# Comprehensive Analysis of PPP-B2b Service and Its Impact on BDS-3/GPS Real-Time PPP Time Transfer
###### Abstract  
2020 saw the official completion of the BDS-3 and the start of the PPP-B2b signal-based real-time precise point positioning (PPP) service to users in China and the neighboring areas. In this work, the quality of PPP-B2b products is first evaluated and compared with real-time products from the CNES and the differential code bias (DCB) from the Chinese Academy of Science (CAS). Then, a detailed performance evaluation of the PPP time transfer based on the PPP-B2b service (B2b-RTPPP) is conducted. Three solutions, namely, GPS-only (G), BDS-3-only (C), and GPS + BDS-3 (GC) B2b-RTPPP solutions, are compared and assessed. The results suggest that for the PPP-B2b products, BDS-3 satellites have better orbit and clock offset quality than GPS satellites, while the opposite is true for CNES products. The quality of the PPP-B2b orbit and clock offset is poorer than those of the CNES. The PPP-B2b DCB shows excellent agreement with the CAS DCB. The accuracy of the B2b-RTPPP solutions is sub-nanosecond level. The accuracy of B2b-RTPPP time transfer with DCB correction is approximately improved by 64% compared with that without DCB correction. The GC B2b-RTPPP solution has the greatest frequency stability, while G B2b-RTPPP solution has the poorest. Considering that the receiver may be blocked, the B2b-RTPPP time transfer performance is also evaluated at different cut-off elevation angles. As the angle increases, the B2b-RTPPP time transfer performance decreases. Additionally, the short-term frequency stability remains constant at different cut-off elevation angles, but deteriorates in the long term, especially when the angle is 40\({}^{\circ}\). BDS-3; GPS; real-time PPP; PPP-B2b; time transfer 14 5366 2  
superiority over IGS ultra-rapid products [12]. The mathematical models of real-time PPP were introduced by [PERSON] et al. [10], and the positioning performance was assessed using raw observations. A novel method of real-time precise timing was improved by [PERSON] et al. employing IGS products [13], and it was shown that the accuracy of the timing solutions can achieve 0.2 ns. However, real-time PPP using IGS RTS relies on the IGS real-time streams through the Internet, which may be disrupted by equipment failure or network outage. In this case, an obvious decline in PPP performance is possible. To solve this problem, some navigation systems provide differential corrections to the broadcast ephemeris, providing access to precise real-time products rather than products via the Internet. Nowadays, the Quasi-Zenith Satellite System can broadcast the centimeter-level augmentation services (CLAS) for Japanese users via the L6 signal [14], while the Galileo High Accuracy Service (HAS) can transmit corrections for GPS and Galileo users [15]. However, neither of them supports the BDS-3. Under this background, BDS-3 was fully completed and officially launched the satellite-based augmentation service in October 2020 to enable the real-time PPP. The BDS-3 consists of three Geostationary Earth Orbit (GEO), 24 Medium Earth Orbit (MEO), and three Inclined Geosynchronous Orbit (IGSO) satellites, with three additional frequency signals, B1C (1575.42 MHz), B2a (1176.4 MHz), and B2b (1207.14 MHz), while retaining the B1I (1561.098 MHz) and B3I (1268.520 MHz) signals of BDS-2 [16]. BDS-3 satellites are installed with more accurate on-board clocks compared to BDS-2, which enables better PPP performance for BDS-3 [17]. The state space representation (SSR) messages are released via the B2b signal through the three BDS-3 GEO satellites, which are available for users in mainland China and its maritime regions, and even the rest of the world [18; 19; 20; 21]. The PPP-B2b service can provide correction messages for GPS LNAV and BDS-3 CNAV1 broadcast ephemeris that contain satellite mask, orbit correction parameters, User Range Accuracy Index (URAI), differential code bias (DCB) correction, and clock correction parameters [22].