# Radiometric Calibration of the Near-Infrared Bands of GF-5-02/DPC for Water Vapor Retrieval
## 3 Methods
### Radiometric Calibration of the DPC at the NIR09 Band  
In this study, the TOA reflectance of the calibrated 865 nm band of the DPC is used to perform radiometric calibration of the 910 nm band of the DPC. When water vapor absorption is not considered, the TOA reflectance observed by the calibrated 865 nm and 910 nm bands of the DPC at the same time and location are equal. Therefore, the conversion relationship between the TOA reflectance of the calibrated 865 and 910 nm bands of the DPC can be expressed using Equation (3). The \(T\) in Equation (3) represents the WVAT of the  
Figure 3: The spectral reflectance data of typical ground objects. Each curve in subfigure (**a**) represents the spectral reflectance of bare soil at different wavelengths; each curve in subfigure (**b**) represents the spectral reflectance of vegetation at different wavelengths; and each curve in subfigure (**c**) represents the spectral reflectance of a rock at different wavelengths.  
910 nm band of the DPC. This is equal to the value of the TOA reflectance of the calibrated 910 nm band of the DPC divided by the TOA reflectance of the calibrated 865 nm band of the DPC, and is negatively correlated with the PWV content. This means that before calibrating the 910 nm band of the DPC, it is necessary to calculate the WVAT of the band at 910 nm for all pixels involved in the calibration.  
It is important to note that the WVAT of the band at 910 nm corresponding to any pixel can be calculated by inputting the satellite zenith angle, solar zenith angle, and water vapor content into an RTM. Since water vapor content is required for calculating WVAT, only the matching results between DPC data and ground-based PWV data are used for the radiometric calibration of the 910 nm band of the DPC, rather than all DPC data. As mentioned above, 70% of the matching results between the DPC data and AERONET PWV data are used for the calibration of the band at 910 nm. For any pixel participating in the calibration of the 910 nm band of the DPC, the WVAT of the pixel at the 910 nm band can be calculated by using the RTM according to the observation geometry of DPC and AERONET PWV data. As the observations without systematic error for the 910 nm band of the DPC can be calculated using Equation (3), the radiometric calibration coefficient for the 910 nm band of the DPC (referred to as \(k2\)) can be computed using Equation (4).  
\[R_{910\ nm,\ i}=R_{865\ nm,\ i}\times T_{i}, \tag{3}\]  
\[k2=\frac{\sum_{i=1}^{N}\left(R^{\prime}_{910\ nm,\ i}/R_{910\ nm,\ i}\right)}{N }, \tag{4}\]  
where \(N\) denotes the count of valid matching results between the observations of DPC and AERONET; \(R_{865\ nm,\ i}\) is the i-th observation of the calibrated 865 nm band of the DPC; \(T_{i}\) is the WVAT of the 910 nm band of the DPC, corresponding to \(R_{865\ nm,\ i}\); \(R_{910 nm,i}\) represents the observation of the calibrated 910 nm band of the DPC, corresponding to \(R_{865\ nm,\ i}\); and \(R^{\prime}_{910 nm,i}\) is the actual observation of the 910 nm band of the DPC, corresponding to \(R_{910 nm,i}\).