# Airborne Radar Staggered PRF Coherent Processing Method for Down-Looking Target Detection
## 4 Simulation Results
### Multi-PRF Result Analysis of Airborne Radar with Orthogonal Signals
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & \begin{tabular}{c} **Number of** \\ **Azimuth Array** \\ **Element** \\ \end{tabular} & \begin{tabular}{c} **Number of** \\ **Elevation Array** \\ **Element** \\ \end{tabular} & \
\begin{tabular}{c} **Azimuth Angle** \\ **Element** \\ \end{tabular} &
\begin{tabular}{c} **Elevation Angle** \\ **Elevation Angle** \\ \end{tabular} \\ \hline Value & 40 & 40 & 30 & 30 \\ \hline \hline \end{tabular}  
\end{table} 
Table 2: Radar antenna parameter. The experiments above confirm the accuracy of Doppler ambiguity and the coherent gain of target energy. In practical applications, performing coherent processing of ambiguity resolution on every range-Doppler cell within the power range requires extensive computation. To reduce the computational load, first-layer detection of the echo signal can be employed. Firstly, the RD results of each PRF signal group are detected using a lower first detection threshold (7 dB). Figure 14 illustrates the schematic diagram of CFAR  
Figure 12: The DFT results of the radar echo signal. (**a**) Results of RD for the PRF1 signal group. (**b**) Results of RD for the PRF3 signal group. (**c**) Results of RD for the PRF4 signal group. (**d**) Results of RD for the PRF5 signal group. Figure 13: The results after multi-DFT de-ambiguous and coherent processing. (**a**) Result of RD coherent processing. (**b**) Range slice containing the target. detection. Then, based on the detection results, Doppler ambiguity resolution and coherent fusion are performed, followed by a second-layer detection on the fusion results. Figure 15 presents the RD result of the first PRF signal group. The detection result using the first detection threshold is depicted in Figure 15b, where numerous false alarms are observed. For ambiguity resolution and coherent fusion processing using the multi-PRF signal group, the simulation applies a second detection threshold (13 dB) to detect the signal after ambiguity resolution. The comparison between the first and second CFAR detections is shown in Figure 16.  
Figure 14: The threshold for the first layer of CFAR detection. Figure 15: The result of the first layer of CFAR detection. (**a**) RD map of the target region. (**b**) CFAR detection results. In Figure 17, coherent processing of the RD results from multiple PRF groups leads to a corresponding increase in the detection threshold. The target energy experiences a five-times SNR gain through coherent processing. Simulation raises the threshold (i.e., the second detection threshold) to 13 dB, surpassing the noise level. Figure 17b displays the detection result of the second detection threshold. After coherent processing and the second detection, only the target signal is output, eliminating all false alarms. Compared to the traditional Multi-PRF method, within the same dwell time, which sequentially transmits five groups of PRF waveforms and performs ambiguity resolution processing after detecting the RD results of each group. Utilizing low PRFs in PRF cross-transmission extends the maximum unambiguous distance than the traditional method, avoiding the division of the entire dwell time into multiple parts. Consequently, the coherent processing time is longer (with a larger slow-time domain sampling width), resulting in improved target energy and Doppler resolution. As shown in the figure above, Figure 18a shows the traditional Multi-PRF at 20 KHz repetition frequency, accumulating 256 pulses, the Doppler resolution is 103 Hz, and Figure 18b shows the use of alternate PRF in this way, and the Doppler resolution is 20 Hz. Figure 16: Comparison of two-layer detection thresholds. Figure 17: The result of the second layer of CFAR detection. (**a**) RD map of the target region. (**b**) CFAR detection results. After coherent processing, the Doppler resolution is less than 20 Hz, and the Doppler resolution is five times higher than that of Multi-PRF.