### Description of BRDF  
To mitigate the inconsistency in the response of the remote sensors between elements, it is required to provide an in solar diffuser reflector-based on-planet calibration; the sun serves as a standard light source and illuminates the diffuser reflector, forming a uniform irradiance source. Additionally, relative corrections are introduced to the remote sensors to be calibrated. The crucial component governing the bidirectional reflectance distribution function (BRDF) of the UVS instrument is the onboard diffuser [4; 5]. This onboard diffuser facilitates the transformation of sun irradiance into radiance, measured subsequently by the instrument. The BRDF (bidirectional reflectance distribution function) represents the micro-increment of surface irradiance reflected in a specific direction, expressed as the ratio between the incremental brightness of the reflected radiation and the incident irradiance. Radiance is used to describe the intensity distribution of a radiation field. It represents the physical quantity of the strength of radiation at a point on a surface radiation source in a specific direction. The unit of radiance is \(\upmu\)W/cm\({}^{2}\)/sr/nm. Irradiance refers to the radiant flux per unit area of the illuminated surface, with the unit being \(\upmu\)W/cm\({}^{2}\). The BRDF of the diffuse reflection plate on the satellite can be defined as follows [6]:  
\[E=\frac{L}{BRDF} \tag{1}\]  
where \(E\) denotes irradiance, and \(L\) denotes radiance, which can be expressed as follows:  
\[L=A\cdot S \tag{2}\]  
where \(A\) denotes the radiance response of the load instrument, and \(S\) denotes the output count of the load detector. The detection signals in the Earth mode are derived through two steps: first, using a solar simulator [7] and a standard diffuse reflector with a known BRDF, and secondly, by switching the instrumental operation mode to the solar mode. In the solar mode, the detection signals are obtained by using a simulated light source to directly illuminate the solar-port diffuse reflector. The bidirectional distribution function of the UVS diffuse reflection is derived by comparing these two signals, as shown in the following formula:  
\[E=\frac{A\cdot S_{1}}{BRDF_{diffuse}} \tag{3}\]  
where \(E\) denotes the input irradiance, \(A\) denotes the instrument radiance response, \(S_{1}\) represents the detecting counts in the Earth mode, and \(BRDF_{diffuse}\) represents the bidirectional reflectance distribution function of the standard diffuser. In the solar mode, the input irradiance can be expressed as follows:  
\[E=\frac{A\cdot S_{2}}{BRDF_{x}} \tag{4}\]  
where \(S_{2}\) represents the detecting counts in the solar operating mode, and \(BRDF_{x}\) represents the bidirectional reflectance distribution function of the onboard diffuser. According to a comparison between Formulas (3) and (4),  
\[BRDF_{x}=\frac{S_{2}}{S_{1}}BRDF_{diffuse} \tag{5}\]  
Throughout the year, as the sun moves along the ecliptic, the angle (\(\alpha\), \(\beta\)) between sunlight and the incidence window's normal constantly changes; \(\alpha\) represents the azimuthal angle of sunlight incidence, and \(\beta\) represents the pitch angle of sunlight incidence. The radiometric calibration of the onboard system is crucial for ensuring the accuracy of target retrieval. The variability in the reflectance of the core component, i.e., the diffuse reflector, directly impacts the precision of radiometric calibration. To mitigate the impact of reflectance variability, a dual diffuse reflector system comprising a working board and a reference board (as shown in Figure 2a) is employed in this instrument. The calibration intervals for the dual diffuse reflector are set at once every 10 days and once per quarter. Each calibration session lasts approximately 2 min. The reference board is utilized to calibrate changes in performance exhibited by the working board.