Vicarious calibration is a well-developed method for electro-optical (EO) sensor calibration that has been used since the early 1980s. The radiometric calibration of reflectance solar band is mainly applied to reflection inversion. In this paper, a radiometric calibration-reflectance inversion (RCRII) model is proposed as an improved vicarious calibration method. Taking the reflectance of grayscale targets with constant spectrum, suitable uniformity, and near-Lambertian characteristics as the known information, the grayscale target calibration is realized, and the initial value of calibration coefficient and offset are calculated. Then, the adjacency effect is evaluated and corrected by reflectance inversion, and the results are fed back to the calibration process to realize the iterative process of calibration inversion rescaling. The results indicate that the absolute difference between the reflectance calculated with the RCRII model and measured reflectance is less than 0.01. By comparing with Sentinel-2A images, it is cross-verified that the difference of radiance between them is within 4%, and the absolute reflectance difference is less than 0.01, in the range of 0.1~0.3 reflectance.

We have demonstrated the fiber Bragg gratings (FBG) fabricated on different fibres by using the excimer laser (193 nm) and phase mask. The FBG is annealed at 400 °C for 70 days. The wavelength shift of FBG at 400 °C has been investigated. After the FBG is annealed for a long time, the wavelength shift of the FBG approaches stability. Then, the relationship between wavelength shift of FBG and annealing time has been obtained.

A partial aperture onboard calibration method can solve the onboard calibration problems of some large aperture remote sensors, which is of great significance for the development trend of increasingly large apertures in optical remote sensors. In this paper, the solar diffuser reflectance degradation monitor (SDRDM) in the onboard calibration assembly (CA) of the FengYun-4 (FY-4) advanced geostationary radiance imager (AGRI) was used as the reference radiometer. It was designed for measuring the partial aperture factor (PAF) for the AGRI onboard calibration. First, the linear response count variation relationship between the two was established under the same radiance source input. Then, according to the known bidirectional reflection distribution function (BRDF) of the solar diffuser (SD) in the CA, the relative reflectance ratio coefficient between the AGRI observation direction and the SDRDM observation direction was calculated. On this basis, the response count value of the AGRI and the SDRDM was used to realize the high-precision measurement of the PAF of the AGRI B1~B3 bands by simulating the AGRI onboard calibration measurement under the illumination of a solar simulator in the laboratory. According to the determination process of the relevant parameters of the PAF, the measurement uncertainty of the PAF was analyzed; this uncertainty was greater than 2.04% and provided an important reference for the evaluation of the onboard absolute radiometric calibration uncertainty after launch.

Factors such as the optical point spread function (PSF), intrapixel sensitivity (IPS), and interpixel crosstalk of an imaging system can cause the response to point sources to vary with position, directly affecting the absolute radiometric calibration accuracy of the remote sensor. This paper investigates an absolute radiometric calibration method based on a multilevel point source array and quantitatively evaluates the differences in response stability between a single point source and a point source array as a function of position. The method improves the stability and accuracy of point source response acquisition through subpixel sampling and averaging, and obtains the calibration coefficients of the imaging system by linear regression on multilevel data, thereby achieving wide-dynamic-range calibration based on the point source array. The test results indicate that the stability of the point source response using a 6 × 6 array is better than 0.21%, while the stability of the calibration coefficient is better than 0.3%. The relative difference between the point source calibration coefficient and the results calculated using conventional extended source calibration methods is within 1.5%, thereby validating the effectiveness of the point source array calibration method. Further optimization based on on-orbit application requirements indicates that a point source array ranging from 2 × 2 to 3 × 3 can reduce response fluctuations in the imaging system by a factor of 6 to 10; point source arrays within this range offer high engineering feasibility while maintaining calibration accuracy. Following an uncertainty analysis, the laboratory measurement uncertainty of the calibration method using a multilevel point source array was found to be 3.2%; when the method was applied to on-orbit absolute radiometric calibration, the estimated uncertainty was approximately 3.9%. The multilevel point source array effectively enhances calibration accuracy while ensuring response stability and accuracy, thereby providing technical support for point-source-based on-orbit absolute radiometric calibration.

A partial aperture onboard calibration method can solve the onboard calibration problems of some large aperture remote sensors, which is of great significance for the development trend of increasingly large apertures in optical remote sensors. In this paper, the solar diffuser reflectance degradation monitor (SDRDM) in the onboard calibration assembly (CA) of the FengYun-4 (FY-4) advanced geostationary radiance imager (AGRI) is used as the reference radiometer for measuring the partial aperture factor (PAF) for the AGRI onboard calibration. First, the linear response count variation relationship between the two is established under the same radiance source input. Then, according to the known bidirectional reflection distribution function (BRDF) of the solar diffuser (SD) in the CA, the relative reflectance ratio coefficient between the AGRI observation direction and the SDRDM observation direction is calculated. On this basis, the response count value of the AGRI and the SDRDM is used to realize the high-precision measurement of the PAF of the AGRI B1 ~ B3 bands by simulating the AGRI onboard calibration measurement under the illumination of a solar simulator in the laboratory. According to the determination process of the relevant parameters of the PAF, the measurement uncertainty of the PAF is analyzed; this uncertainty is better than 2.04% and provides an important reference for the evaluation of the onboard absolute radiometric calibration uncertainty after launch.

In the solar reflective band, the on-board calibration method based on a solar diffuser (SD) is the mainstream calibration method. However, the calibration accuracy is limited by the measurement accuracy of the SD bidirectional reflectance distribution function (BRDF) in the laboratory. The limitations of the light sources and detectors hinder accurate BRDF measurements in the short-wave infrared band, particularly beyond 1700 nm. In this study, the angle integration-band crossing transfer (AIBCT) method is proposed for SD BRDF measurement to avoid the influence of the Fourier spectrometer's signal processing and detector nonlinearity in the absolute measurement. The method utilized the relationship between the BRDF and directional-hemispherical reflectance, and calculated the spectral distribution of the SD BRDF at the standard in-plane geometry with an 0° incidence zenith angle and a 45° reflection zenith angle (0:45) by integration. The absolute measurement result of the 0:45 SD BRDF at 1150 nm was used as the standard for transfer to the short-wave infrared band. The SD BRDF at 0:45 was generalized to other geometries through relative measurements based on the reciprocity theorem. The mean relative error of the results of the AIBCT and absolute measurement methods was less than 0.3 % in the range of 1100 nm to 1600 nm. Analysis of the results showed that the AIBCT method’s measurement uncertainty (coverage factor k = 2) of the 0:45 SD BRDF is better than 0.84 % at 1100 nm to 2300 nm and 1.05 % at 2300 nm to 2500 nm. Furthermore, the AIBCT method was found suitable for the ultraviolet band.