## 3 AS01 SAR Satellite Data Experiment and Analysis  
### AS01 SAR Satellite Data Processing Experiment  
The AS01 SAR satellite, launched by Skysight Technology Co., Ltd. in Taiyuan, China in July 2023, is equipped with a lightweight X-band two-dimensional phased array SAR payload, and with a maximum resolution of 0.5 m. The AS01 satellite adopts a variety of imaging modes and has different resolution and width imaging capabilities, such as sliding spotlight, strip-map, and terrain observation by progressive scans SAR (TOPSAR). Noise equivalent sigma zero (NESZ) of AS01 satellite is less than \(-21.6\) dB, which can provide high-quality imaging of weakly scattered targets. This study utilizes ocean scene images acquired by the AS01-SAR satellite on 16 February 2024, employing the sliding spotlight imaging mode for experimental analysis. According to the product metadata, the range signal bandwidth is 600 MHz, with an incidence angle of 30.57\({}^{\circ}\), and the ground spatial resolution is 0.5 m. The full-aperture SAR image processing utilizes an azimuth signal bandwidth of 15,333 Hz, with the antenna beam rotating 3\({}^{\circ}\) in the azimuth direction, resulting in an azimuth resolution of 0.31 m. SLC data are used as input, and the frequency spectrum is segmented to generate sub-aperture image sequences. Figure 4 below shows the sub-aperture images, with the horizontal axis representing the range direction and the vertical axis representing the azimuth direction. Since the ship is moving along the azimuth direction, no image displacement of the ship target is observed. In the provided images, Figure 4a represents the original full-resolution SAR image captured by the satellite, while the other images depict sub-aperture images generated using azimuth domain segmentation. Through azimuth domain segmentation, both targets and wakes are imaged sequentially by each sub-aperture, with variations in imaging time and angle. During this timeframe, the motion of the ocean surface results in short correlation times, approximating the ocean surface as noise. However, a high correlation still exists for ship targets and large-scale texture features, such as ship wakes or large waves. In the sequence of images, there is a moving ship target, leading to poor imaging effects due to the target's motion, resulting in significant azimuth defocusing in the full-resolution image. Additionally, the motion of the target on the ocean surface generates wake phenomena, visible as relatively bright wake features in the image. However, coherent speckle noise affects the clarity of the wakes in the image. A comparison of the sub-aperture images reveals significant differences in the ship target itself and ocean wakes at different times and azimuth angles. Despite these differences, the linear wake features in the images are generally preserved. Figure 4: Sub-aperture image sequence generation. To enhance the clarity of the wakes and mitigate the effects of noise, the proposed high-resolution SAR wake enhancement algorithm employs histogram intensity normalization to preprocess the image sequence. This step eliminates differences in pixel grayscale values caused by variations in antenna orientation. Subsequently, filtering is applied along the pixel time sequence to reduce random errors, resulting in the enhanced image shown below. In Figure 5, (a) and (b), respectively, represent the original SAR wake image and the result image after equivalent sequence enhancement processing. The horizontal axis represents the range direction, while the vertical axis represents the azimuth direction. Figure 5c,d depicts the corresponding statistical histograms, where the horizontal axis represents grayscale distribution in dB, and the vertical axis represents the histogram statistics for each grayscale. In Figure 5a, coherent noise is evident, and the features of the ship wake are not continuous. Speckle noise and strong reflections caused by complex sea conditions disrupt the wake image, with some areas being obscured by wave noise.