# An Improved Predicted Model for BDS Ultra-Rapid Satellite Clock Offsets
## 2 BDS Satellite Orbits and Clock Status  
The satellite clocks in orbit with different orbital heights have different periodic characteristics. The performance and service life of the satellite clocks in different periods are also not similar. The atomic clock types of the BDS satellites in orbit are summarized in Table 1. The satellite orbit constellation geometry is shown in Figure 1.  
From Table 1 it can be seen that there are in total, 20 BDS on-board satellites, of which 15 are BDS-2 satellites, and five are new BDS-3 test satellites. All the BDS satellites are currently equipped with Rb atomic clocks except for satellite C32 (which uses a Hydrogen (H) atomic clock). The longest-serving satellite is C01, which has been working for more than seven years.  
It can be seen from Figure 1 that the service areas of the BDS-2 satellites focus on the Asia-Pacific region; five GEO satellites are running in a latitude and longitude within the grid perturbation, while six IGSO satellites are running around the \"8\"-shaped trajectory in the Asia-Pacific region. This can guarantee the number of visible satellites in the Asia-Pacific region, while three MEO satellites are running globally. The geometric configurations of three types of satellites also create the diversity of  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Generation** & **Lunch Date** & **Orbit** & **Status** & **PRN** & **Equipment Clock [1]** & **Serve Time (Month)** \\ \hline \multirow{4}{*}{BDS-1} & 2000.10.31 & GEO & Not usable & S-1 & Rb & None \\  & 2000.12.21 & GEO & Not usable & S-2 & Rb & None \\  & 2003.05.25 & GEO & Not usable & S-3 & Rb & None \\  & 2007.02.03 & GEO & Not usable & S-4 & Rb & None \\ \hline \multirow{4}{*}{BDS-2} & 2007.04.14 & MEO & Not usable & None & Rb & None \\  & 2009.04.15 & GEO & Not usable & C02 & Rb & 69 \\  & 2010.01.17 & GEO & normal & C01 & Rb & 86 \\  & 2010.06.02 & GEO & normal & C03 & Rb & 81 \\  & 2010.08.01 & IGSO & normal & C06 & Rb & 79 \\  & 2010.11.01 & GEO & normal & C04 & Rb & 76 \\  & 2010.12.18 & IGSO & normal & C07 & Rb & 75 \\  & 2011.04.10 & IGSO & normal & C08 & Rb & 71 \\  & 2011.07.27 & IGSO & normal & C09 & Rb & 68 \\  & 2011.12.02 & IGSO & normal & C10 & Rb & 64 \\  & 2012.02.25 & GEO & normal & C05 & Rb & 62 \\  & 2012.04.30 & MEO & normal & C11, C12 & Rb & 59 \\  & 2012.09.19 & MEO & normal & C13, C14 & Rb & 53 \\  & 2012.10.25 & GEO & normal & C02 & Rb & 52 \\  & 2015.03.30 & IGSO & normal & C15 switched to C13 & Rb & 23 \\  & 2016.06.12 & GEO & Not usable & G7 & Rb & 8 \\ \hline \multirow{4}{*}{BDS-3 Test} & 2015.07.25 & MEO & normal & C33, C34 & Rb & 19 \\  & 2015.09.30 & IGSO & normal & C32 & H & 17 \\  & 2016.02.01 & MEO & normal & C35 & Rb & 12 \\  & 2016.03.30 & IGSO & normal & C31 & Rb & 11 \\ \hline \hline \end{tabular} \({}^{1}\) [[http://www.csno-tarc.cn/](http://www.csno-tarc.cn/)]([http://www.csno-tarc.cn/](http://www.csno-tarc.cn/)).  
\end{table}
Table 1: The types and ages of BDS on-board satellite clocks (the list of events as of March 2017).  
Figure 1: Satellites’ orbits of BDS.  
the clock periodic characteristics. The satellite and clock types in Table 1 and Figure 1 are beneficial to the classification of the satellite clock prediction models below.  
This paper mainly discusses the effect of satellite clock modeling on prediction precision. In the process of clock offset estimation, the satellite orbits and clock parameters are typically estimated simultaneously. Therefore, the satellite orbit solutions have some effects on the satellite clock estimations. Due to the precise orbit determination and time synchronization method, the orbital period and type have a direct impact on the characteristics and performance of satellite clocks. In this paper, the impact of the BDS satellite clock on its heterogeneous constellation's orbit is analyzed in Section 4.