# Real-Time Precise Point Positioning during Outages of the PPP-B2b Service
## 1 Introduction  
Precise point positioning (PPP) using global navigation satellite systems (GNSSs) has become a popular technique for various applications, including positioning and atmospheric monitoring, because of its high precision, proficiency, stability, and flexibility. Because the accuracy of PPP largely depends on the accuracy and availability of satellite orbit and clock products, various efforts have been made to produce high-precision satellite orbit and clock products for PPP applications. In recent years, the Multi-GNSS Experiment (MGEX), initiated by the International GNSS Service (IGS), has provided precise orbit and clock products for GNSS users [1, 2]. To overcome the latency problem of IGS precision products and meet real-time PPP application requirements, the IGS launched the Real-time Pilot Project (RTPP) in 2007 to provide real-time service (RTS) [3, 4, 5]. With access to the RTS, users can receive the orbit and clock corrections to the broadcast ephemeris in the form of a state space representation (SSR) based on the Radio Technical Commission forMaritime Services (RTCM) and the networked transport of the RTCM via the internet protocol (NTRIP). To date, more than ten institutions, including the GeoForschungsZentrum (GFZ), Center National d'Etudes Spatiales (CNES), and Wuhan University (WHU), have been providing SSR products, promoting the application of real-time PPP in many fields, such as high-precision positioning and atmospheric remote sensing. However, because the IGS real-time precision orbit and clock correction products are broadcast using the internet, poor internet performance or its unavailability will lead to the loss of SSR corrections, resulting in an interruption or at least harmed PPP calculation, which limits its use in areas without stable internet access [6]. In recent years, the GNSSs has undergone significant advancements. The Next Generation Control Segment (OCX) for GPS will be responsible for controlling all modern and legacy GPS satellites, managing all civil and military navigation signals, and providing improved cybersecurity and resilience for GPS operations [7]. Galileo's High-Accuracy Service (HAS) will offer PPP with an accuracy of 20-40 cm globally with 5 min convergence. In addition, regional broadcasts in Europe will be corrected and targeted to converge within 100 s [8]. GLONASS plans to add an IGSO regional component and improve its global geo-distribution control network, currently only covering Russia [9]. China completed the construction of the BDS-3 and began providing positioning, navigation, and timing services to global users in 2020. BDS-3 is composed of three GEO satellites, three IGSO satellites, and twenty-four MEO satellites [10;11]. A prominent feature of BDS-3 is its real-time decimeter-level PPP service via B2b signal (PPP-B2b) based on high-precision satellite orbit and clock corrections, broadcast in real time by the GEO satellites [12;13]. As [PERSON] et al. [14] pointed out, the three-dimensional orbit accuracy of the overlapping arc is about 60 cm when only using regional station observation to determine the satellite orbit; due to the BDS-3 satellite being equipped with inter-satellite links (ISL). Through ISL measurements, the orbit accuracy is about 30 cm and 24-h orbit prediction accuracy also improved from 140 cm to 51 cm; through laser observation evaluation, the radial accuracy can reach 10 cm. In addition, compared with the IGS RTS broadcast via the internet, the orbit and clock corrections in PPP-B2b messages are broadcast on the B2b signal through GEO satellites, which are not restricted by internet conditions. The three BDS-3 GEO satellites operate in orbits with an altitude of 35,786 km and latitudes of 80\({}^{\circ}\)E, 110.5\({}^{\circ}\)E, and 140\({}^{\circ}\)E. Figure 1 shows the Earth coverage areas of the three GEO satellites, which indicates that they cover most of Asia, Oceania, Europe, and Africa.