales are typically traceable to UTC (module whole seconds) to better than 50 nanoseconds. To be useful for precision navigation solutions, this error needs to be reduced to below 5 nanoseconds. Therefore in 2004, GPS and Galileo agreed to develop and jointly broadcast a GPS-to-Galileo Time Offset (GGTO) message, which user receivers may use for system-to-system navigation timing traceability. Working in cooperation, USNO (representing GPS) and ESA (Galileo Project) have agreed upon several methods to compute and coordinate the GGTO values. During the initial stages of the coordination, and throughout Galileo's In-Orbit Validation (IOV) campaign, the different methods will provide validation to the GGTO computations, ensuring the most accurate results. One of the techniques to be employed by USNO will utilize a GPS/Galileo combined receiver. For its proper application, the special GGTO monitoring receiver must be precisely calibrated to account for its internal time delays among all of the GPS and Galileo channels. In September of 2012, USNO and ESA teamed up to perform calibrations of the USNO and ESA GGTO receivers using a combined GPS/Galileo multi-constellation simulator (Spirent), located at the European Space Research and Technology Centre (ESTEC) in Noordwijk, Netherlands. This paper details the procedures and the results of the experiment.