# Considering Inter-Frequency Clock Bias for BDS Triple-Frequency Precise Point Positioning
## 3 Results  
### Space Segment and Navigation Signals  
Up to 12 June 2016, 18 BDS-2 satellites and 5 BDS-3 satellites have been launched. Currently, three BDS-2 satellites, namely M1, G2 and M5, are no longer transmitting. In addition, the BDS-2 satellite G7 and BDS-3 satellite M3-S are in the phase of flight tests. As of October 2016, there are 14 \begin{table}
\begin{tabular}{l l l} \hline \hline
**Item** & **Models and Strategies** & **Spectral Density and Precision** \\ \hline Observations & Undifferenced code and carrier & Code observations: 0.6 m \\  & phase observations & Phase observations: 0.004 m \\ Signal selection & B1/B2 and B1/B3 & - \\ Observation weighting & Elevation-dependent weight & - \\ Elevation mask angle & 10\({}^{\circ}\) & - \\ Sampling interval & 30 s & - \\ Satellite orbit & Fixed using the products from MGEX & - \\ Satellite clock & Fixed using the products from MGEX & - \\ Phase wind-up effect & Corrected & - \\ Earth rotation parameter & Fixed & - \\ Station displacement & Solid Earth tides, ocean tide loading, pole & - \\  & tides, IERS Convention 2003 & - \\ Relativistic effect & IERS Convention 2003 & - \\ Phase center offset & Satellite: corrected using IGS values & - \\  & Receiver: corrected using GPS values & \\  & from IGS & \\ Phase center variation & Satellite: neglected; Receiver: corrected & - \\  & using GPS values from IGS & \\ Ionospheric delay & First-order effect eliminated by & - \\  & ionospheric-free linear combination & - \\ \hline  & Dry part: corrected using Saastamoinen & \\  & tropospheric model & \\ Tropospheric delay & Wet part: estimated; Mapping functions: & \\  & GMF [33] & \\  & Tropospheric ZWD: modeled as a random & \\  & walk (RW) process & \\ \hline Receiver coordinate & Static: estimated and modeled as constants & Kinematic: 10\({}^{2}\) m\({}^{2}\)/s \\  & Kinematic: estimated and modeled as a & \\  & RW process & \\ Receiver clock & Estimated and modeled as a RW process & 10\({}^{5}\) m\({}^{2}\)/s \\ Phase ambiguity & Estimated and modeled as constants & - \\ Terrestrial frame & ITRF2008 & - \\ Time system & GPS Time & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data processing models and strategies for BDS triple-frequency PPP. BDS-2 satellites and 4 BDS-3 satellites in full operation, including 5 GEO, 8 IGSO and 5 MEO satellites. The information on the 18 operational satellites is included in Table 2. The GEO and IGSO satellites operate in orbit at an altitude of 35,786 km with a period of revolution of 23 h 56 min. The IGSO satellites have an inclination of 55\({}^{\circ}\), while the GEO satellites exhibit a non-zero inclination of 0.7\({}^{\circ}\)-1.7\({}^{\circ}\). The MEO satellites are in a nearly circular orbit at an altitude of 21,528 km and an inclination of 55\({}^{\circ}\), and complete the orbit in approximately 12 h 53 min. A PRN switch from C15 to C13 for I6 spacecraft was carried out on 11 October 2016. The 24-h ground tracks of BDS-2 and BDS-3 satellites available for positioning on 13 July 2016, are shown in Figure 1. The ground tracks of IGSO satellites describe two similar loops in the shape of figure-of-eight. The average difference between longitudes of the two loops is roughly 30\({}^{\circ}\). For main-land China, IGSO satellites provide an optimum coverage. The satellites with inclined geosynchronous orbits show an improved availability in terms of high elevation angles. When users are in densely populated areas, the so-called \"urban canyon\" problem can be alleviated due to the presence of IGSO satellites. The ground tracks are confined from approximately 76.2\({}^{\circ}\)E to 138.0\({}^{\circ}\)E longitude and 57.5\({}^{\circ}\)S to 57.5\({}^{\circ}\)N latitude for the eight IGSO satellites. Five GEO satellites distributed in the Indian and Pacific oceans over the Equator further complement the IGSO constellation. As such, the users in Asian-Pacific regions can observe enough satellites.