# Accuracy Analysis of GNSS Hourly Ultra-Rapid Orbit and Clock Products from SHAO AC of iGMAS
## 2 Data, Methods, and Strategy
The undifferenced (UD) ionosphere-free (IF) combinations for double frequency observations, such as L1 (1575.42 MHz) and L2 (1227.60 MHz) frequencies for GPS, GLONASS L1 (1602.00 MHz) and L2 (1246.00 MHz), Galileo E1 (1575.42 MHz) and E5a (1176.45 MHz), \begin{table}
\begin{tabular}{c c c c} \hline  & GPS & GLONASS & Galileo & BDS \\ \hline Thread & GPS & GLONASS+GPS & Galileo+GPS & BDS+GPS \\ \hline Undifferenced ionosphere-free combination & GPS: L1,L2 & GLONASS: L1,L2 & Galileo: E1,E5a & BDS: B1,B2 \\ \hline Elevation angle cutoff & & GPS: L1,L2 & GPS: L1,L2 & GPS: L1,L2 \\ \hline Sampling interval & & & 7 degrees \\ \hline  & & & 450 s for 6-hourly updated products \\ \cline{2-3}  & & & 600 s for 1-hourly updated products \\ \hline ISB/IFB & & Estimated as constant \\ \hline Satellite clock error & & Estimate every sampling interval relative to reference clock \\ \hline Receiver clock error & & Estimate every sampling interval relative to reference clock \\ \cline{2-3}  & A receiver clock fixed as a time reference (such as ALGO, PTBB, WTZR, NRC1, PIE1, ONSA) \\ \hline Orbits & Estimated with 5 parameters for solar radiation pressure modeling \\ \hline station displacement & & Solid earth tide, pole tide, ocean tide loading \\ \hline Earth gravity & & EGM 12X12 \\ \hline RHC phase & & Phase wind-up applied \\ rotation correction & & \\ \hline Ambiguities & Fixed \\ \hline Satellite phase & & igs14.atx from IGS & PCO/PCV for BDS-2 [14] \\ center corrections & & \\ \hline Station phase & & igs14\_absolute.atx from IGS \\ \hline  & & Initial ZTD model: Saastamoinen model \\ \cline{2-3}  & & Mapping function: GMF \\ \cline{2-3}  & & Interval: 1-h for ZTD, 12-h for gradient \\ \hline  & & Initial value: downloaded from IERS rapid solution \\ \cline{2-3}  & & Constrain: strong constrain \\ \hline Reference frame & & ITRF 2014 \\ \hline \end{tabular}
\end{table} 
Table 1: Strategy for hourly GNSS products generation. BDS-2 B1 (1561.098 MHz) and B2 (1207.14 MHz), were formed to eliminate the first order of ionospheric delays. The cutoff elevation angle of the observation data was selected at 7 degrees. The sampling intervals were 450 and 600 s for 6-hourly (6H) products and 1-hourly (1H) products, respectively, in order to improve the data processing efficiency. Then, the orbit and clock parameters were estimated from the carrier phase and pseudorange observations. The clock of satellites and receivers were estimated at every sampling interval. Moreover, a reference clock from a station was fixed as the time reference, which was usually connected with a hydrogen atomic clock and showed better performance. The force models, such as the solar radiation pressure (SRP) with five parameters, Earth gravity model (EGM) with degree and order 12, solid earth tide, pole tide, M-body gravity model (DE405), and phase win-up corrections [15] were applied for orbit estimation. The ocean tide loading corrections of each station were retrieved from the Onsala Space Observatory (OSO), which is the component of the Swedish National Infrastructure [16]. The ambiguity solutions needed to be determined in order to improve the accuracy of the parameters estimation [17]. However, the float ambiguity solutions were calculated for GLONASS and BDS GEO, which were hard to fix because of the distinctive signal transmit mechanism for GLONASS and special satellite slot for BDS GEO. The phase center correct (PCC) must be considered for improving the accuracy of the products. PCC files of the antenna of the stations and GPS/GLONASS/Galileo satellites can be downloaded from IGS [18; 19], while PCC of BDS are referred to reference [14]. [PERSON] established the ZTD model, which was called the Saastamoinen model by many researchers [20]. [PERSON] et al. (2011) assessed the [PERSON] model and found the accuracy of the model was about 4 cm [21]. [PERSON] studied and established the global mapping function (GMF), which was used by many researchers and engineers [22; 23; 24; 25]. The Saastamoinen model and GMF were used to estimate ZTD in this contribution.