Abstract
Abstract. To examine the calibration performance of the Meteosat-8/9 Spinning Enhanced Visible Infra-Red Imager (SEVIRI) 0.640-μm and the Multi-functional Transport Satellite (MTSAT)-1R 0.724-μm channels, three calibration methods are employed. Total eight months during the 2004–2007 period are used for SEVIRI, and total seven months during the 2007–2008 period are used for MTSAT-1R. First, a ray-matching technique is used to compare Meteosat-8/9 and MTSAT-1R visible channel reflectances with the well-calibrated Moderate Resolution Imaging Spectroradiometer (MODIS) 0.646-μm channel reflectances. Spectral differences of the response function between the two channels of interest are taken into account for the comparison. Second, collocated MODIS cloud products are used as inputs to a radiative transfer model (RTM) to calculate Meteosat-8/9 and MTSAT-1R visible channel reflectances. In the simulation, cloud three-dimensional (3-D) radiative effect associated with subgrid variations is taken into account using the lognormal-independent column approximation (LN-ICA) to minimize the simulation bias caused by the plane-parallel homogeneous assumption. Third, an independent method uses the typical optical properties of deep convective clouds (DCCs) to simulate reflectances of selected DCC targets. Although all three methods are not in perfect agreement, the results suggest that calibration coefficients of Meteosat-8/9 0.640-μm channels are underestimated by 6–7%. On the other hand, the calibration accuracy of MTSAT-1R visible channel appears to be variable with the target reflectance itself because of an underestimate of calibration coefficient (up to 20%) and a non-zero space offset. The results further suggest that the solar channel calibration scheme combining the three methods in this paper can be used as a tool to monitor the calibration performance of visible sensors that are particularly not equipped with an onboard calibration system.
Highlights
Radiometric calibration converts the digital form of raw satellite data into physically meaningful radiances or reflectances
In this paper we examined the performance of operational calibration of Meteosat-8/9 Spinning Enhanced Visible Infra-Red Imager (SEVIRI) 0.640-μm and MTSAT1R 0.724-μm visible channels using three calibration methods
Moderate Resolution Imaging Spectroradiometer (MODIS) 0.646-μm channel is used as a reference, and reflectances are compared between MODIS and SEVIRI, and MODIS and Multi-functional Transport Satellite (MTSAT)-1R only over oceanic regions
Summary
Radiometric calibration converts the digital form of raw satellite data into physically meaningful radiances or reflectances. In the pre-launch stage of the satellite program, the calibration coefficient, which is the ratio for converting the digitized raw counts to radiometric quantities, can be estimated from laboratory experiments (Barnes et al, 1998; Bruegge et al, 1998; Johnson et al, 1999). This factor may change after launch in the space environment. The sensor degrades with time, implying that operational updates of the calibration coefficient are required for reliable satellite measurements. To compensate for the limitations of the onboard calibration system, vicarious methods are required to monitor sensor capability
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