# Analysis of BDS/GPS Signals' Characteristics and Navigation Accuracy for a Geostationary Satellite
## 2 System and Methods Description  
In the GEO satellite of TJS-5 experiment, the altitude of the receiver is higher than that of the MEO navigation satellite, as shown in Figure 1. Therefore the GNSS receiver receives the signals from the other side of the earth and the elevation of this receiver is negative. A high-gain antenna in form of bifilar helix mounted in a deployable structure is oriented toward the center of the earth. The gain of this antenna is more than 7 dB within an angle of \(-30^{\circ}\)-+\(30^{\circ}\) according to the measured data. It should be noted that the BDS GEO/IGSO satellites and receiver are on the same orbital plane over the Asia Pacific. The elevation of these BDS GEO/IGSO satellites is beyond the beam range of the receiving antenna (\(-30^{\circ}\)-+\(30^{\circ}\)). In this situation, whether BDS GEO/IGSO satellites signal can be received or not depends on the signal transmission distance, the side-lobe gain of the navigation satellite antenna and the receiving antenna gain. A high-sensitivity receiver are used for acquisition and tracking the weak navigation signals in this experiment. Table 1 provides some information about this GNSS receiver. The architecture of this high-sensitivity spaceborne receiver is shown in Figure 2. The filter, low noise amplifier (LNA), and the RF-front ends downconvert the navigation signals to intermediate frequency (IF) signals which are sampled at 56.8 MHz. The RF-front is driven by a stable, low-phase noise, 10 MHz oven-controlled crystal oscillator (OXCO). The high sensitivity fast acquisition and tracking are performed in the digital application specific integrated circuit (ASIC), which is embedded in a rad-hard system on chip (SOC). Raw measurements (pseudorange, carrier phase, ephemeris, time, etc.) are generated and used for the single point position and real-time orbit determination by the CPU in this SOC. The receiver can track up to eight GPS satellites and eight BDS satellites with 16-channel hardware correlators simultaneously. A real-time orbit determination filter based on an extended Kalman filter (EKF) in this receiver works with pseudorange observations and dynamic models. Figure 1: Reception geometry for MEO, IGSO and GEO navigation satellites and receiver in GEO mission. This receiver can process L1 C/A signals of 32 GPS satellites of the PRN G01-G32. Moreover, this receiver can be compatible with the BeiDou regional system (BDS-2) and the BeiDou global system (BDS-3), which have three orbit types: medium earth orbit (MEO), inclined geostationary orbit (ICSO), and GEO [25; 26]. The receiver can process the B1I signals of 37 BDS satellites of the PRN C01-C37. Because C15, C17, C18 and C31 satellites are out of service, there are 33 BDS satellites which can be tracked by the receiver, including 5 BDS-2 GEO, 7 BDS-2 IGSO, 3 BDS-2 MEO, and 18 BDS-3 MEO satellites, as listed in Table 2. \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Value** \\ \hline Compatible frequency & BDS B1I, GPS L1 C/A \\ Number of channels & 8 for BDS, 8 for GPS \\ Original observation types & Carrier phase, pseudorange and C/N\({}_{0}\) \\ Acquisition sensitivity & 28 dB-Hz \\ Tracking sensitivity & 24 dB-Hz \\ OXCO accuracy & 0.5 ppm \\ OXCO Allan variance & \(\leq\)1\(\cdot\)10\({}^{-11}\) s\({}^{-1}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Primary parameters of the TJS-5 spaceborne receiver.