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.

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 angle θ of 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 angle θ, as shown in Equation 6.

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.

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%.