Initial and comprehensive analysis of PPP time transfer based on Galileo high accuracy service

European Galileo officially provided global users with the initial high accuracy service (HAS) through Galileo satellites for free on January 24, 2023. The emergence of the Galileo HAS provides the possibility of a globally stable real-time precise point positioning (PPP) time transfer that does not depend on a network. The coverage and service availability (the proportion of epochs that can support PPP solution) of the HAS, accuracy of the satellite orbit and clock offset, and accuracy of HAS time transfer were comprehensively analyzed using real-time and satellite-broadcast HAS data. Twenty-three global time links, including time-keeping laboratories, were established to assess the accuracy of time comparison using the HAS product. The results showed that the average numbers of GPS and Galileo satellites with valid HAS corrections worldwide were 9.44 and 7.95, respectively, and the average service availability of GPS-only and Galileo-only PPP using the HAS product reached 99.9% and 99.6%, respectively, in most areas. Taking post-processing satellite products from GeoForschungZentrum as a reference, the radial errors of the HAS orbit product are concentrated within 5 cm, and the accuracy of the corrected Galileo satellite clock offset was twice that of ephemeris. Further, in the 7200-12295 km-long baselines, the mean standard deviation values of HAS GPS-only, Galileo-only, and GPS/Galileo time comparison result errors were 0.19-0.29 ns, 0.13-0.24 ns, and 0.11-0.21 ns, respectively. In general, HAS time transfer is available globally.