## 4 Atmospheric Correction Results and Analysis
### Validation of Reflectance Through In Situ Measurement Comparison  
To validate the atmospheric correction algorithm described in Section 3, reflectance synchronization experiments were conducted in Dunhuang, Gansu Province, and Hefei, Anhui Province, China. The surface reflectance values obtained after atmospheric correction were compared with in situ measured reflectance to evaluate the accuracy of the correction. A surface reflectance validation experiment was conducted in Dunhuang, Gansu Province, China, from 23 January to 3 February 2021. The primary surface type at the Dunhuang site is the Gobi Desert, with coordinates at (40.0926\({}^{\circ}\)N, 94.3934\({}^{\circ}\)E). A secondary site featuring a high-reflectance surface covered with bright pebbles is located at (40.4750\({}^{\circ}\)N, 94.3806\({}^{\circ}\)E). Figures 10 and 11 show the images before and after atmospheric correction, with red-marked areas indicating the regions where ground experiments were conducted. After atmospheric correction, the atmospheric effects were effectively removed, enhancing image details and surface textures. A similar experiment was conducted in Hefei, Anhui Province, China, from 22 March to 26 March 2021. The experimental site included a wheat field at (31.5234\({}^{\circ}\)N, 117.3376\({}^{\circ}\)E) and a river water site at (31.5281\({}^{\circ}\)N, 117.3413\({}^{\circ}\)E). Figure 12 compares the pre- and post-correction images, with red marked and blue marked areas representing the ground measurement regions for the wheat field and river water, respectively. The corrected images show significant improvements in visual quality, with clearer surface features and reduced atmospheric interference. Figure 9: The contrast, clarity, and their improvements of the multispectral images of Xian Ning, Hubei Province, China, before and after atmospheric correction from the HJ-2A satellite. (**a**) Contrast. (**b**) Clarity. Figure 11: Comparison of pre- and post-atmospheric correction for HJ-2B satellite multispectral image at Northern high-reflectance site in Dunhuang, China. The red-marked area represents the ground measurement region at the high-reflectance site. (**a**) Before atmospheric correction. (**b**) After atmospheric correction. Figure 10: Comparison of pre- and post-atmospheric correction for an HJ-2B satellite multispectral image at the Dunhuang site in China. The red-marked area represents the ground measurement region at the Dunhuang site. (**a**) Before atmospheric correction. (**b**) After atmospheric correction. During the ground experiment, surface reflectance was measured using a FieldSpec hand-held spectroradiometer during HJ-2A/B satellite overpasses. The FieldSpec, a product of Analytical Spectral Devices (ASD), USA, features high spectral resolution and portability, enabling precise in situ measurements across various surface types. The measurement method involved setting up multiple measurement points across the test area and employing a walking approach to ensure uniform spatial coverage. At each measurement point, five sets of data were collected to calculate the average error. During the data processing phase, the measurements from individual points were first averaged, followed by averaging the results from all points within the experimental area. Invalid data were excluded using a variance-based threshold to minimize the impact of outliers. The processed surface reflectance results are presented in Figure 13, which shows the averaged reflectance values across the experimental area. Figure 12: Comparison of pre- and post-atmospheric correction for an HJ-2A satellite multispectral image at the suburban area of Hefei, Anhui Province, China. The red-marked and blue-marked areas represent the ground measurement regions for the wheat field and river water, respectively. (**a**) Before atmospheric correction. (**b**) After atmospheric correction. Figure 13: The reflectance curve from the ground-based synchronized measurements. (**a**) Dunhuang, Gansu, China. (**b**) Hefei, Anhui, China.