Geostationary Environment Monitoring Spectrometer

GEMS instru­ment is basically a step-and-stare scan­ning UV-visible imaging spectrometer, with scanning Schmidt telescope and Offner spectrometer. A UV-enhanced 2D CCD takes images, with one axis spec­tral and the other north-south spatial, with east-west scanning over time. On-orbit calibrations are planned, making daily solar measurements and weekly LED light source linearity checks. For the solar calibration, there are two transmissive diffusers, a daily working one and a reference diffuser used twice a year to check the degradation of the working one. Dark current measure­ments are planned twice a day, before and after the daytime imaging. To avoid dark current issues and random telegraph signal (RTS), the CCD is cooled to temperatures well below 0° C. Spectral stability is required to be better than 0.02 nm over daily observation, stray light less than 2 percent after correction, polarization sensitivity less than 2 percent at the instrument level, and the instrument sys­tem level MTF better than 0.3. Preflight calibration have been carried out including BTDF of diffuser, bandpass. Straylight, polarization, dark current, smear signal and so on, to be reflected to its L0-1b algorithm.

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  (Image courtesy : KARI/BATC)

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  (Image courtesy : KARI/BATC)

The GEMS system employs a 1 K × 2 K pixel CCD detector that operates from 300 - 500 nm, which at a minimum, enables NO2, SO2, HCHO, O3, and aerosol retrieval. The telescope projects the slit field of view onto the Earth, and the full field of regard is achieved via a 2-axis onboard scan mirror. Images from each scan mirror position are co-added on-board and co-added images are then transmitted to ground. Ground processing spatially bins and geo-locates each co-added image.