# Radiometric Calibration of the Near-Infrared Bands of GF-5-02/DPC for Water Vapor Retrieval
## 4 Results  
### Radiometric Calibration Results of the DPC at the NIR08 Band  
The errors in the observations of the 865 nm band of the DPC include both random error and systematic error. It is important to note that random error cannot be corrected, while systematic error can be corrected. In this study, we aim to correct the systematic error in the observation data of the 865 nm band of the DPC by using the observations of the 865 nm band of the POSP, rather than the random error. We use \(k1\) to characterize the systematic bias in the observations of the 865 nm band of the DPC. Due to the very close SRFs of the DPC and POSP at the 865 nm band, and the minimal variation in the spectral reflectance of different ground objects near the wavelength of 865 nm, the surface reflectance of the DPC and POSP at the 865 nm band calculated using Equation (1) is highly correlated (R\({}^{2}\) = 1.00). In addition, the analysis results of the simulated TOA reflectance of the POSP and DPC at the 865 nm band under different aerosol contents, observational geometries, and surface types show that the simulated observation of these two bands is highly correlated (R\({}^{2}\) = 1.00), and the difference between them can be ignored. In other words, the TOA reflectance observed by the 865 nm band of the POSP and the DPC calibrated at the 865 nm band at the same time and location are equal. This indicates that it is feasible to directly use the observation data of the 865 nm band of the POSP to perform radiometric calibration on the 865 nm band of the DPC. The matching results between the observations of the POSP and DPC at the 865 nm band are illustrated in Figure 4a. The observations of these two bands are highly correlated (R\({}^{2}\) = 0.95). This implies that calibrating the 865 nm band of the DPC using the 865 nm band of the POSP is feasible. The analysis based on the actual observations indicates that the calibration coefficient for the 865 nm band of the DPC (i.e., \(k1\)) is 1.07. Due to the negligible impact of random errors in the observations of the 865 nm band of the POSP on the calculation result of \(k1\), we did not consider the influence of the uncertainty of the observations of the 865 nm band of the POSP during the cross-calibration process. Next, we used an experiment to show that the random observation error of the 865 nm band of the POSP only has a negligible effect on the calculation results of \(k1\). We added random errors of 1%, 3%, and 5% to the POSP data, shown in Figure 4a, respectively. The matching results between the POSP data with the added random errors and the DPC data are illustrated in Figure 4b-d. By analyzing Figure 4, it can be found that the calculation result of \(k1\) remains unchanged as the random error of the observations of the POSP increase. In this work, \(k1\) is used to characterize the systematic bias in the observations of the 865 nm band of the DPC, and is equal to the mean of the ratio between the actual observation of the DPC at the NIR08 band (i.e., \(R^{\prime}_{865 nm}\)) and the actual observation of the POSP at the NIR08 band (i.e., \(R_{865 nm}\)). The method used to calculate the uncertainty of \(k1\) is as follows. The distribution of the values of \(R^{\prime}_{865 nm}/R_{865 nm}\) is shown in Figure 5. The mean (i.e., expected value) and standard deviation of the normal distribution corresponding to \(R^{\prime}_{910 nm}/R_{910 nm}\) are 1.07 and 0.06, respectively. This indicates that the uncertainty of \(k1\)  
Figure 4: Matching results between the actual TOA reflectance data of the DPC at the NIR08 band and the TOA reflectance data of the POSP at the NIR08 band. (**a**) Actual TOA reflectance data of the POSP at the NIR08 band. (**b**) TOA reflectance data of the POSP at the NIR08 band after adding 1% random error. (**c**) TOA reflectance data of the POSP at the NIR08 band after adding 3% random error. (**d**) TOA reflectance data of the POSP at the NIR08 band after adding 5% random error. is 0.06.