ASTER round-robin radiometers for the preflight cross-calibration of EOS AM-1 instruments

The combined use of data of different sensors on satellites for the Earth observation have been desired. For that purpose the radiometric and spectral calibration of the sensors in the preflight phase are very important. The EOS AM-1 to be launched in 1998 has five sensors on it, that is, the ASTER, MODIS, MISR, MOPITT and CERES. For the cross comparison of their working standard large integrating spheres (LIS), round-robin measurements are planned in the visible and near- and shortwave-infrared region. The round-robin (RR) radiometers are developed in the NRLM for the this purpose, especially among the ASTER, MODIS and MISR. Three RR radiometers have the center wavelengths similar to the ASTER VNIR bands 1 to 3. The other two have the center wavelengths of the ASTER SWIR bands 4 and 6. The round-robin method is as follows. At first the NRLM characterizes the spectral responsivities of the RR radiometers and the spectral radiances of a transfer integrating sphere (TIS), Next the RR radiometers see the TIS. Then the RR radiometers and TIS are transported to the calibration site of each instrument. Then the stability of the RR radiometers will be checked against the TIS. Finally the RR radiometers see the LIS. Using the relative spectral radiance of the LIS, the authors can calculate the spectral radiance of the TIS averaged at each band. The detectors of the RR radiometers are silicon photodiodes for the VNIR region and germanium and indium-arsenide photodiodes for the SWIR region. The RR radiometers for the VNIR region have two interference filters, one for defining the band shape and the other for suppressing the out-of-band response.<<ETX>>