## 1 Introduction  
The US Global Positioning System (GPS) and the Russian GLObal NAvigation Satellite System (GLONASS) are going through the process of modernization nowadays. GPS constellation is composed of 32 operational satellites, including five BLOCK IIIA satellites (G04, G11, G14, G18, G23) with the capability of transmitting L2C and L5 signals. For the GLONASS constellation, there are 21 operational satellites used for navigation, including three new GLONASS-K1 satellites (R09, R11, R22) with the capability of transmitting both FDMA and CDMA signals. Meanwhile, the emerging satellite navigation systems, i.e., European Galileo Navigation Satellite System (Galileo), Chinese BeiDou Navigation Satellite System (BDS) and Japanese Quasi-Zenith Satellite System (QZSS), have just fulfilled the constellation networking phase. Concerning BDS, with a constellation consisting of four in medium earth orbit (MEO), five in inclined geosynchronous orbit (IGSO) and five in geostationary orbit (GEO), BDS-2 has been offering positioning, navigation and timing (PNT) services for the Asia-Pacific region since the end of 2012. BDS-3 has been officially providing global PNT services since 31 July 2020 with a constellation of 24 MEO, 3 IGSO and 3 GEO satellites [1,2]. With 24 full operational capability (FOC) satellites and 4 in orbit validation (IOV) satellites, Galileo has officially started offering PNT services since the end of 2016. For QZSS, four IGSO and one GEO satellites have been deployed. The operational status of the GNSS constellations are summarized in Table 1, as of July 2022. Increasingly perfect multi-GNSS bring great potential for more reliable and precise global capable real-time PPP services [3; 4; 5]. High-accuracy, stable real-time GNSS satellite orbit and clock corrections are the footstone for real-time PPP and PPP-RTK applications [6; 7; 8; 9; 10; 11; 12], especially satellite orbits. Ultrarapid orbit determination and real-time filtering are the two prevailing methods for GNSS satellite real-time POD. POD based on the real-time filtering method has drawn increasing attention from research institutes and commercial companies. Jet Propulsion Laboratory (JPL) developed the real-time GIPSY (RTG) software using a square root information filter (SRIF). RTG has been employed in many real-time service systems with an accuracy of 5 cm for the user range error (URE) as of 2020 [13]. Auto-BAHN was developed by Newcastle University by using an extended Kalman filter (EKF) [14]. The Kalman filtering algorithm was also used by Centre National D'Etudes Spatiales (CNES) for multi-GNSS orbit and clock estimation [15]. Multi-GNSS Advanced Demonstration Tool for Orbit and Clock Analysis (MADOCA) was developed by Japan using the EKF method, and then employed for providing augmentation services for QZSS [16]. [PERSON] et al. [17] and [PERSON] et al. [18] have also done some research on filtering POD of multi-GNSS constellations. Currently, several Multi-GNSS Experiment (MGEX) [19] and International GNSS Monitoring and Assessment System (iGMAS) [20] analysis centers have provided multi-GNSS ultrarapid orbit products or demonstrated their processing systems, e.g., GFZ [21], Wuhan University [22; 23; 24; 25], Innovation Academy for Precision Measurement Science and Technology (APM, CAS) and Shanghai Astronomical Observatory (SHAO, CAS). Wuhan University [26], GFZ and SHAO have also demonstrated their hourly ultrarapid multi-GNSS satellite orbits. Ultrarapid orbits have been widely used in the International GNSS Service (IGS) Real Time Service (RTS), which was formally launched in 2013 [27; 28; 29]. The RTS currently provides a GPS-only service. Multi-GNSS orbit and clock products are undergoing development and testing phases, including a number of streams from analysis centers and an experimental combination solution.