Calibration of superconducting submillimeter-wave limb-emission sounder (SMILES) on the ISS

Abstract

The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) is a space-station-borne limb sounder for the stratospheric and mesospheric observations using frequency bands around 625 and 650 GHz. SMILES was developed cooperatively by National Institute of Information and Communications Technology (NICT) and Japan Aerospace Exploration Agency (JAXA). SMILES operated from October 2009 to April 2010 on the International Space Station (ISS). The calibration process of the observed submillimeter spectra is continuously improved also in the cooperation of NICT and JAXA. This paper gives an overview of the SMILES calibration, including intensity and spectral calibrations and the field-of-view calibration. The largest error source in the calibration of the spectrum is the uncertainties in the linearity correction of the receiver gain and the spectral response function of spectrometer channels. The efforts of our calibration improvement are focused on these calibrations. The linearity correction is based on the results of the gain nonlinearity measurement in prelaunch tests. The correction is modified so as to be consistent with in-orbit measurements. The spectral response functions of the spectrometers are estimated also from the in-orbit experiments. The tangent-height precision was another calibration issue that needed improvement in the preliminary version of a product of calibrated spectra. The improvement of the tangent-height precision will contribute the accuracy improvement in the volume-mixing-ratio product through a reduction in error by misplacement of the tangent height for each limb measurement.