# Analysis of BDS/GPS Signals' Characteristics and Navigation Accuracy for a Geostationary Satellite
## 5 Conclusions  
For the GEO satellite of the TJS-5 mission, we use a high sensitivity receiver with a high-gain antenna to realize real-time navigation. In this study, we analyze the inflight data generated by this sensitivity receiver, which can track weak BDS and GPS signals. We investigate the GNSS signal characteristics, including observations quantity and distribution, availability, PDOP, observations accuracy. It is found that when BDS and GPS are combined, the number of navigation satellites tracked will increase significantly and the PDOP can be reduced obviously. Although the number of BDS satellites tracked is less than that of GPS, it makes a positive contribution to the improvement of PDOP and navigation solutions. In order to analyze the observations distribution characteristics, the number of observations with respect to the C/N\({}_{0}\) was analyzed. Most of the GPS and BDS observations are concentrated in C/N\({}_{0}\) range of above 30 dB-Hz and below 35 dB-Hz, except BDS-3 MEO satellite observations. We use epoch difference arithmetics to analyze observations noises. The standard deviation of the pseudorange noises are less than 4 m, and the corresponding carrier phase noises are mostly less than 8 mm. The pseudorange and carrier phase noises of BDS measurements are lower than those of GPS measurements. We give the navigation performance using only GPS observations and GPS + BDS observations combination at different weights. As for the single-epoch least square solutions, remarkable improvements in position and velocity accuracy are made with the combined GPS and BDS observations. Through comparisons with the precision reference orbits, when combining GPS observations with BDS observations at the weight of 1:1, the RMS of these position solutions accuracy can improve from 32.1 m to 16.5 m and the corresponding velocity accuracy can improve from 0.238 m/s to 0.165 m/s. Especially in x-axis direction, the position accuracy can improve from 31.0 m to14.6 m. We discuss the accuracy influence when BDS GEO/IGSO and MEO combined with GPS respectively. In this case, when BDS IGSO and GEO observations involved, there is an obvious improvement in position and velocity. As for the real-time orbit determination solutions, the RMS of position accuracy is 5.55 m and that of velocity accuracy is 0.697 mm/s when GPS and BDS pseudorange combined at a weight of 1:1. When we increase the weight of BDS observations, although the accuracy in x-axis direction has some improvements, the accuracy in y-axis direction and z-axis direction become worse. We need to do further research into some aspects to improve the accuracy in future high-earth-orbit missions. A comprehensive utilization of GPS, GLONASS, BDS and GALILEO system should be taken into account to improve orbit determination accuracy by increasing the number of available satellites and reducing PDOP. In addition, carrier observations should be considered to increase navigation accuracy, because the accuracy of carrier observations is higher than that of pseudorange observations. For the high-orbit satellites with orbit maneuver, the research on how to use IMU to realize integrated navigation is necessary. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Results**} & \multicolumn{6}{c}{**Weight (GPS + BDS)**} \\ \cline{3-6}  & & **GPS** & **1:2** & **1:1** & **2:1** & **4:1** \\ \hline \multirow{4}{*}{Position (m)} & X & 7.24 & 4.09 & 4.30 & 5.88 & 6.81 \\  & Y & 2.53 & 3.59 & 2.87 & 1.70 & 1.84 \\  & Z & 1.81 & 2.18 & 2.01 & 1.68 & 1.55 \\  & 3D & 7.88 & 5.86 & 5.55 & 6.34 & 7.22 \\ \hline \multirow{4}{*}{Velocity (mm/s)} & X & 0.629 & 0.502 & 0.491 & 0.547 & 0.601 \\  & Y & 0.739 & 0.384 & 0.361 & 0.537 & 0.668 \\ \cline{1-1}  & Z & 0.302 & 0.334 & 0.340 & 0.312 & 0.295 \\ \cline{1-1}  & 3D & 1.02 & 0.715 & 0.697 & 0.827 & 0.946 \\ \hline \hline \end{tabular}
\end{table}