Research On The Lunar Calibration Technologies Of Optical Remote Sensing Radiometers

On-orbit calibration is an important guarantee for the quantitative application of optical remote sensing radiometers.The stability of lunar surface,the absence of atmosphere on the Moon,and the similarity of dynamic range between lunar radiance and the measuring capacity of existing optical remote sensing radiometers,make the Moon a potential reference for on-orbit calibration of optical remote sensing radiometers.For the solar reflective bands,the domestic researches started relatively late,which needs to be further supplemented and deepened.For the thermal emissive bands,there are only a few studies at home and abroad,and they all belong to indirect lunar calibration,which is cross calibration between different bands or different gains.There is no attempt to use the Moon as a direct reference for the calibration of thermal emissive bands.In order to solve these problems,a variety of theories and methods relating to lunar calibration technologies are discussed systematically,including: the method of data processing for lunar calibration of solar reflective bands,researches of lunar temperature model and thermal radiation model,and the solution to the core technical problems of ground-based lunar observation,etc.When the geometry between the Sun,the Moon and observer is certain,the lunar reflective radiance or irradiance remains unchanged.Using this characteristic,the method of lunar calibration of solar reflective bands is studied.In fact,the geometry of the three is always fluctuating.The lunar observation sequence is invariable in theory only after the normalization of distance correction,phase correction and libration correction.The actual attenuation of the lunar observation sequence reflects the attenuation of the response rate of the remote sensing radiometer.Taking FY-2E as an example,the lunar calibration algorithm for the visible band of the geostationary radiometer is mainly studied.It is found that the annual attenuation rate of the visible band of FY-2E was about 2.17%,which provides reference for the systematic correction of the radiometric calibration of the radiometer.The Moon is also a potential reference for optical remote sensing radiometers in thermal emissive bands.In order to get the thermal radiation characteristics of the Moon,the distribution of the surface temperatures needs to be studied.Based on a standard one-dimensional thermal-diffusion model,the qualitative relationships between the surface temperatures and the solar radiation,the reflectance of the lunar surface and the thermal physical properties of the lunar regolith are discussed.The daytime temperatures of the Moon are mainly controlled by the solar radiation and the reflectance of the lunar surface,while the nighttime temperatures of the Moon are sensitive to the thermal physical properties of the lunar regolith.Based on the nadir observations of the Lunar Reconnaissance Orbiter Diviner lunar radiometer experiment,spatial and temporal distributions of the surface temperatures of the Moon has been obtained.Finally,the analytic functions of surface temperatures of the lunar equator are derived,and the fitting residuals are within 2.5%.The roughness of the lunar surface makes the effective solar radiation on the lunar surface not completely changed with the selenographic latitude and longitude.In addition,due to the lack of atmosphere and the low density of lunar regolith,the lunar surfaces are highly insulated.These cause the local temperatures of the lunar surfaces to be controlled by their orientation with respect to the Sun.The scale of the temperature contrast is far smaller than the instantaneous field of view of the lunar orbit instrument or the earth orbiting remote sensing radiometer.The proportion of warm surface and cool surface within the field of view is different under different viewing angles,making different brightness temperatures.Based on Diviner's off-nadir observational data,the directional characteristics of thermal emissive radiation of the lunar surface are studied;the variation of brightness temperature with emission angle has been derived;the thermal emission model of the Moon has been preliminarily established.The rationality of the lunar thermal emission model is verified by an example of the lunar image taken by the MWIR band(3.5~4.0?m)of FY-4A.There is a difference about-4.25% between the model predicting radiance and the measured radiance from FY-4A.The method of directly using the lunar thermal radiation as a reference for the calibration of remote sensing radiometers is also discussed.No matter how the lunar calibration is implemented,it is necessary to know reflective or thermal emissive characteristics of the Moon in advance,and the ground-based lunar observation is an effective way to achieve this goal.There are differences in the paths,geometrical angles and spectral responses between the ground-based lunar observation and the satellite observation.The ground-based lunar observations need a series of processing to be used as a reference for the calibration of remote sensing radiometers.The calibration of ground-based instrument is discussed;the influent factors of atmospheric correction are analyzed by modeling and simulation;the algorithm of spectral correction is described.The ultimate goal of ground-based lunar observations is to establish a model of the lunar reflective and thermal emissive radiation.The model takes the observational time,the selenographic coordinates of the observer and the Sun as input variables,and calculates the exact amount of lunar radiation detected by the observer at any location,achieving the purpose of on-orbit radiometric calibration for remote sensing radiometers.Through the analysis of the long-term ground-based lunar observation data,the coefficients of the model can be continuously iterated and updated,and the precision of the model can be improved gradually.The researches provide theoretical analysis and method guidance for the application of lunar calibration in several ways,and have certain values in scientific research and engineering application.