# Accuracy Analysis of GNSS Hourly Ultra-Rapid Orbit and Clock Products from SHAO AC of iGMAS
## 2 Data, Methods, and Strategy
The interval of ZTD was estimated by piecewise constant (PWC), with a 1-h interval, while the interval of horizontal gradient was 12 h.  
The initial parameters of ERP were achieved from International Earth Rotation and Reference Systems Service (IERS) rapid products, which were strongly constrained during the parameters estimation. Moreover, the ERP parameters were also updated during the running CSPT. The reference frame of the orbit and clock products was based from the ITRF2014. The initial coordinates and constrain information of the stations from IGS/MGEX were downloaded from IGS weekly final solution independent exchange (SINEX) format solutions. However, the initial coordinates of stations from iGMAS or CMONOC could be extracted from the final solution of SHAO AC of iGMAS with loose constrains. Figure 2 shows the slide window for hourly ultra-rapid orbit and clock products. The slide window was 6 h for 6-hourly GNSS orbit and clock productions, which were similar to IGS ultra-rapid products, while a 1-h slide window was applied for 1-hourly productions. Moreover, Figure 2 shows the ultra-rapid productions were comprised of a 24-h observation session and a 24-h prediction session. Figure 2: Slide window for hourly ultra-rapid obit and clock products generation. Some studies found that a long arc-length, such as a 25-40 h length, could get a better prediction orbit [8; 11; 13]. However, time consumption is an issue for the latency of the products. Thus, in this study, we discussed a new prediction strategy which fit two-day orbits to integrate the orbit for the prediction orbit, while the prediction clock was computed by a modified Auto Regressive Integrated Moving Average (ARIMA) method [26]. The daily-observed data are not suited to generate the ultra-rapid orbit and clock productions for their long latency, and the fixed whole day observation session. Thus, the hourly observation data are extremely important, and combined for each station, according to the given session set by users to generate the hourly ultra-rapid productions. The raw code and phase observation data needed to be preprocessed and quality controlled before taking part in the parameters estimation. Thus, the format and integrity of the hourly observation data files must be checked after being uncompressed, and then combined into a new file for each station. Similar to observation data file combinations, the hourly broadcast files were also uncompressed and combined by our script, because the combined broadcast files from IGS/MGEX may sometimes be postponed, or show the wrong format, which would decrease the stability and continuity of our routine data processing procedure. It also should be mentioned that there are two servers used in this study, including eight Central Processing Unit (CPU) cores with basic frequency, 2.4 GHz and 128G memory, respectively, connected with the shared storage array (SSA) by a fiber optic cable. The phase and code hourly observation and navigation data files from IGS/MGEX/iGMAS/CMONOC were downloaded by multi-threads arranged in the first server. The CSPT routines for precise orbit determination (POD) ran in the second server, which could copy the observation, navigation, and other data files from SSA to the local storage hard disk, and then process the data. Figure 3 shows the flow chart of hourly orbit and clock product generation. The control information and initial files, such as configuration files, ERP, ocean tide loading files, and so on, were prepared first. Then the hourly phase and code observation files were preprocessed and combined for one station by one thread. The threads of each station ran in parallel mode. The navigation files were also preprocessed, to transfer the version from 2.0 to 3.0, in order to get the unified format, and then checked and deleted the unhealthy satellites. Quality control is important for the observation of each station. The millisecond jump of the observation data would be corrected, and cycle jumps needed to be checked and marked.