Abstract
Based on simulated reflectance, deep convective clouds (DCC) can be used as an invariant target to monitor the radiometric response degradation of the FY-3A/MERSI (Medium Resolution Spectral Imager) reflective solar bands (RSBs). The long-term response of the MERSI RSBs can easily be predicted using a quadratic fit of the monthly DCC mean reflectance, except for bands 6 and 7, which suffer from instrument anomalies. DCC-based degradations show that the blue bands (λ < 500 nm) and water-vapor bands have degraded significantly, whereas for near-infrared bands, the total degradations in four years are within 3% (excluding bands 3 and 20). For most bands, the degradation rates are greatest during the first year in orbit and decrease over time. The FY-3A/MERSI degradation results derived from DCC are consistent within 2.5%, except for bands, 11, 18 and 19, when compared with Aqua/MODIS(Moderate Resolution Imaging Sepetroradiometer) inter-calibration, multi-site invariant earth target calibration and the CRCS(Chinese Radiometric Calibration Site) Dunhuang desert vicarious calibration methods. Overall, the 2σ/mean degradation uncertainty for most MERSI bands was within 3%, validating the temporal stability of the DCC monthly mean reflectances. The DCC method has reduced the degradation uncertainties for MERSI water vapor bands over other methods. This is a significant advantage of the DCC calibration method. The saturation of some MERSI bands may hinder the effectiveness of the DCC calibration approach.
Highlights
Considering that some space-borne instruments do not carry onboard calibration devices for reflective solar bands (RSBs) or even if they do, these devices, when exposed to the space environment and high energy solar radiation, usually may be subject to deterioration
The results show that the total degradations of FY-3A/Medium Resolution Spectral Imager (MERSI) by three kinds of monthly deep convective clouds (DCC)
The radiative transfer simulation results show that the DCC reflectances are stable for cloud optical depths greater than 200 and independent of tropospheric aerosol optical depth
Summary
Considering that some space-borne instruments do not carry onboard calibration devices for reflective solar bands (RSBs) or even if they do, these devices, when exposed to the space environment and high energy solar radiation, usually may be subject to deterioration. It is required to update the calibration coefficient frequently by all kinds of means of in-flight vicarious calibration (VC). Based on whether the vicarious calibration is conducted on the ground test site or not, the VC calibration can be divided into site-based calibration and non-site-based calibration [1,2,3]. Non-site-based VC calibration without ground reflectance and atmospheric measurements performed simultaneously with the satellite overpass started in the 1990s. The calibration coefficient is often obtained by several vicarious calibrations after launch. These methods include stable desert target, snow ice, synchronous in-flight calibration and the inter-calibration using other sensors with high accuracy calibration [4,5,6,7,8,9]
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