In-flight absolute radiometric calibration of MODIS using the irradiance-based method

In order to reduce the calibration uncertainty of the reflectance-based method brought by the assumption of the aerosol model, the irradiance-based method, known as improved reflectance-based method, was proposed. The irradiance-based method is described in this paper. The radiometric calibration field campaign was performed at Dunhuang test site on 27 August, 2014. A hyperspectral irradiance meter (HSIM) developed by Anhui Institute of Optics and Fine Mechanics (AIOFM) was used to measure the diffuse-to-global spectral irradiance ratio. The irradiance-based method and the reflectance-based method were performed to calibrate the first four bands of Moderate Resolution Imaging Spectroradiometer (MODIS). The results of two methods were compared with result of MODIS on-board calibrator. The comparison shows that the result of irradiance-based method has a good consistency with on-board calibration and reflectance-based method results. The difference of calibration coefficients between irradiance-based and on-board method was less than 1.4%. Due to the limitations of the irradiance-based method, a clear sky and stable atmospheric condition is required for the entire half of the calibration day to provide the data necessary for the extrapolation of diffuse-to-global ratio in viewing direction. A study on the effects of aerosol mode assumption on the final apparent reflectance was performed on both the irradiance-based method and the reflectance-based method by selecting different aerosol modes to predict the apparent reflectance. The results show that aerosol mode assumption has a great effect on the reflectance-based method, however slight effect on the irradiance-based method.