Research On Improved Vicarious Calibration Method Based On Multi-Grayscale Targets

Radiometric calibration technology is an important way to realize the quantification of remote sensing information by using remote sensing technology.It is an indispensable key link in remote sensing system.During the on-orbit operation of the remote sensor,it is necessary to continuously monitor and correct changes in its radiation response to ensure the data quality of the remote sensor throughout its onorbit lifetime.On-orbit calibration technologies such as vicarious calibration,on-board calibration and pseudo-invariant site calibration have played an important role,but the accuracy of on-orbit radiometric calibration of optical remote sensors has always been a scientific concern in the field of quantitative remote sensing.Reflectance-based method is a classical vicarious calibration method,in which site reflectance measurement is an important factor affecting the uncertainty of this calibration method,and its contribution is about half of the calibration uncertainty.The measurement of site reflectance has many difficulties,such as large time span,low measurement density,small measurement range,etc.It is difficult to achieve highprecision measurement,which has become a key factor restricting the improvement of vicarious calibration accuracy.At the same time,the number of large area uniform sites available in China is limited,and the site reflectance is low,so single point extrapolation is required when calculating high reflectance targets.The dark current offset of the remote sensor needs to be obtained by observing the cold space,which is inconsistent with the state of the earth observation,further affecting the improvement of the accuracy of the on-orbit radiometric calibration.Aiming at the problems existing in the current vicarious calibration method,this paper starts from the site reflectance,remote sensor dark current,diffuse-to-global irradiance ratio and other influencing factors involved in the calibration method,carries out the research on the improved vicarious calibration theoretical model in the solar reflection band,and carries out the site experiment verification and application.On the basis of summarizing and analyzing the existing vicarious calibration theories and methods,this paper proposes an improved vicarious calibration method combining calibration and application,and establishes a radiometric calibrationreflectance inversion iterative model based on grayscale target.The calibration coefficient and dark current value of the camera are calculated in real time by the slope and intercept of the grayscale target linear regression,and the insufficient reflectance measurement range is compensated by the reflectance inversion,and the results are fed back to the calibration process,realizing the iterative solution of the calibration coefficient,the camera dark current and the equivalent environmental reflectance.The influence of target and environmental reflectance,atmospheric transmittance,radiative transfer calculation and other factors on the calibration method is quantitatively analyzed.The results show that the uncertainty of the improved calibration method is 3.8%.From the theoretical analysis and preliminary experimental results,the accuracy of the improved calibration method is about 1%higher than that of the reflectancebased method.In order to verify the iterative model of radiometric calibration-reflectance inversion,several site calibration tests were carried out for the PRSS-1 optical satellite.First of all,the calibration coefficients of uniform site and grayscale targets are calculated directly using the reflectance-based method.The results of the two methods are quite different.After dark current correction of the uniform site image count value.the difference between the two experiments is within 3.5%.This result shows that the camera does have dark current offset,and this factor will cause large errors in the calibration results of uniform site in low-end applications.It is necessary to evaluate and calculate the dark current when the camera is observing the earth.Then,the radiometric calibration-reflectance inversion iterative model is applied to the experimental image.The initial value of the iteration is the calibration coefficient,the dark current and the measured environmental reflectance.The iteration ends when the relative error of the calibration coefficient is less than 0.1%and the difference of the retrieved pixel reflectance is less than 0.001.The results show that the absolute difference between the target inversion reflectance and the measured reflectance is within 0.014.The adaptability of the model in different environments is analyzed through the influence of the initial parameters of the model on the results.Finally,it is verified by comparing Sentinel-2 satellite images.The calibration coefficients obtained from the grayscale target method and the radiometric calibration-reflectance inversion iterative model are respectively applied to PRSS-1 satellite,and cross verified with Sentinel-2 satellite in Gobi.The results show that the difference of radiance can be reduced to 3.7%after the application of the model;The radiometric calibrationreflectance inversion iterative model is also applied to the experimental image data,and the reflectance inversion model is applied to Sentinel-2 satellite L1 C image to compare the inversion reflectance difference.The results show that the absolute difference between them is within 0.012 in the range of 0.1-0.3 reflectance.In this paper,based on the theory and method of site vicarious calibration,an iterative model of radiometric calibration and reflectance inversion is established.In the way of combining calibration and application,the quantitative remote sensing of high-resolution optical satellite remote sensor based on the integration of calibration and inversion of grayscale target is realized,which is expected to further improve the in-orbit calibration accuracy of China’s medium and low-resolution optical remote sensor based on large area uniform site.