## 2 Instrument, Model, and Method
### Instrument
However, this active CCC and passive radiative cooling scheme cannot hold the PST stable, and it will vary with the temperature of the external conditions. Actually, the temperature drift of the cry radiator in one orbit is expected to be generally less than \(\pm 3\)\({}^{\circ}\)C, and thus the PST will fluctuate synchronously in this small range. Moreover, the temperature difference between the photosensitive surface \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Swath & km & \(\geq\)800 \\ Instantaneous Field of View (IFOV) & \({}^{\circ}\) & 0.52 \\ Resolution at nadir & km & \(\leq\)60645 \\ Wavelength bands & nm & 410, 443, 555, 670, 865,910, 1380, 1610, 2250 \\ Polarization analyzers’ relative azimuths & \({}^{\circ}\) & 0, 45, 90, 135 \\ Radiometric accuracy & \% & \(\leq\)5 \\ Polarimetric accuracy & / & \(\leq\)0.005 (p \(<\) 0.3) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main specifications of the PSAC. Figure 2: Schematic diagram of the optical path of the PSAC. 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 \(-32.5^{\circ}\)\(\sim+32.5^{\circ}\) for cross-track scanning. of the detector and the heat sink will be very limited; it can be achieved only by the active cooling of the TECs inside the detector. Therefore, it will be difficult to obtain the low PST required for the on-orbit (PST \(\approx\)\(-\)65 \({}^{\circ}\)C) operation under higher ambient temperatures, such as PSAC laboratory-based radiometric test conditions. The temperature comparison of detector assembly during two operation conditions is presented in Table 2. There is a PST difference of approximately 35 \({}^{\circ}\)C when PSAC is in on-orbit operation (PST \(\approx\)\(-\)65 \({}^{\circ}\)C) compared with its laboratory-based radiometric test (PST \(\approx\)\(-\)30 \({}^{\circ}\)C). Generally, the photoelectric characteristics of the detector are always affected by the PST. The spectral photosensitivity of the detector of 2250 nm channels at two temperatures provided by the manufacturer is shown in Figure 4. The PST difference between the on-orbit operation (PST \(\approx\)\(-\)65 \({}^{\circ}\)C) and laboratory-based radiometric test (PST \(\approx\)\(-\)30 \({}^{\circ}\)C) is expected to reduce its responsivity by more than 1%. As seen in Table 1, the PSAC radiometric accuracy specification is better than 5%. In addition, as the manufacturer of the PSAC, referring to MODIS which is the typical remote sensor for land-atmosphere radiation detection [22], we expect the radiation accuracy of the PSAC to reach 3.5%. To ensure the detection accuracy of atmospheric parameters, it is necessary to perform PSAC RTDC. \begin{table}
\begin{tabular}{c c c} \hline  & **Laboratory-Based** & **On-Orbit Operation** \\  & **Radiometric Test** & **PST** \\ \hline Main structure temperature & \(\approx\)20 \({}^{\circ}\)C & \(\approx\)20 \({}^{\circ}\)C \\ Heat sink temperature & \(\approx\)20 \({}^{\circ}\)C & \(\approx\)\(-\)15 \({}^{\circ}\)C \\ PST & \(\approx\)\(-\)30 \({}^{\circ}\)C & \(\approx\)\(-\)65 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table} 
Table 2: PSAC SWIR detector assembly temperature comparison under two operating environments. Figure 3: The SWIR detector assembly. Two SWIR detectors in the same band are mounted back to back on a heat sink. The SWIR detectors and cold shield are mounted on the heat sink and connected to the cry radiator through the heat pipe.