| With the rapid development of quantitative remote sensing,the calibrationaccuracy of remote sensors has become higher and higher.Measurement and study on direction reflection characteristics of radiometric calibration site and applied in the direction of the surface reflectance correction is helpful to improve the accuracy of field calibration.To improve the site BRDF measuring precision,reduce the measuring time and the artificial intervention degree,this paper carried out the high precision automation BRDF measurement system control software design and thorough analysis and research on the measured data of Dunhuang Radiometric Calibration Site.In this paper,the development and current situation of BRDF measurement systems at home and abroad were investigated.In the premise of ensuring the accuracy of measurement,automation and portability were the current development trends of measurement system.The automatic measurement function of the BRDF measurement system depends on the effective design of the control software.In view of the domestic advanced high precision automation BRDF measuring system for the design of control software,combining with the characteristics of measuring system and field site measurement requirements,the control software was described in detail the performance and functional requirements,the overall design scheme and the composition of each functional module.The control software was tested,and the results showed that the software can complete the automatic measurement function of the field BRDF in various modes,and can process and visualize the measured data.As an internationally-recognized national-level radiometric calibration site,Dunhuang Radiometric Calibration Site had been successfully used for absolute radiometric calibration of domestic satellites such as FY series and ZY series.An on-orbit vicarious calibration experiment for FY-4A was formally performed at Dunhuang Radiometric Calibration Site in late April,2017.In order to meet the needs of the directional corrections in the sensor viewing angle of surface reflectances,the Dunhuang Gobi surface directional reflectances were measured using a fixed-point measurement instrument.The effects of view zenith angle,relative azimuth,solar zenith angle and spectral wavelength on the BRDF characteristics of the site were analyzed.At the same time,in order to quantitatively describe the directional reflection characteristics of Dunhuang field,the RossThick-LisparseR kernel-driven model wasused to fit the measured data,the parameters of bidirectional reflectance distribution function model were obtained,and the goodness of fit of the model was evaluated.The linear regression analysis was performed between the model calculated data and measured data.The adjusted R-Square was greater than 0.8.and the present model had proven to provide a good fit to measured data.The anisotropy factors(ANIF)in the solar principal plane were obtained by utilizing model and compared with historical data of moderate-resolution imaging spectroradiometer BRDF/Albedo model parameters(MODIS BRDF)product.The relative deviation of band 2 and band 4 were below 1.56% and the band 5 was below 3.12% when the view zenith angle was less than 40 degrees.The results demonstrate that the constructed model can pretty describe the directional reflection characteristics of Dunhuang site,and verify the reliability of measured data.In addition,the spring model and the autumn model of the MODIS BRDF product were also compared.The relative deviation of the ANIF on the principal plane can reach 7.42%,indicating that there are differences in the directional reflection haracteristics of the Dunhuang Radiometric Calibration Site in different seasons.A site-specific BRDF model for different seasons should be established to meet the calibration requirements in multi-season vicarious calibration work. |
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