# Characteristics of BeiDou-3 Experimental Satellite Clocks
## 2 Data Collection  
The BeiDou clock offset is one of the products of the Precise Orbit Determination (POD) calculated from a modified version of the Positioning and Navigation Data Analyst (PANDA) software developed by Wuhan University, China [16]. International GNSS Monitoring and Assessment System (iGMAS) [17] and IGS Multi-GNSS Experiment (MGEX) tracking network stations are used to collect \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Pseudo Random Noise (PRN)** & **Type** & **Launch Date** & **Clock** \\ \hline C31 & IGSO01-S & 2015.03.30 & Rb \\ C32 & IGSO02-S & 2015.09.30 & PHM \\ C33 & ME001-S & 2015.07.25 & Rb \\ C34 & ME002-S & 2015.07.25 & Rb \\ C35 & ME003-S & 2016.02.01 & PHM \\ \hline \hline \end{tabular}
\end{table} 
Table 1: On-broad clock type of BeiDou-3 experimental satellites on 1 October 2017. PHM—passive hydrogen maser. BeiDou-3 observations for the time span day-of-year 254 to 281, of the year 2017. The distribution of the tracking stations is shown in Figure 1. The green dots represent the selected global stations, which can be capable of tracking the BeiDou-2 satellites. The red dots represent the regional stations, which are mainly distributed in the Asia Pacific region and have access to the BeiDou-3 signal. As a result, there is a loss percentage from about 11% to 15% of the BeiDou-3 MEO satellites in the calculated clock offsets, because of the insufficient measurements. We calculated the global and regional clock offsets from the global stations and regional stations, respectively. If not explicitly stated, the global clock offsets have been selected for the following analyses. Note that the red stations can only receive and identify the B1 and B3 signal both adopted in BeiDou-2 and BeiDou-3. Therefore, the following characteristics of the BeiDou clocks are analyzed based on the B1 and B3 signal. The two-step method to determinate the BeiDou orbit and clock could be described as follows: To begin with, the parameters that are same as those of the BeiDou system, such as the station coordinates, receiver clocks, and tropospheric delay, are estimated from the GPS observations using precise point positioning. Then, these parameters are fixed so as to determine the BeiDou satellite orbit and clock [18; 19]. The Phase Center Offset (PCO) and Phase Center Variation (PCV) correction provided by Wuhan University are used for BeiDou-2 satellites [20], while only the PCO correction provided by the Operation Control Department (OCD) is used for the BeiDou-3 experiment satellites. The details of the POD processing strategy can be found in Table 2. \begin{table}
\begin{tabular}{l l} \hline \hline
**Correction** & **Model** \\ \hline Observation type & Code and carrier observations of B1 and B3 \\ Measurements combination & Ionosphere-free combination \\ Elevation cutoff & \(10^{\circ}\) \\ Sampling rate & 300 s \\ Weight & A priori precision of 0.002 m and 0.2 m for phase and code observables, and elevation-dependent weighting factor \\ Station coordinates & Estimated in step one and fixed in step two \\ Receiver clocks & Estimated in step one and fixed in step two \\ Inter system biases & Estimated in step two \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Processing strategy for the POD. Figure 1: Distribution of the tracking stations used in this study. Stations capable of tracking BeiDou-3 observations are marked in red. The other stations are marked in green.