## 2 Instrument, Model, and Method  
### Instrument  
The Chinese environmental protection and disaster monitoring satellite constellation Huanjing Jianzai-2 (HJ-2) A/B satellites were launched on 27 September 2020. HJ-2A and HJ-2B, with the same configuration, run in a sun-synchronous orbit, and the local time of descending node is about 10:30 a.m. They are arranged in a 180\({}^{\circ}\) phase to obtain a higher time resolution [3]. Each satellite carries a 16 m Charge-Coupled Device (CCD) camera, a Hyperspectral Imager (HSI), an Infrared Spectroradiometer (IRS), and the PSAC. The PSAC provides the other remote sensors with the atmospheric parameters required for atmospheric correction to further improve the application efficiency of remote sensors and the level of remote sensing quantification. The main structure of the PSAC is shown in Figure 1 [3]. The measurement principle of the PSAC is the same as that of the American Aerosol Polarimetry Sensor/Glory Mission (APS/Glory) [20]. It adopts a simultaneous polarization measurement scheme based on a combination of split-aperture and split-amplitude components and acquires 9 bands and 4 polarization analyzers' relative azimuths (for a total of 36 channels) to obtain spectral intensity information simultaneously in the field-of-view range of \(-32.5^{\circ}\)-\(+32.5^{\circ}\) for cross-track scanning. The schematic diagram of the optical path of the PSAC is shown in Figure 2. The PSAC adopts two dual detectors per band to measure \(I_{0}\), \(I_{45}\), \(I_{90}\), and \(I_{135}\). These four linearly polarized units with different polarizations analyze relative azimuths. Additionally, the main specifications of the PSAC are shown in Table 1.  
In principle, the PSAC can determine the first three Stokes parameters in the Equation (1) of the target linear polarization state [2]:  
\[\mathbf{S}= \begin{bmatrix}I\\ Q\\ U\\ V\end{bmatrix} = \begin{bmatrix}I_{0}+I_{90}\\ I_{0}-I_{90}\\ I_{45}+I_{135}\\ V\end{bmatrix} = \begin{bmatrix}I_{45}+I_{135}\\ I_{0}-I_{90}\\ I_{45}+I_{135}\\ V\end{bmatrix}  \tag{1}\]  
where \(\mathbf{S}\) represents the Stokes vector; \([I,Q,U,V]\) are the four Stokes parameters of a plane wave; \(I\) is the light intensity information, which is the sum of the responses of two channels with polarization azimuths that differ by \(90^{\circ}\); \(Q\) is the linear polarization in the horizontal direction; \(U\) is the linearly polarized component at an angle of \(45^{\circ}\) from the horizontal direction; and \(V\) is the circularly polarized component, which is much smaller than the other three components in natural scenes and is often ignored [21]. Figure 1: Main structure of the PSAC [3]. As detailed in Table 1, the PSAC is equipped with 2250 nm polarization channels for the observation and acquisition of intensity and polarization information of the underlying surface for better land-atmosphere decoupling of other channels [6]. Previous studies on PSAC have shown that the temperatures of the heat sink and the photosensitive surface of the detectors in 2250 nm channels need to be kept below \(-20\)\({}^{\circ}\)C and \(-50\)\({}^{\circ}\)C, respectively, to better suppress noise and background radiation [7; 8]. A schematic diagram of the SWIR detector assembly structure is shown in Figure 3. Two SWIR detectors in the same band are mounted back to back on the heat sink, and the cold shield is installed in front of the detectors. The only SWIR detector assembly consisting of these components is connected to the radiator plate through the heat pipe. The SWIR detector components adopt an active-passive temperature control scheme--that is, an active CCC scheme based on Thermo-Electric Coolers (TECs) built into each detector and a passive cooling scheme based on a radiator plate connected to the external heat sink of the SWIR detectors, which can reduce the PST fluctuation with high control accuracy, low power consumption, and low computing resource requirements.