# Radiometric Calibration of the Near-Infrared Bands of GF-5-02/DPC for Water Vapor Retrieval
The results, calculated based on error propagation theory, showed that the errors of the calibration coefficients calculated using this method ranged from 1.24% to 4.76% [6]. A similar method was also employed by [PERSON] et al. to recalibrate long time series data derived from four bands of the DPC [7]. [PERSON] et al. used two calibration methods, the sun glint inter-band calibration method and the Rayleigh calibration method, to calibrate the intensity bands of DPC [8]. In the study, the Rayleigh calibration method was initially utilized for calibrating the bands at 443, 490, 565, and 670 nm in clear oceanic regions worldwide. Subsequently, the recalibrated band at 565 nm was used as a reference for calibrating the other four DPC bands (443, 490, 670, and 865 nm) in the sun glint areas of the ocean. The assessment results showed that the errors in the calibration coefficients calculated using the Rayleigh calibration method ranged from 1.3% to 4%, and the calibration error would increase with the increase in wavelength. In addition to the above methods, a cross-calibration method between different satellites has been employed for the calibration of the five non-absorption bands (443,490, 565, 670, and 865 nm) of the DPC by [PERSON] et al. [9]. They conducted the calibration of these five DPC bands at a desert calibration site in North Africa by using MODIS data as a reference. The uncertainty in the calibration coefficients obtained through this method ranged from 2.4% to 3.7%. Although researchers have developed multiple in-orbit calibration methods for the intensity bands of the DPC, there remains a lack of calibration algorithms for the band at 910 nm of the DPC. Because of the significant impact of water vapor on the observation of this band and the obvious difference in the spatiotemporal distribution of water vapor, calibrating the 910 nm band of the DPC is more challenging compared to the non-absorption bands of the DPC. Although it is feasible to use ocean or desert calibration fields to calibrate the DPC, the calibration process requires high-quality water vapor distribution data. Due to the difficulty of obtaining high-quality water vapor distribution data in unmanned and remote desert or ocean calibration sites, it is difficult to ensure the calibration accuracy of the water vapor absorption (WVA) band of the DPC. For instance, [PERSON] et al. conducted radiometric calibration of the Medium-Resolution Imager (MERSI) onboard the FengYun-3 satellite using multiple calibration sites, including desert and ocean sites [10]. Owing to constraints in the quality of PWV data utilized for calibration input, the assessment resultsrevealed that the error of the calibration coefficient for the WVA band of the MERSI exceeds that of the other bands. This is also the reason why researchers have not yet performed radiometric calibration of the 910 nm band of the DPC. Since the POSP is equipped with an onboard radiometric calibrator, its observations can be used for the radiometric calibration of the observations of other sensors with similar channels (380, 410, 443, 490, 670, 865, 1380, 1610, and 2250 nm). In this study, the observation data of the 865 nm band of the POSP are used for the radiometric calibration of the 865 nm band of the DPC. It has been proven that the TOA reflectance for the bands at 865 and 910 nm are strongly correlated without considering the effects of the WVA [11]. In this work, the relationship between them is used to calibrate the 910 nm band of the DPC. The 865 nm band of the POSP is firstly used for the calibration of the 865 nm band of the DPC. After that, the observation data of the 910 nm band of the DPC without systematic radiometric calibration errors can be simulated using the observation data of the 865 nm band of the DPC and the WVAT calculated based on ground-based PWV data. It is worth noting that several ground-based networks can provide PWV data with a high accuracy, such as the Aerosol Robotic Network (AERONET) and SuomiNET [12].