This study analyzes the response changes of the Medium Resolution Spectral Imager II (MERSI-II), which is an optical instrument on the Fengyun-3D satellite, during different periods since liftoff. The study was based on the Dunhuang radiometric calibration site. The solar reflective channels, assessed using the reflectance-based method, showed a degradation of MERSI-II. The degradation rate of change was found to be higher in the beginning of the mission and to be decreasing with time. The latest calibration results show that the decrease in instrument response between 440 and 900 nm in the near-infrared band was relatively small compared to the previous official calibration coefficient, with an average attenuation rate of ∼5.3 % . It was observed that the MERSI-II shortwave channel had a large attenuation of 21%, especially in the 412 nm band. The calibration method and parameter processing of the reflectance-based method are introduced in detail. The accuracy of the calibration process was tested using Moderate Resolution Imaging Spectroradiometer, an internationally recognized high-precision satellite instrument, which determined the calibration error to be <4 % . The experimental results contribute to the improvement in the accuracy of satellite observation and the design of subsequent equipment.
Tetrolets are Haar-type wavelets whose supports are tetrominoes which are shapes made by connecting four equal-sized squares. A fusion method for polarization images based on tetrolet transform is proposed. Firstly, the magnitude of polarization image and angle of polarization image can be decomposed into low-frequency coefficients and high-frequency coefficients with multi-scales and multi-directions using tetrolet transform. For the low-frequency coefficients, the average fusion method is used. According to edge distribution differences in high frequency sub-band images, for the directional high-frequency coefficients are used to select the better coefficients by region spectrum entropy algorithm for fusion. At last the fused image can be obtained by utilizing inverse transform for fused tetrolet coefficients. Experimental results show that the proposed method can detect image features more effectively and the fused image has better subjective visual effect