Real-time undifferenced precision time transfer of multi-GNSS and multi-frequency

Abstract Currently, the four major Global Navigation Satellite Systems (GNSS) are capable of supporting multi-frequency signals, and Real-time (RT) State Space Representation (SSR) correction products have been enriched and improved. Multi-GNSS, multi-frequency observations and multiple satellite RT SSR correction products present new opportunities to implement precise RT time transfer. This study extends the traditional post-processing dual-frequency undifferenced (UD) Precise Point Positioning (PPP) time transfer to RT multi-frequency scenarios, establishes models for dual-frequency, triple-frequency, quad-frequency and penta-frequency multi-GNSS PPP time transfer, and evaluates the product availability, orbit accuracy and clock offset accuracy of RT SSR products from BKG, CNE, DLR, ESA, GFZ, GMV, WHU, and IGS. A multi-frequency and multi-GNSS RT PPP time transfer experiment was conducted by selecting data from four stations, BRUX, PTBB, ONSA and SPT0, with precise atomic clocks mounted for one week. The accuracy of various SSR products in RT PPP time transfer was compared, and the performance of BDS/Galileo combination dual-frequency to penta-frequency RT UD PPP time transfer was evaluated. The experimental results show that the standard deviation and frequency stability difference of RT PPP time transfer among each SSR product is relatively small, especially after combining multi-systems. The frequency stability tends to be entirely consistent. In general, the WHU, GFZ, GMV, and WHU SSR products offer notable advantages in terms of accuracy and frequency stability for GPS, GLONASS, Galileo, and BDS RT PPP time transfer, respectively. Multi-GNSS and multi-frequency signals have both enhanced RT PPP time transfer performance, with a particularly noticeable improvement in terms of accuracy and short-term frequency stability. The effect of multi-GNSS on RT PPP time transfer is more pronounced than that of multi-frequency alone. Considering the higher costs associated with multi-frequency GNSS receivers, prioritizing multi-GNSS for RT PPP time transfer is recommended.