Internal‐blackbody calibration (IBBC) approach and its operational application in FY‐2 meteorological satellites

Fengyun-2 (FY-2) are the first-generation geostationary meteorological satellites (GEOMS) in China, two of which, FY-2E and FY-2G, are the operating workforce to support the dual-satellite observation mode. The on-board blackbody preliminarily used for stability monitoring of the instrument's radiometric response was equipped behind its fore-optics due to the technical limitations in the 1990s with what was referred to as an internal-blackbody (IBB) and operated at a unique pre-set temperature. In this article, a novel IBB calibration (IBBC) approach is proposed and the radiometric contributions of the main optical elements as well as the IBB itself are modelled and extracted. This is the most distinguishing difference from the traditional full-path blackbody calibration. The IBBC software has been in service for more than 2 years. Compared with the high spectral resolution reference instruments recommended by the Global Space-based Inter-Calibration System, the monthly mean calibration biases of the long-wave infrared band (IR1: 10.8 mu m) and water vapour band (IR3: 6.95 mu m) are shown to be lower than 1.5 K for FY-2E and lower than 1.0 K for FY-2G (for approximate to 90% of cases). In particular, the annual mean biases for the FY-2G IR3 band can maintain a good state of 0.4 K, which is nearly identical to that of other high-class in-orbit satellites, e.g. MTSAT-2 and MSG-1. Likely impacted by ice contamination, the calibration biases of the long-wave split-window infrared band (IR2: 12 mu m) of FY-2G could be as large as -2.3 K and similar phenomena once occurred in GOES-13 and MSG-1. This is expected to improve in the near future. In general, the calibration accuracies of FY-2G IR1 and IR3 bands have become lower than 1 K, which is significantly superior to those of other first-generation GEOMS, such as GMS-5, Meteosat-7 and GOES 4-7, whose calibration accuracies are around 2-3 K. The quality of the data could be greatly enhanced so as to benefit the global observation system.