星上定标器漫透射板外场标定技术研究

Outfield Calibration Technique for Diffuse Transmission Plate of Onboard Calibrator

在轨辐射定标是极轨气象卫星光谱成像仪定标的重要组成部分,是实现高定量化遥感的重要技术手段。星上定标器以太阳作为长期稳定的参考光源,以定标器漫透射板作为基准传递媒介,漫透射板的标定精度直接决定了星上定标精度。根据风云三号E星轨道特点和光谱成像仪工作模式,星上定标器采用了漫透射板这一特殊形式。针对星上定标器的高精度性能测试技术开展研究,提出了一种基于标准光谱仪的外场漫透射板标定方法,以解决漫透射板双向透射分布函数(BTDF)所需的大口径、高亮度均匀测试光源以及高精度自动控制覆盖测试角度矩阵等标定难题。所提方法以外场太阳作为参考光源,标准光谱仪作溯源传递,秒级高精度自动跟踪转台作角度变换,获得漫透射板的BTDF。结果表明,该方法能获得准确的漫透射板BTDF,标定精度优于2.5%,满足星上定标器的在轨定标精度要求。仪器在轨测试阶段的星上定标系数与同类载荷交叉定标的偏差优于2%。

Objective On-orbit radiation calibration is part of spectral imager calibration of polar-orbiting meteorological satellites,and also an important technical means to achieve highly quantitative remote sensing data.Onboard calibrators take the sun as a long-term stable reference light source and the diffuse transmission plate of the calibrators as the reference transmission medium,the calibration accuracy of which directly determines the onboard calibration accuracy.According to the orbital characteristics of the Fengyun-3E satellite and the working mode of the spectral imager,the onboard calibrator adopts the special form of diffuse transmission plate.Meanwhile,we study the high-precision performance testing technology of the calibrator and propose an outfield calibration method for diffuse transmission plate based on a standard spectrometer.We solve the calibration problems such as large-aperture and high-brightness uniform test light source and high-precision automatic control overlay of test angle matrix in the bidirectional transmittance distribution function(BTDF)of diffuse transmission plates.Fengyun-3E satellite is in the morning and dusk polar orbit,and its spectral imager employs scanning mirror to observe cold space,earth,diffuse transmission plate,and black body in turn.As an onboard radiation reference in the solar reflection spectral band,the diffuse transmission plate can receive direct light from the sun to form a near-Lambertian light source.The imager scanning mirror observes the diffuse plate in the normal direction to obtain radiation input and complete the radiation calibration of the solar reflection band.The imager calibration in orbit is shown in Fig.1.Methods The whole calibration test project includes two aspects of diffuse plate BTDF,spectral dimension distribution and angular dimension distribution.1)Spectral dimension distribution test:under the perpendicular incidence of the sun(normal direction,zenith angle is 0),different radial brightness is obtained by the changing atmospheric quality at different times of the day for BTDF spectral distribution data of 0 incident zenith angle of the diffuse plate.When the sun shines on the diffuse plate,the standard spectrometer obtains a mathematical model of the diffuse plate outgoing radiation brightness,as shown in Equation 5.2)Angular dimension distribution test:during the period of slow changes in the sun zenith angle at noon,the incidence angleθof the solar rays changes by adjusting the pitch angle and horizontal angle of the scaler to obtain the relative distribution of the diffuse plate BTDF under different solar incidence angleθ,as shown in Equation 6.Results and Discussions The test data of Figs.6-8 show that the atmospheric transmittance on the test day is high and stable,with high linearity of the fitting curve.The spectral transmittance of the diffuse transmission plate in the range of 400-1000 nm is flat(except for atmospheric absorption peaks such as 760 nm and 940 nm),and the transmittance value can meet the design expectations and on-star calibration design.Figs.9-11 demonstrate that in the pitch direction and horizontal direction,the BTDF does not change significantly with the angle and is close to 1,indicating that the diffuse transmission plate has better Lambertian properties.According to the results of the current on-orbit test,the overall deviation between the calibration coefficient obtained by the onboard scaler and the cross-calibration coefficient of the imager from the international load is better than 2%,revealing that the on-orbit application of the scaler has reached the design expectations.Conclusions We propose an outfield radiation calibration method of diffuse transmission plate based on standard spectrometer.The test site is selected in the stable atmospheric transmittance area at high altitude and high latitude.The spectral distribution and angle distribution of the diffuse plate BTDF are obtained through the reasonable design of the test process and time period selection.The method adopts the outfield sun as the reference light source,the standard spectrometer for traceability transmission,and the 2D high-precision automatic tracking turntable for angle transformation to obtain the BTDF of the diffuse transmission plate.The results show that this method can obtain an accurate BTDF of the diffuse transmission plate,and the calibration accuracy is better than 2.5%,which can meet the on-orbit calibration accuracy requirements of the on-orbit calibrator.The consistency of the onboard calibration coefficient of the instrument in the orbit test stage with the cross-calibration of similar loads is better than 2%.