Abstract
The Moderate-Resolution Wide-Wavelength Imager (MWI), onboard the Tiangong-2 (TG-2) Space Lab, is an experimental satellite sensor designed for the next-generation Chinese ocean color satellites. The MWI imagery is not sufficiently radiometrically calibrated, and therefore, the cross-calibration is urgently needed to provide high quality ocean color products for MWI observations. We proposed a simple and effective cross-calibration scheme for MWI data using well calibrated Moderate Resolution Imaging Spectroradiometer (MODIS) imagery over aquatic environments. The path radiance of the MWI was estimated using the quasi-synchronized MODIS images as well as the MODIS Rayleigh and aerosol look up tables (LUTs) from SeaWiFS Data Analysis System 7.4 (SeaDAS 7.4). The results showed that the coefficients of determination (R2) of the calibration coefficients were larger than 0.97, with sufficient matched areas to perform cross-calibration for MWI. Compared with the simulated Top of Atmosphere (TOA) radiance using synchronized MODIS images, all errors calculated with the calibration coefficients retrieved in this paper were less than 5.2%, and lower than the lab calibrated coefficients. The Rayleigh-corrected reflectance (ρrc), remote sensing reflectance (Rrs) and total suspended matter (TSM) products of MWI, MODIS and the Geostationary Ocean Color Imager (GOCI) images for Taihu Lake in China were compared. The distribution of ρrc of MWI, MODIS and GOCI agreed well, except for band 667 nm of MODIS, which might have been saturated in relatively turbid waters. Besides, the Rrs used to retrieve TSM among MWI, MODIS and GOCI was also consistent. The root mean square errors (RMSE), mean biases (MB) and mean ratios (MR) between MWI Rrs and MODIS Rrs (or GOCI Rrs) were less than 0.20 sr−1, 5.52% and within 1 ± 0.023, respectively. In addition, the derived TSM from MWI and GOCI also agreed with a R2 of 0.90, MB of 13.75%, MR of 0.97 and RMSE of 9.43 mg/L. Cross-calibration coefficients retrieved in this paper will contribute to quantitative applications of MWI. This method can be extended easily to other similar ocean color satellite missions.
Highlights
The newest-generation Chinese ocean color experimental sensor, the Moderate-Resolution Wide-Wavelengths Imager (MWI), onboard the Tiangong-2 (TG-2) space lab was launched on 15 September 2016
We evaluated the accuracy of the cross-calibration coefficients by comparing the Top of Atmosphere (TOA) radiance simulated from synchronized Moderate Resolution Imaging Spectroradiometer (MODIS) imagery with the TOA radiance from the calibration coefficients
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Summary
The newest-generation Chinese ocean color experimental sensor, the Moderate-Resolution Wide-Wavelengths Imager (MWI), onboard the Tiangong-2 (TG-2) space lab was launched on 15 September 2016. The MWI uses a push-broom imaging system similar to the Medium Resolution Imaging Spectrometer (MERIS) onboard the ENVISAT satellite and the Ocean and Land Color Instrument (OLCI) onboard the Sentinel-3A satellite. It has a 100 m spatial resolution and 14 bands in the visible and near-infrared spectrum, two shortwave infrared bands and two thermal infrared bands [2]. MWI has a general field of view (FOV) angle of 42◦ and a swath width of 300 km [3]
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