## 3 Multi-GNSS Data and Products  
### Multi-GNSS Orbit and Clock Products from IGS RTS  
In order to meet the requirements of RT GNSS precise applications, the IGS Real Time Working Group (RTWG) has officially provided the RT service (RTS) since 2013. The multi-GNSS orbit and clock products from RTS include precise orbit and clock corrections, which largely depend on the IGS infrastructure of the global RT network stations, the data centers, and the analysis centers that offer high-precision GNSS data products. To date, eight analysis centers are capable of providing the IGS-RT orbit and clock products, including BKG (Bundesamt fur Kartographie und Geodasie), CNES (Centre National d'Etudes Spatiales), DLR (Deutsches Zentrum fur Luft- und Raumfahrt), ESA (European Space Agency), GFZ (Deutsches GeoForschungsZentrum), GMV (GMV Aerospace and Defense), and WUHAN (Wuhan University). Those analysis centers broadcast RT orbit and clock corrections through different mount points. Most of them provide GPS-only single-system or GPS/GLONASS dual-system RT orbit and clock products, while only a few analysis centers  
\begin{table}
\begin{tabular}{c l} \hline
**Item** & **Strategies** \\ \hline Estimator & All multi-GNSS observations are processed together in one sequential least square estimator. \\ Sources of satellite orbits \& clocks & As shown in Table 2 \\ Observations & Carrier phase and pseudorange observations; \\  & GPS + GLONASS + Galileo + BDS, about 80 navigation satellites \\ Signal selection & GPS: L1/L2; GLONASS: L1/L2; Galileo: E1/E8a; BDS: B1/B2 \\ Sampling rate & 5 \\ Elevation cutoff & 7\({}^{\circ}\) \\ Weight for observations & The variance component estimation weighting method \\ Satellite orbit & Fixed \\ Satellite clock & Fixed \\  & Initial model (ZHD estimated using Saastamoinen model based on GPT2) + random-walk \\  & process (process noise: 5 mm/h\({}^{1/2}\)) \\  & Topospheric gradients & No \\  & Mapping function & Global Mapping Function (GMF) \\  & Phase-window effect & Corrected \\  & Receiver clock & Estimated, white noise \\  & ISB and IFB & Estimated as constant, GPS as reference \\  & Station displacement & Solid Earth tide, pole tide, ocean tide loading, IERS Convention 2010 \\ Satellite antenna phase center & Corrected using MGEX and IGS values \\ Receiver antenna phase center & Corrected \\  & Station coordinate & Fixed to coordinates of weekly solution/kinematic estimated \\  & Phase ambiguities & Constant for each continuous arc, without ambiguity resolution \\ \hline \end{tabular}
\end{table}
Table 1: Processing strategies for the real-time (RT) multi-Global Navigation Satellite Systems (GNSS) zenith total delays (ZTD) estimation.  
(e.g., CNES and GFZ) are able to provide the GPS/GLONASS/Galileo/BDS four-system products. Table 2 lists the IGS-RT orbit and clock products from each analysis center in details.  
Currently, the RTS products, including the precise satellite orbit/clock corrections on the broadcast ephemeris, are distributed as the RT data streams (i.e., IGS01/IGC01, IGS02, and IGS03) and are expressed within the International Terrestrial Reference Frame 2008 (ITRF08). Additionally, the products can be accessed through internet without special licensing [26]. The accuracies for the orbits and clocks are 5 cm and 0.3 ns, respectively [27]. As an example, GFZ, as one of the IGS-RT data analysis centers, is able to offer the RT satellite orbit/clock products of all four systems to users with accuracy at cm level [14].