![]() ![]() ![]() Furthermore, because FY-3D is on a sun-synchronous quasi-polar Earth orbit (836 km) with a local time of 14:00 on ascending node, the biases consistently appeared when the satellite flew towards the end of a descending orbit with a solar zenith angle (SZA) between 100° and 120°. However, in the winter of 2019, it was found that several channels in the MWIR band and most channels in the SWIR band on FOV-3 presented significantly large biases in comparison with the other three FOVs. Since the launch of the FY-3D satellite on 15 November 2017, the HIRAS has operated properly on-orbit for more than three years. Both calibration data sets together with the ES interferogram measurements are downloaded to the ground, and then the radiometric calibration for atmospheric radiance determination is performed using the processing algorithm which is developed based on the interferometer complex calibration model. ![]() The ICT and DS views are served as warm calibration point and cold calibration point, respectively. One typical cross-track scan with a 10-s repeat interval includes 29 Earth scene (ES), 2 DS, plus 2 ICT views. HIRAS is calibrated by periodically viewing an onboard blackbody or the so-called internal calibration target (ICT) combined with a cold deep-space view (DS). The four FOVs of a FOR will be labeled as FOV-1, FOV-2, FOV-3, and FOV-4 in the following. The combined 2 × 2 FOVs define the field-of-regard (FOR) of about 52.4 km in diameter at nadir. The ground footprint of each detector’s field-of-view (FOV) is 16 km in diameter at nadir. There are four circular detectors for each band arranged as a 2 × 2 array. ![]() The full spectral resolution (FSR) for all the three bands is 0.625 cm −1 corresponding to the double-sided interferograms with maximum optical path differences (MPD) of ☐.8 cm. It measures the atmospheric upwelling infrared radiance over three spectral ranges: the long-wave infrared (LWIR) band from 650 to 1135 cm −1, the mid-wave infrared (MWIR) band from 1210 to 1750 cm −1, and the short-wave infrared (SWIR) band from 2155 to 2550 cm −1. The Hyperspectral Infrared Atmospheric Sounder (HIRAS) onboard the FY-3D satellite is a scanning Michelson interferometer with a full cross-track scan angle of ±50.4° for the Earth view. The results show that the large biases of the radiance brightness temperature (BT) caused by the contamination are ameliorated greatly to the levels observed in the normal conditions. The effect of the correction is assessed by comparing the recalibrated HIRAS radiances with those measured by the Cross-track Infrared Sounder onboard the Suomi National Polar-orbiting Partnership Satellite (SNPP/CrIS) via the extended simultaneous nadir overpasses (SNOx) technique and by checking the consistency among the radiance data from different HIRAS detectors. The historic ES observations are recalibrated after the contaminated DS spectra correction. As for the historic contaminated data, a correction method is applied to detect the anomalous data by checking the continuity of the DS spectra and then replace them with the proximate normal ones. The solar light intrusion issue was addressed on 13 December 2019 when the DS view angle of the scene selection mirror (SSM) was adjusted from −77.4° to −87°. While in the winter and spring seasons, the HIRAS detector-3 DS view is susceptible to solar stray light intrusion when the satellite flies towards the tail of every descending orbit, and as a result, the measured DS spectra are contaminated by the stray light pseudo spectra, especially in the short-wave infrared (SWIR) band. The DS spectra stability in the moving average window is crucial to the calibration accuracy of ES radiances. The deep-space (DS) view spectra are used as a cold reference to calibrate the Hyperspectral Infrared Atmospheric Sounder (HIRAS) Earth scene (ES) observations. ![]()
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