# Integrated Precise Orbit Determination of Multi-Gnss and Large LEO Constellations
## 2 Constellations and Observation Simulation  
### Constellation Design  
With the full operation of the Galileo and BDS constellations, there will be more than 120 GNSS satellites in the sky by 2020. As of August 2019, Galileo had reached its nominal constellation, with only back-up satellites to be launched in the future, while the BDS constellation, which is still under construction, has 19 BDS-3 satellites providing navigation services. Therefore, in order to perform the integrated POD with the whole constellation of the four systems, the orbits of the GPS, GLONASS, Galileo, and BDS satellites have to be simulated based on the nominal constellation configurations [22; 23; 24; 25] of the four systems. The nominal GPS constellation consists of 24 medium earth orbit (MEO) satellites unevenly distributed in six planes. MEO satellites are also employed by GLONASS and Galileo using a 24/3/1 Walker constellation to provide global coverage. In addition, six additional MEO satellites serve as in-orbit spare satellites for the Galileo constellation. Different from the other three constellations, BDS-3 is made up of 24 MEO satellites, 3 geostationary earth orbit (GEO) satellites, and 3 inclined geosynchronous orbit (IGSO) satellites. The three GEO satellites are located at 80\({}^{\circ}\)E, 110.5\({}^{\circ}\)E, and 140\({}^{\circ}\)E, while the IGSO satellites operate in an orbit with an inclination of 55\({}^{\circ}\); the right ascension of the ascending nodes (RAAN) is 118\({}^{\circ}\)E. Table 1 presents detailed information on the GNSS constellations.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**GNSS** & **GPS** & **GLONASS** & **Galileo** & & **BDS** & \\ \hline
**Orbit type** & MEO & MEO & MEO & GEO & IGSO & MEO \\ \hline
**Satellites number** & 24 & 24 & 30 & 3 & 3 & 24 \\ \hline
**Altitude** & 20,180 km & 19,100 km & 23,220 km & 35,786 km & 35,786 km & 21,528 km \\ \hline
**Inclination** & 56\({}^{\circ}\) & 64.8\({}^{\circ}\) & 56\({}^{\circ}\) & 0\({}^{\circ}\) & 55\({}^{\circ}\) & 55\({}^{\circ}\) \\ \hline \multirow{2}{*}{**Constellation**} & Distributed & Walker & Walker (24/3/1) + 6 & Located at 80\({}^{\circ}\)E, & RAAN of & Walker \\  & in 6 planes & (24/3/1) & backups & 110.5\({}^{\circ}\)E, and 140\({}^{\circ}\)E & 118\({}^{\circ}\)E & (24/3/1) \\ \hline
**Total number** & 24 & 24 & 30 & 30 & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Nominal constellation configuration of GPS, GLONASS, Galileo, and BDS.  
To comprehensively evaluate the impact of LEO configuration on GNSS POD, six LEO constellations with different configurations are designed and simulated. Table 2 lists the LEO constellation parameters. Nearly-polar and sun-synchronous orbits are selected in our study to investigate the influence of the LEO orbit type, which are two typical orbit types for many LEO missions. The nearly-polar orbit we choose has the same orbit inclination, i.e., 84.6 degrees, as the Iridium satellite constellation. Considering the fact that computation time rises sharply for the integrated POD due to huge amounts of observations to be processed and unknown parameters to be recovered, only the constellations with satellites number of 60, 66, 96 are adopted. The Chinese Hongyan project, which has been announced by the China Aerospace Science and Technology Corporation (CASC) for the purpose of global communication and LEO-augmented positioning, proposes a constellation made up of 60 LEO satellites as its backbone, while the 66-LEO constellation is employed by the Iridium satellite constellation for global coverage. In order to assess the influence of LEO altitude on the LEO-augmented GNSS POD, the 60-LEO constellation is simulated with altitudes of 600 km, 1000 km, and 1400 km. Figure 1 presents a sketch of the designed LEO constellations.