## 4 Conclusions  
In this study, we calibrated the bidirectional reflectance distribution function (BRDF) by simulating incidence angles in orbit and testing the BRDF for various angles.  
In Section 3, we utilized a transfer formula to calculate the relationship between the UVS BRDF under different incidence angles. The results indicate that, at specific incidence angles, the BRDF exhibits wavelength-dependent and spatial distribution characteristics, leading to varying BRDF characteristics corresponding to each pixel on the CCD image plane. From the results, it is evident that the diffuser's dependence on wavelength is more pronounced in the 250-300 nm compared to the 300-500 nm. The BRDF characteristics of the diffusers of the UVS were evaluated. Based on radiometric calibrations involving multiple azimuth and elevation angles, a BRDF model was established for each pixel, revealing coefficients for the two diffusers in UVS. From the perspective of spatial distribution at a specific angle, the changes in BRDF are not entirely uniform across a wide field of view, and this phenomenon may be attributed to the intrinsic characteristics of the diffuse reflection board material. Given that all pixels on the UVS have been calibrated, these wavelength-dependent and spatially dependent characteristics can be adjusted through calibration coefficients.  
We conducted a comprehensive analysis and precise calculation of the uncertainties inherent in the calibration process. The accuracy levels of UVS in the UV1, UV2, and VIS channels were 2.162%, 2.162%, and 2.173%, respectively. The coefficients obtained from the fitting serve as the baseline coefficients for the in-orbit testing of the UVS. The UVS BRDF calibration was conducted under simulated in-orbit conditions, including the environment and operational modes, so the obtained calibration coefficients can be directly used for the initial in-orbit phase. Subsequent validation and analysis will be carried out using the corresponding solar data. Future studies are warranted to improve the BRDF model based on in-orbit comparison results.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **UV1** & **UV2** & **VIS** \\ \hline Source non-uniformity & 1.5\% & 1. 5\% & 1.5\% \\ Standard diffuser plate uncertainty & 1\% & 1\% & 1\% \\ UVS non-stationarity & 0.42\% & 0.42\% & 0.47\% \\ Source non-stationarity & 1\% & 1\% & 1\% \\ Solar simulator distance error & 0.5\% & 0.5\% & 0.5\% \\ Total & 2.162\% & 2.162\% & 2.173\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Diffuse reflector calibration uncertainty.  
**Author Contributions:** Writing--original draft preparation: [PERSON]; on-ground calibration: [PERSON] and [PERSON]; instrument designing: [PERSON]; calibration data-processing program: [PERSON] and [PERSON]; writing--review and editing: [PERSON] and [PERSON]; data validation: [PERSON] and [PERSON]. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by the Civil Space Program [D010206].  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the author.  
**Acknowledgments:** The authors wish to thank the Project team, [PERSON] and [PERSON] from the Beijing Institute of Space Mechanics and Electricity (BSME).  
**Conflicts of Interest:** Author [PERSON] and [PERSON] was employed by the company DFH Satellite Co. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.