# Weak and Dynamic GNSS Signal Tracking Strategies for Flight Missions in the Space Service Volume
## 5 Simulation and Experiment Results
### Scenario Settings  
In order to identify the tracking quality of our proposed strategies, we establish a scenario of a lunar upper stage on a HwaCreat(tm) GNSS signal simulator (produced by Beijing Hwa Creat Technology Corporation Ltd., Beijing, China). In the scenario, the lunar exploration probe operates in its Earth phasing orbit [4], whose trajectory traverses both TSV and SSV in a geostationary transfer orbit (GTO). One certain GPS satellite is chosen as the signal source, and the signal emitted from the satellite is used for analysis when it is visible to our object vehicle. The simulated scenario is drawn in Figure 5. We can see the lunar upper stage receives the signal emitted by the GPS satellite and transmitted over the limb of the Earth, which is consistent with the basic characterization of SSV navigation. The simulation time lasts for 320 s, which is divided into four phases. During the simulation time, the lunar probe operates at an altitude of about 35,000 km, and transfers from its original phasing orbit to another phasing orbit by means of an orbital maneuver. In the first 50 s, the GPS satellite is not visible to the lunar probe. From the 51 st s to 300 th s, the GPS satellite enters the visible area of the lunar probe. In this period, the probe operates under gravity, and completely free from its own drag force. In the third phase, from the 301 st s to the 310 th s, the onboard motor starts to work and generates a ramp LOS jerk. The value of the LOS jerk reaches its maximum at the 310 th s and keeps constant in the fourth phase. The dynamic performance requirements for the GNSS receiver of the lunar upper stage can be seen in Table 2. If the Doppler shift caused by the LOS velocity and acceleration are completely compensated by the third-order PLL, the bearable jerk of 4 g/s becomes the most important requirement. Thus, we set the maximal LOS jerk at 40 m/s\({}^{3}\) with a little excess. The mission control sequence (MCS) of the scenario is shown in Table 3.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Dynamic Type** & **Sustainable Range** \\ \hline Velocity & 0–12 km/s \\ Acceleration & 0–15 g \\ Jerk & 0–4 g/s \\ \hline \hline \end{tabular}
\end{table}
Table 2: The requirements of bearable dynamics for onboard GNSS receiver of the lunar probe.  
Figure 5: Schematic diagram of the simulation scenario.  
Figure 6: The variation of C/N\({}_{0}\) and LOS jerk for the established scenario. (**a**) Global view; and (**b**) close-up view of the second phase.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Phase** & **Time Span** & **Visibility of GPS Satellite** & **Orbital Maneuvering Status** \\ \hline I & 0–50 s & Not in view & No maneuver is made \\ \hline II & 51–300 s & In view & No maneuver is made \\ \hline III & 301–310 s & In view & Make amaneuver with a ramp LOS jerk (the peak is 40 m/s\({}^{3}\)) \\ \hline IV & 311–320 s & In view & Make amaneuverwith a constant LOS \\  & & & jerk of 40 m/s\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: The mission control sequence (MCS) of the lunar probe during the simulation time.