# A High Dynamic Weak Spread Spectrum Signal Acquisition Strategy Based on Iterative Local Search
## 4 Experiments and Results  
Experiments were performed to evaluate the acquisition strategy based on ILS. In a typical tracking loop, the coherence integration time is 1 ms, and the tolerance range of the Doppler frequency error is within \(\pm\)500 Hz, while the tolerance range of the code phase error is within \(\pm\)0.5 chip. Based on this premise, the ILS acquisition strategy was implemented to improve the performance of the parallel code phase algorithm for the high dynamic weak spread spectrum signals of the space station, and the acquisition time was evaluated based on the operating frequency of a typical FPGA chip XC7K325 TFFG900-2. The simulation compared the computational burden and the sensitivity of the acquisition algorithm under different dynamic conditions before and after the acquisition strategy was changed. In order to verify the superiority of this method by comparison, we carried out experiments on the acquisition strategy based on prior information, which is widely used in practical engineering; this strategy reduces the acquisition difficulty by providing Doppler information to the acquisition algorithm through external devices (inertial navigation system, ephemeris, etc.). The error range of the INS velocity measurement for different grades is 0.1085 m/s-45.09 m/s [19]; the corresponding Doppler estimation error range in the space station experiment is 8.6 Hz-3.9 kHz. The velocity error of the ephemeris solution is usually less than 0.2 m/s [20]; the corresponding Doppler estimation error is less than 17.3 Hz. As shown in Figure 7, in the experiment, the high dynamic weak spread spectrum signal is transmitted by the Ground Testing System of the microwave link payload of the China Space Station, and the signal is collected by the Radio Frequency (RF) signal collection system (Figure 8). The superiority of the algorithm was verified by processing the data collected by the computer. The Ground Testing System mainly consists of the Menlo Optical frequency comb system, signal analog source, and frequency conversion channel; the optical frequency comb system provides high-precision time-frequency reference for the signal analog source and frequency conversion channel (Figure 9). Figure 8: Portable ultra – wideband RF signal high –speed collecting, recording, and playback system (the system needs to connect to the reference clock signal). Figure 7: Structure of experimental system. (1) Experiment of the space station spread spectrum signal acquisition: The ILS strategy was used to improve the parallel code phase search, and the dynamic maximum signal of the space station was simulated and captured. Table 1 shows the dynamic simulation signal parameters used for the space station, which correspond to the dynamic maximum signal (as shown in Figure 2, the Doppler second-order change rate is the largest; the Doppler change rate is nearly the largest). When the parallel code phase search was performed using the iterative local search strategy to capture the signal, the coherent integration time and the number of non-coherent integrations of a single search must be dynamically adjusted to reduce the overall acquisition time and to improve the accuracy of the Doppler search. The parameters of the preset four iterative local searches are shown in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Simulation Parameters** & **Values** \\ \hline Initial Doppler (kHz) & \(300\) \\ Initial Doppler change rate (kHz/s) & \(-10\) \\ Second-order change rate of Doppler (Hz/s\({}^{2}\)) & \(-150\) \\ Spreading code rate (MHz) & \(51.15\) \\ Sampling rate (MHz) & \(300\) \\ SNR (dB) & \(-34\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: China Space Station analog signal parameters. Figure 9: The Ground Testing System of China Space Station. The signal acquisition results are shown in Figure 10. Owing to the dynamic characteristics of the signal, the Doppler frequency of the four-segment signal varied significantly.