Abstract
During the early stage of the G satellite of the Fengyun-2 series (FY-2G), severe cold biases up to ~2.3 K occur in its measurements in the 12.0 μm (IR2) band, which demonstrate time- and scene-dependent characteristics. Similar cold biases in water vapor and carbon dioxide absorption bands of other satellites are considered to be caused by either ice contamination (physical method) or spectral response function (SRF) shift (empirical method). Simulations indicate that this cold bias of FY-2G indeed suffers from equivalent SRF shift as a whole towards the longer wavelength direction. To overcome it, a novel approach combining both physical and empirical methods is proposed. With the possible ice thicknesses tested before launch, the ice contamination effect is alleviated, while the shape of the SRF can be modified in a physical way. The remaining unknown factors for cold bias are removed by shifting the convolved SRF with an ice transmittance spectrum. Two parameters, i.e., the ice thickness (5 μm) and the shifted value (+0.15 μm), are estimated by inter-calibration with reference instruments, and the modification coefficient is also calculated (0.9885) for the onboard blackbody calibration. Meanwhile, the updated SRF was released online on 23 March 2016. For the period between July 2015 and December 2016, the monthly biases of the FY-2G IR2 band remain oscillating around zero, the majorities (~89%) of which are within ±1.0 K, while its mean monthly absolute bias is around 0.6 K. Nevertheless, the cold bias phenomenon of the IR2 band no longer exists. The combination method can be referred by other corrections for cold biases.
Highlights
Radiometric accuracy is critical for space-to-Earth observation aimed at certain quantitative applications, i.e., weather, climate, and environmental monitoring and forecasting
According to the principle given by Equation (1), the (SRFITS ) with a 5 μm ice layer is confirmed to be the best choice for the physical method
The first one is called the physical method, which deals with the spectral response function (SRF) variation caused by the thin ice-layer condensed on the surfaces of cooled optics and is applied in the SRF correction of the Meteosat Second Generation (MSG)-1/SEVIRI 13.3 μm band
Summary
Radiometric accuracy is critical for space-to-Earth observation aimed at certain quantitative applications, i.e., weather, climate, and environmental monitoring and forecasting. For a usual reflective solar or thermal emissive band, the spectral response function (SRF), which restrains the incident radiation within the specified spectrum, is one of the most important radiometric features influencing the radiometric accuracy of space-borne sensors. 2017, 9, 553 other ones so as to meet the requirements with high confidence, e.g., climate change detection and calibration accuracy validation [1]. Due to their unprecedented hyper-spectral characteristics, it is possible for these reference sensors (i.e., IASI) to qualify SRF differences and related uncertainties to alleviate other sensor (i.e., High-resolution Infrared Radiation Sounders, HIRS) inter-satellite biases [2]
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