Nonlinear characteristics of FY-3D microwave radiation imager and an optimal calculation method

Abstract

The Earth’s radiation was observed by a microwave radiation imager(MWRI) on board FengYun-3D(FY-3D) satellite at 10.65 GHz, 18.7 GHz, 23.8 GHz, 36.5 GHz, and 89.0 GHz with dual polarization. The nonlinearity of this payload, as an important parameter in calibration algorithms, is represented by the nonlinear parameter u from vacuum calibration ground tests. Therefore, an accurate knowledge on the nonlinear characteristics of MWRI is required to achieve precise remote sensing.The nonlinear parameter applied in the calibration algorithm is usually averaged over a set of u, which is calculated at corresponding observed brightness temperatures in the range between 95 K and 298 K. A t-distribution test method is proposed to screen u and further optimize the nonlinearity calculations in this study instead of conventional empirically filtering of u before averaging. The t-method examines the validity of u values at each observed brightness temperatures, and its effectivity is proven in this study.Nonlinear fitting brightness temperatures, as well as means and standard deviation of residual error, were calculated using empirically filtering method and the t-method to demonstrate the results in the nonlinearity calculation. The nonlinear parameter of MWRI is a physical parameter of the instrument, and the nonlinearity of up to 2 K at 10 GHz is mainly introduced by higher-order harmonic during detection.The nonlinear brightness temperature calculated using the t-test distribution method is improved by approximately 0.04 K with better fitting results than the ground tests, especially at 10 GHz.According to the results presented in this study, the nonlinear parameter is independent of observed brightness temperatures, and its nonlinearity is correlated to the instrument’s working condition. The proposed algorithm using the t-test distribution method can improve the nonlinearity fitting results and calibration precision. This method provides accurate nonlinear parameter for on-orbit calibration and can play a role in the total life cycle of MWRI after being launched.