Gain Nonlinearity Calibration of Submillimeter Radiometer for JEM/SMILES

Correcting the gain nonlinearity of an atmospheric sounder is important because nonlinearity causes a systematic error in the measured atmospheric emission. In spectroradiometers consisting of a nonlinear broadband receiver followed by a nonlinear spectrometer, the distortion of the received atmospheric spectrum due to nonlinearity should be corrected using the linearity information both on the broadband receiver and on the spectrometer. It is necessary to measure the input-output relation of two parts, the broadband and narrow-band sections, of the spectroradiometer. We propose a nonlinearity measurement method in which the input-output relations of two sections can be measured using broadband noises as input. The method is advantageous for satellite-borne sounders, which do not always measure each section separately. We successfully applied the method to the radiometer for the Japanese Experiment Module/Superconducting Submillimeter-Wave Limb-Emission Sounder.