| High precision on-board calibration is an important method for quantification of remote sensing data.Sun illuminated solar diffuser(SD)made by Teflon with known bidirectional reflectance distribution function(BRDF)and a ratioing radiometer(RR)simultaneously track the degradation of SD can achieve high precision on-board calibration with entire aperture,field of view and optical path.This technology is the main development trend of on-board calibration at VIS-NIR region.Precision of performance evaluation for on-board ratio-radiant calibrator plays an important role in its in-orbit performance.Based on this background,on-board ratio-radiant calibrator and method of its performance evaluation is studied in this paper.In this paper,working principle of on-board ratio-radiant calibrator is introduced and design schemes of important parts are elaborated.According to the working principle and physical model of on-board ratio-radiant calibrator,the main source of uncertainty of on-board calibration comes from the uncertainty of real time BRDF of SD,the key lies in the precision of SD in-orbit degradation correcting made by RR.Test requirement of performance parameters for the calibrator is analyzed,and the key feature parameters of RR are distinguished,including bands ratioing sun view factor,ratio of SD view to sun view,dynamic range and signal-to-noise ratio(SNR).Test method and process are established,and performance evaluation for RR,SD and on-board ratio-radiant calibrator is put forward.Look-up-table of bands ratioing sun view factor of RR is acquired using halogen lamp as light source,and the uncertainty is within 0.18%.With relative lamp-illuminated plaque system,the ratio of SD view to sun view is verified,and uncertainty of the ratio in orbit is estimated to be within 0.4%.With calibrated integrating sphere that has large aperture and multiple energy level,dynamic range,SNR and non-stability of RR are tested.Combining with the test result of SD and entire calibrator,uncertainty of SD stability monitoring performed by RR is within 1.3%,uncertainty of real time BRDF of SD is 1.78%,and combined uncertainty of absolute radiometric calibration of on-board ratio-radiant calibrator is better than 3%.In this paper,method of performance evaluation for on-board ratio-radiant calibrator is studied systematically.Method and procedure of the test of the calibrator are proposed.By making performance test in component level and entire calibrator,calibration coefficient related parameters are acquired and uncertainty of radiometric calibration are evaluated.Design of on-board ratio-radiant calibrator and the method of perform evaluation are proved to be reasonable,which provide valid data basis for application of on-board ratio-radiant calibrator and estimation for precision of on-board calibration. |
