## 2 Dechirp Influence on Squint Sliding-Spot Imaging Mode  
We simulated the squint sliding-spot echo data of 18 point targets in the target ground area, and the SAR imaging parameters used in the simulation are shown in Table 1. Figure 1 indicates the satellite-to-ground position relationship and the distribution of targets. The satellite descends from north to south and flies in the left side looking, and performs sliding-spot imaging on the target area by turning backward with the azimuth angle from small to large. After the echo data are range-compressed, the SAR signal of any target can be expressed as  
\[\widehat{ss}(\tau,\eta)=p_{r}\bigg{[}\tau-\frac{2r(\eta)}{c}\bigg{]}\cdot w_{a }(\eta)\cdot exp\bigg{\{}-j4\pi f_{0}\frac{r(\eta)}{SOL}\bigg{\}} \tag{1}\]  
Here \(f_{0}\) is the radar center frequency, \(c\) is the wave speed, \(\eta\) is the azimuth time, \(\tau\) is the slant range delay time, \(p_{r}()\) is the range-compressed pulse, \(w_{a}()\) is the azimuth antenna modulation, and \(r(\eta)\) is the slant range history between SAR and the target. The signal represented by this formula is shown in Figure 2. We can see that the large range of antenna rotation angle and the squint angle of view lead to serious range walking in the echo slant \begin{table}
\begin{tabular}{c c} \hline
**Satellite orbit altitude** & 600 km \\
**Squint scanning angle range** & 4.93–13.13 degrees \\
**Center side-looking angle** & 27 degrees \\
**Range bandwidth** & 800 MHz \\
**Range sampling rate** & 1000 MHz \\
**Satellite platform velocity** & 7.62 km/s \\
**Pulse repetition frequency (PRF)** & 5182 Hz \\
**Imaging time** & 15.26 s \\
**Range sampling point number in the** & 25,600 \\
**imaging area** & 46,272 \\
**Azimuth sampling point number in the** & \\
**imaging area** & 46,272 \\ \hline \end{tabular}
\end{table} 
Table 1: Spaceborne SAR squint sliding-spot imaging simulation parameters. Figure 1: The satellite-to-ground position relationship in the simulation and the distribution of simulation points. range history of each target. Perform range FFT (Fast Fourier Transform) on Equation (1) to obtain  
\[\widetilde{SS}(f_{\tau},\eta)=W_{r}(f_{\tau})\cdot w_{a}(\eta)\cdot exp\left\{-j \frac{4\pi(f_{0}+f_{\tau})}{c}r(\eta)\right\} \tag{2}\]  
where \(W_{r}(f_{\tau})\) is the pulse range spectral envelope. It can be seen that in the high-resolution squint sliding-spot SAR mode, the total azimuth bandwidth may exceed several PRFs, resulting in a severe azimuth aliasing phenomenon. If no effective measures are taken, the SAR image quality will be seriously affected: on the one hand, the target focusing quality will seriously degrade; on the other hand, a large number of blurred images will be caused. Thus, compared with the traditional side-looking strip or low-resolution sliding-spot modes, the imaging algorithm for the squint sliding-spot mode has great changes. Current algorithms [20; 21] commonly adopt the azimuth dechirp processing in the range frequency and azimuth time domain to correct the curved range history, move the doppler center of all signals to zero, and reduce the doppler bandwidth to a PRF, which can solve the problem of insufficient PRF. That is, perform the phase correction by multiplying Equation (2) by a chirp factor \(\widetilde{\text{Q}}(f_{\tau},\eta)\) as follows  
\[\widetilde{\text{Q}}(f_{\tau},\eta)=exp\left\{-j\frac{4\pi(f_{0}+f_{\tau})}{c }\text{D}(\eta)\right\} \tag{3}\]  
\(\text{D}(\eta)\) in this factor is a function of \(\eta\) as an independent variable, and its form is not completely fixed. It is calculated from real-time parameters such as satellite position, attitude, and antenna pointing. For example, \(\text{D}(\eta)\) is defined as follows in [20]  
\[\widetilde{\text{Q}}(f_{\tau},\eta)=exp\left\{-j\frac{4\pi(f_{0}+f_{\tau})}{c} \text{D}(\eta)\right\} \tag{4}\]  
where \(r_{d}(\eta)\) is the range history between the antenna and the rotation center, and \(\overline{r_{d}(\eta)}\) is the mean value of \(r_{d}(\eta)\).