## 2 Materials and Methods
### The Processing Strategies
#### 2.2.2 Partial Available Strategies and the Flowchart of the New Prototype Software Platform  
There is research that has showed that the change of satellite orbit model can remarkably change the results of Galileo satellite POD [40], and [PERSON] et al. summarized the five-system satellite attitude models [41]. In this paper, the yaw-steering is assumed for all satellites, which is not yet correct for BDS and QZSS satellites. Besides, the ECOM 5-parameter model will be used for all satellites in the next section, but the model was not designed for Galileo, BDS, and QZSS satellites. The quality of the ultra-rapid orbits mostly depends on the satellite attitude models and the SRP model; therefore, the state of the new prototype software platform can be considered as preliminary.  
Furthermore, during the processing of the five-system ultra-rapid POD in our new prototype software platform, the length of the arc and the setting of the stochastic pulses referred to the method proposed by [PERSON] et al. [27]. The 06 (UTC), 12 (UTC), 18 (UTC), and 24 (UTC) ultra-rapid orbits used 54-h, 60-h, 66-h, and 72-h observations, respectively. Besides this, 3-5 sets of stochastic pulses were used in each arc. The cut-off angle was set to 3 degrees, and the data sampling rate was 180 s. The available partial processing strategies are shown in Table 1, and Table 2 summarizes the observation types of each satellite navigation system selected from the observation files. In our software platform, the five-system observation and navigation files were processed simultaneously in response to MGEX's call for the joint processing of multi-GNSS, which is also needed for the five-system ultra-rapid POD using the one-step method. In addition, the flowchart of the new prototype software platform is shown in Figure 1.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Items** & **Description** \\ \hline Processing scheme & IF double-difference network processing \\ Solar radiation & ECOM 5-parameter model \\ Sampling interval & 180 s \\ Cut-off angle & 3 degrees \\ Tropospheric delay & GPT model, GMF mapping function, two-hourly ZTDs [42; 43] \\ A priori ERP & IERS Bulletin A and continuous piecewise linear function [44] \\ Attitude & Yaw-steering always assumed for all sat \\ Ambiguity & Real constant value for each ambiguity arc \\ PCO/PCV(Sat) & GPS, GLONASS: PCO and PCV from IGS; Galileo: PCO and PCV from ESA; \\  & BeiDou, QZSS: PCO from IGS MGEX \\ PCO/PCV(Rec.) & GPS, GLONASS: ICS PCO and PCV; new GNSS: adopted from GPS L1 and L2 \\ Update frequency & 6 h \\ A priori orbits & Broadcast ephemeris \\ A priori coordinates & Previous 3-day solution, strong constraint (0.01, 0.01, 0.01) m \\ Earth gravity & EGM2008 12 \(\times\) 12 [45] \\ Solid earth tide and pole & IERS2010 [46] \\ tide & FES2004 [47] \\ Ocean tide & IAU2000 [48] \\ N-body gravitation & Sun, moon, and other planets (DE405) \\ Relativity effect & IERS2010 [46] \\ \hline \hline \end{tabular}
\end{table}
Table 1: The partial strategies of the new prototype software platform. IF: Ionosphere-free.  
Figure 1: The flowchart of the new prototype software platform. Obs: observation; Nav: navigation.  
The above section introduces the models, methods, and processing strategies used in the new prototype software platform. Although the double-difference IF combined observation equations are constructed in the same navigation satellite systems, all satellites and these equations are solved together in a least square estimator.