## 3 Methods
### Resolution Study and Peaks Identifying Methods  
In this part, we study the resolution performance of the BDS signal. As shown in Table 1 in the simulation section, the B3I signal is transmitted with a bandwidth of 20.46 MHz. Figure 3 shows the autocorrelation curves of the B3I signal and two chirp signals with bandwidths of 10.23 MHz and 20.46 MHz.  
In Figure 3, we find that the main lobe of the B3I signal is wider than that of the chirp signal at 20.46 MHz. According to Formula (18), where C is the electromagnetic wave velocity and \(B\) is the signal bandwidth, the range resolution of the two chirp signals can be calculated as 29.32 m. The resolution of the B3I signal is approximately 30 m, as shown in Figure 3.  
\[\Delta r=C/B \tag{18}\]  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Signals** & **B3I** \\ \hline Bandwidth (B)/MHz & 20.46 \\ \hline Carrier frequency \(f_{0}/\mathrm{Hz}\) & \(1.268\times 10^{9}\) \\ \hline Satellite1 PRN & 19 \\ \hline Satellite2 PRN & 26 \\ \hline Satellite3 PRN & 21 \\ \hline Satellite4 PRN & 22 \\ \hline Receiver label/km & (0, 0, 6.5) \\ \hline Receiver velocity/m/s & 250 \\ \hline Receiver velocity angle & \(\varphi=135^{\circ},\theta=90^{\circ}\) \\ \hline Simulation serial number & 1 & 2 \\ \hline Accumulation time/s & 0.128 & 10 \\ \hline \hline \end{tabular}
\end{table}
Table 1: System parameter.  
Figure 3: Autocorrelation curves of GNSS B3I and chirp signals.  
To further study the resolution of the B3I signal, we simulated two-point target detection with interval of 29 m and 33 m by using the B3I signal, setting the chirp signal at a 10.23 MHz bandwidth as a comparison. Figure 4a shows the detection peaks of the GNSS B3I signal and chirp signal reflected by two-point targets with a distance of 29 m. We found that the solid line peaks overlap completely, which means that the B3I signal cannot distinguish the two-point targets. The chirp signal, with a bandwidth of 10.23 MHz, can clearly detect two-point target peaks. Next, in Figure 4b, where the distance between two-point targets is 33 m, two signals show two target peaks, and the sidelobe of the B3I signal in the center is higher, while its sidelobes on both sides are lower than those of the chirp signal.  
Next, we studied the sidelobe between the two target peaks of the B3I signal detection curves. Figure 5 shows the normalized sidelobe peak curves between two target peaks of the GNSS B3I signal as the distance between two targets. We found that the sidelobe peaks gradually decreased until the distance was more than 30 m, and in the range of 30-60 m, they had a linear decline as the distance increased. The results show that the GNSS B3I signal can realize a clear target peak resolution with an interval of targets near 30 m. After further simulation, we found that 33 m is the suitable interval for B3I signals with clear and smaller peak resolutions.  
In BDS-R detection, the echo intervals are smaller than the resolution of the B3I signal, and in the case of bistatic detection, intervals of targets vary with the bistatic angle. To acquire a clear and stable resolution, we propose sampling the echoes with an interval of 33 m, and the RCS data for comparative verification can also be processed with an interval of 33 m.  
Figure 4: Two-point target detection main lobe curves of GNSS B3I and chirp signals: (**a**) The distance between the two targets is 29 m; (**b**) the distance between two targets is 33 m.