The Role of Radiometric Calibration for the Vegetation Indices of CBERS-02 WFI

摘要

Wide Field Imager (WFI), one payload of the China Earth Resources Satellite (CBERS), has two channels, red and near infrared ones. In our article, the WFI was cross-calibrated with the MODIS on TERRA. The results showed that the offsets of these two bands were very big: 45 and 30 digital counts respectively. It was the first time in our China that we could know something about the radiometric coefficients of WFI and could make some quantitative applications. We calculated the NDVI from different WFI images with and without calibration coefficients and compared them. It was difficult to draw a single correlative line among their scattering plot because a turning point existed there, where when the NDVI from digital count was equal to -0.2. However, after subtracting the offsets, the NDVI calculated from the digital numbers were correlated well to those calculated from the TOA reflectance. The correlation wasn't linear, but nonlinear. The degree of fitting using the quadratic polynomial function can reach 99% In our article, the NDVI were also derived from the corresponding images of the MODIS, which were used to compare with the NDVT of WFI images with and without radiometric calibration. After applying the radiometric coefficients and offsets, the values of the NDVI from WFI unage were very close to those derived from MODIS images. The histograms of the NDVI derived from the TOA reflectance of the MODIS and WFI images were similar. The comparison demonstrated that the WFI images with radiometric calibration can be applied to the quantitative application of vegetations.