Investigation On Space Cryogenic Radiometer Used As Spectrum Radiation Standard On-orbit

High-quality space remote sensing data are needed to support scientific research such as global warming,extreme disaster prediction and high-resolution earth observation.The optical remote sensing load is affected by the launching process and space environment.The high-precision laboratory radiation scale established before launching will drift or attenuate,and the reference traceability link will be broken.At the same time,the existing on-orbit radiometric calibration methods lack high-precision traceable radiometric calibration standard.Therefore,the absolute accuracy of a large number of remote sensing data obtained by optical remote sensing load is lower.The urgent needs of quantitative remote sensing cannot be satisfied.Referring to the technical route of establishing and transferring ground radiation standard,the on-orbit radiometric calibration transfer link based on the space cryogenic absolute radiometer is proposed,which will improve the accuracy of on-orbit radiometric calibration by an order of magnitude.Space cryogenic radiometer is a kind of electrical substitution radiometer,and operates at the low temperature of 20 K.Based on photoelectric equivalence,the unknown optical power can be measured by accurately measuring the equivalent electrical power.For the future space applications,it is necessary to study the design requirements and uncertainty allocation of the space cryogenic absolute radiometer.The ground-based prototype is developed through detailed design of optoelectronic,electromechanical and thermoelectric devices.The performance of the prototype is investigated by test experiments,and the absolute measurement accuracy of the prototype is verified by indirect comparison.Firstly,the basic working principle of space cryogenic radiometer is introduced.According to the characteristics of the thermal transmission structure of cryogenic radiometer detector,the three-dimensional thermal conduction model of the detector is simplified,and the simplified thermal conduction differential equation is solved.Then the response calculation formula and time constant calculation model which characterize the performance of the cryogenic radiometer detector are obtained.In order to achieve the equivalent of the photoelectric power response and the electric heating response of the detector,the basic design requirements of the space cryogenic radiometer are summarized.The calculation model of detector absorption ratio including diffuse reflection,mirror reflection and attenuation is established to guide the optimization design of the high absorption cavity detectors.Considering the calibration and redundancy design,in order to achieve wide dynamic range of optical radiation power calibration,high power cavity for total solar calibration and high sensitivity low power cavity for spectral radiation calibration are designed.Based on the analysis results of measurement influence and limitation factors,the basic design requirements of space cryogenic radiometer are summarized and put forward,taking into account time constants,absorption rate,sensitivity,refrigeration capacity,refrigeration temperature,instrument size,quality and other limitations.Secondly,the key technolgies of the space cryogenic absolute radiometer are researched.A Stirling type pulse tube refrigerator is used to realize the low temperature environment in 20 K temperature range.The blackbody cavity blackening material adopts the black paint with diffuse reflection as the main material.The absorption ratio is better than 0.999 by designing the length and opening ratio of the cavity.At the same time,the attenuation of the absorption ratio of the cavity is two orders of magnitude smaller than that of the surface of the cavity.The preparation process of the blackbody cavity is optimized for improving the consistency of the blackbody cavities.The diaphragm components are designed,the radiation exchange between blackbody cavity,window and diaphragm system are investigated.The structure of cryogenic absolute radiation detector is designed.The temperature control system of the thermal connection components is used for suppressing the noise of Stirling type pulse tube cooler and improve the thermal stability of the blackbody cavity.The background noise of stray light and thermal radiation is reduced by the diaphragm system.The sensitivity of blackbody cavity is optimized to 0.4 K/mW by the thermal structure design.The key technologies are investigated,such as the background radiation exchange analysis and control,the precise measurement of electric power and temperature,and the establishment of high stability low temperature thermal environment.Finally,the performance test and results of the space cryogenic absolute radiometer are investigated,and the absolute accuracy of measurement of light radiation power is evaluated.The validity of uncertainty evaluation results is verified by the indirect comparison.The performance of the space cryogenic absolute radiometer at low temperature are tested,such as the temperature stability,the thermoelectric repeatability,the sensitivity and laser power measurement repeatability.According to the “JJF 1059.1-2012 Evaluation and Representation of Measurement Uncertainty”,the synthetic relative standard uncertainty of the space cryogenic absolute radiometer is 0.029%,which is synthesized by A and B standard uncertainties.The uncertainty evaluation results were verified by indirect comparison with the national standard cryogenic radiometer.According to the “JJF1117-2010 Metrological Comparison”,the normalized deviation En=0.4<1 verifies the validity of the accuracy evaluation results of the space cryogenic absolute radiometer.