Prototype Geostationary Interferometric Infrared Sounder

Prototype Geostationary Interferometric Infrared Sounder is a geostationary nadir infrared sounder flown on FY-4A for atmospheric temperature and humidity sounding and wind-profile derivation from water-vapour features ^[1][2]. It is not an imager: its core measurement is hyperspectral MWIR/TIR interferometric sounding, supported by a visible cloud-detection channel ^[1][3]. The FY-4A configuration spans LWIR 700-1130 cm^-1, MWIR 1650-2250 cm^-1, and a 0.55-0.90 um visible channel; in wavelength terms, that is 8.85-14.2 um for LWIR, 4.44-6.06 um for MWIR, and 0.55-0.90 um visible context ^[1]. Ground sampling is 16 km at sub-satellite point for the infrared sounding channels and 1 km for the visible cloud channel ^[1]. For FY-4A, spectral resolution is 0.625 cm^-1; broader FY-4 generation values include 0.8 cm^-1 and 1.6 cm^-1, so those values are generation-specific when comparing FY-4A and later GIIRS products ^[1][3]. The instrument is most relevant where geostationary atmospheric profiles, moisture fields, and water-vapour wind support matter more than fine spatial imaging ^[1].