# A High Dynamic Weak Spread Spectrum Signal Acquisition Strategy Based on Iterative Local Search
## 4 Experiments and Results
As the iterative search progressed, the scope of the Doppler search was gradually narrowed, and the time required for a single search was gradually reduced. Ultimately, the maximum dynamic signal of the space station can be captured within 3 s. The correlation peak was prominent, and the code phase search was accurate. According to Equation (12), the Doppler frequency deviation for transmission to the tracking loop can be deduced as \(\left|x_{fe}\right|\leq\frac{250.+50}{0.112}\times 0.064+50\approx 221.4\) Hz; according to Equation (13), the code phase deviation for transmission to the tracking loop can be deduced as \(\left|x_{\tau e}\right|\leq\frac{51.15\times 10^{6}}{26\times 10^{9}} \times(0.5\times\frac{250.+50}{0.112}\times 0.064^{2}+50\times 0.064)\approx 0.02\) chip, which satisfies the tracing loop requirement. Using the preset iteration parameters, the acquisition time of the improved parallel code phase search algorithm based on the ILS strategy was simulated under different space station ranging dynamic signal conditions, including the maximum Doppler, maximum Doppler change rate, and maximum Doppler second-order change rate of three cases. The simulation results are shown in Figure 11; as the Doppler increases, the overall acquisition time of the algorithm increases. As the Doppler change rate increases, the convergence rate of the single acquisition time will slow down. However, the search time of a single iteration will still tend to converge ultimately. According to Equation (12), the Doppler frequency error \(x_{f_{e}}\) transmitted to the tracking loop also converges. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Iterative** & **Coherent** & **Incoherent** & **Doppler Search** \\
**Search Times** & **Integration Time/ms** & **Integral Number** & **Step Value/Hz** \\ \hline
1 & 0.5 & 8 & 2000 \\
2 & 1 & 4 & 1000 \\
3 & 2 & 2 & 250 \\
4 & 4 & 1 & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: ILS parameters. Figure 10: ILS simulation results (the variation of Doppler and code phase information obtained through four iterative searches). (2) Comparison experiment: In the initial visible arc segment of the space station, the maximum signal Doppler is 600 kHz and the minimum SNR is \(-34\) dB. When the traditional parallel code phase search was used to capture dynamic signals, the step of the Doppler search was less than 500 Hz to meet the requirement of the tracking loop, and the overall integration time of the algorithm was at least 4 ms to satisfy the requirement of weak signal acquisition. Under these conditions, traditional parallel code phase searching requires at least 2400 frequency bands to search fixed 300 kHz Doppler signals. When the integration time of a single acquisition is constant, the phase search of the parallel code using the ILS strategy requires two iterative rounds. The first Doppler search steps 2 kHz and searches 600 frequency bands, and the second Doppler search steps 500 Hz and searches 8 frequency bands. As shown in Table 3, after the ILS strategy is adopted, the computation amount of the algorithm is reduced by 75%. A Monte Carlo experiment was performed to test the acquisition probability before and after changing the strategy under different dynamic conditions. Each result was based on 1000 experiments, when the acquisition results satisfy the conditions shown in Equation (14), the acquisition is judged to be successful, and the number of successful acquisitions n is obtained at the end of the experiment. The acquisition success probability \(P\) can be given by:  
\[P=\frac{n}{1000} \tag{15}\]  
In the experiment, in order to satisfy the demand of the tracking loop, which requires \(\left|x_{fe}\right|<500\) Hz and \(\left|x_{\tau e}\right|<0.5\) chip, under the condition of the same coherent integration time, the three methods were tested as follows. The integration time of the traditional parallel code phase acquisition algorithm is 4 ms, and the frequency search step is 500 Hz.