Preflight calibration of the Chinese Environmental Trace Gases Monitoring Instrument (EMI)

Abstract

An environmental trace gases monitoring instrument (EMI) is a nadir-viewing wide-field imaging spectrometer, which aims to quantify the global distribution of tropospheric and stratospheric trace gases, and is planned to be launched on 9 May 2018. The selected wavelength bands for EMI are ultraviolet channels: UV1 (240-315nm), UV2 (311-403nm) and visible channels: VIS1 (401-550nm), and VIS2 (545-710nm). The spectral resolution is 0.3-0.5nm, and the swath is approximately 114° wide to achieve a one-day global coverage. The preflight calibration of the EMI is discussed in this paper. A tunable laser and rotating platform are adopted for an EMI wavelength calibration of the entire field of view. The accuracy of the wavelength calibration is less than 0.05nm. In addition, the solar calibration mode shows the same results compared with Earth observation mode. A thermal vacuum test is performed to investigate the influence of in-orbit thermal vacuum conditions on the EMI, and EMI spectral response changes with pressure, optical bench temperature, and charge-coupled device (CCD) detector temperature are obtained. For a radiometric calibration of UV1, a diffuser plate with a 1000 W xenon lamp, which produces sufficient UV output, is selected. An integrating sphere system with tungsten halogen lamp is selected for the UV2, VIS1, and VIS2. The accuracies of radiance calibration are 4.53% (UV1), 4.52% (UV2), 4.31% (VIS1), and 4.30% (VIS2). The goniometry correction factor and irradiance response coefficient of the EMI are also calibrated on the ground for an in-orbit calibration of the solar. A signal-to-noise ratio (SNR) model of the EMI is introduced, and the EMI in-orbit SNR is estimated using the SNR and MODTRAN radiance models.