## 4 Numerical Simulations
### Typical Extended Scene Simulation  
To further validate the proposed framework's performance faced with the situation of more realistic scene, we carry out a typical extended scene simulation. One SAR image is taken as the radar cross section for simulation. The whole scene covers an area of 1000 m \(\times\) 5000 m, where around 95.3 % are areas of pure-seawater surface and the rest of them are ship targets. The simulation parameters are the same as in the Table 3.  
Our framework is designed not only from the perspective of software (simply reducing the computation time) but also from the perspective of the imaging concept, the hardware, and the hardware-software feedback, to better reveal the advantages that it can bring. For this extended scene simulation, we additionally compare it with another similar framework with Cartesian fast-factorized back projection (CFBP) [35]. Figure 16a shows the result of the current imaging framework. Figure 16b,c show the results of the framework with CFBP and the proposed imaging framework. Compared with the results of the current imaging framework and the framework with CFBP, our framework uses the system's resources just on the targets' areas instead of on the water area that provides litter information. Second, our result presents target-oriented variable resolutions in the azimuth direction instead of constant full resolution. Visually, although only part of the azimuth aperture's data are accumulated, all the ships in the result look almost the same as the full-resolution results. Specifically, the effective aperture length for each ship, from the left to the right, is only 0.4, 0.42, 0.54, and 0.65, respectively, verifying the adaptive ability of our framework. In addition, besides the traditional imaging result that can be obtained, as shown in Figure 17, the detection result can also be obtained through our framework. This provides an end-to-end way for maritime surveillance that simplifies the current procedures. At the end of this subsection, we conclude the performance of our framework in the numerical simulations in Table 5, which indicates that for this typical scenario, at least 30% consumption of receiving resources and 99% consumption of computation time are saved. In contrast, the CFBP's result only saves 67.7% consumption of computation time, less than ours; and there is no consumption of receiving resources reduced. This is because the design of CFBP's framework has not fully considered either the features of the HRWS SAR system or the features of the maritime surveillance application. In addition, it is still whole-scene-oriented, without the adaptive hardware receiving and feedback. The ships are all located in the same area that the minimum receiving window and aperture in the two directions can cover. In the range direction, the ships are spread out in the receiving window, which makes the relative sampling bandwidth reduction less than the last simulation, and the scene's width is half of the last one. Thus, the relative reduced proportion of receiving resources in the range direction is less. The situation is the same in the azimuth direction. However, the processing computation time is still reduced quite a lot (99.1%), because only the areas of ships are imaged and only around 50% of the aperture data are accumulated on average.