## 2 HJ-2 A/B Satellites and Sensors  
China's HuanjingJianzai-2 (HJ-2) series satellites are sun-synchronous orbit satellites with an orbital altitude of 645 km. They are designed to replace the HJ-1A/B satellites, which have exceeded their operational lifespan, and are primarily used for disaster prevention and mitigation as well as environmental protection. The HJ-2A/B satellites are the first two in the HJ-2 series, equipped with the same four payloads, and have a descending node local time of approximately 10:30 a.m. The HJ-2A and HJ-2B satellites are arranged in a 180\({}^{\circ}\) phase configuration to achieve higher temporal resolution imagery. Their MSC and IRS can cover the entire Chinese region within two days, providing efficient data support for applications such as natural disaster monitoring, land use macro-monitoring, water resource management and protection, crop monitoring and yield estimation, and earthquake emergency response and disaster relief [19; 20]. To support these diverse applications, the Chinese HuanjingJianzai-2 (HJ-2) A/B satellites are equipped with a Multispectral Camera (MSC), a hyperspectral imager (HSI), an infrared spectroradiometer (IRS), and a Polarized Scanning Atmospheric Corrector (PSAC). Table 1 provides an overview of the basic parameters of these sensors. For clarity in this paper, bands 4 (B4) and 5 (B5) of the MSC have been swapped to present the wavelengths in ascending order, facilitating easier analysis and comparison. The PSAC is designed with nine spectral bands covering the visible to short-wave infrared range, each with four polarization detection channels. It is also equipped with a comprehensive on-board radiometric and polarization calibration system, achieving a polarization detection accuracy of 0.005 and a radiometric calibration accuracy of 7%. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & **MSC** & **HSI** & **IRS** & **PSAC** \\ \hline \multirow{8}{*}{Wavelength bands} & B1: 0.45–0.52 \(\upmu\)m & & B1: 0.63–0.69 \(\upmu\)m & B1: 0.400–0.420 \(\upmu\)m \\  & B2: 0.52–0.59 \(\upmu\)m & & B2: 0.73–0.77 \(\upmu\)m & B2: 0.433–0.453 \(\upmu\)m \\  & B3: 0.63–0.69 \(\upmu\)m & & B3: 0.78–0.90 \(\upmu\)m & B3: 0.545–0.565 \(\upmu\)m \\  & B4: 0.69–0.73 \(\upmu\)m & 0.45–0.92 \(\upmu\)m (\(\Delta\lambda\) = 5 nm) & B4: 1.19–1.29 \(\upmu\)m & B4: 0.660–0.680 \(\upmu\)m \\  & B5: 0.77–0.89 \(\upmu\)m & 0.90–2.50 \(\upmu\)m (\(\Delta\lambda\) = 14 nm) & B5: 1.55–1.68 \(\upmu\)m & B5: 0.845–0.885 \(\upmu\)m \\  & & 0.90–2.50 \(\upmu\)m (\(\Delta\lambda\) = 14 nm) & B6: 2.08–2.35 \(\upmu\)m & B6: 0.900–0.920 \(\upmu\)m \\  & & & B7: 3.5–4.8 \(\upmu\)m & B7: 1.35–1.40 \(\upmu\)m \\  & & & B8: 10.5–11.4 \(\upmu\)m & B8: 1.58–1.64 \(\upmu\)m \\  & & & B9: 11.5–12.5 \(\upmu\)m & B9: 2.21–2.29 \(\upmu\)m \\ \hline \multirow{2}{*}{Nadir pixel resolution} & \multirow{2}{*}{16 m} & 48 m (0.45–0.92 \(\upmu\)m) & 48 m (B1–B5) & \multirow{2}{*}{6 km} \\  & & 96 m (0.90–2.50 \(\upmu\)m) & & \\ \hline \multirow{2}{*}{Swath width} & Swath width & 800 km & 96 km & 720 km & 800 km \\ \cline{2-2}  & \multirow{2}{*}{62.6\({}^{\circ}\)} & \multirow{2}{*}{8.52\({}^{\circ}\)} & \multirow{2}{*}{60\({}^{\circ}\)} & \multirow{2}{*}{65\({}^{\circ}\)} \\  & & & & \\ \hline \multirow{2}{*}{Calibration accuracy} & Absolute: \(\leq\)7\% & Absolute: \(\leq\)7\% & B1–B6: absolute & Radiance: \(\leq\)7\% \\  & Relative: \(\leq\)3\% & Relative: \(\leq\)3\% & \(\leq\)7\%; relative \(\leq\)3\% & DOLP: \(\leq\)0.005 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key instrument parameters of the four sensors on HJ-2A/B satellites. These capabilities are used to obtain synchronized results for cloud detection, atmospheric aerosol, and water vapor content [21; 22]. The MSC consists of four CCDs, arranged in a stitched configuration, with a total swath width of 800 km and a total field of view angle of 62.6\({}^{\circ}\). Each CCD has five bands, and compared to the HJ-1A/B satellites, an additional red-edge band has been added for monitoring the growth of green vegetation.