| Since twenty-first Century,Natural disasters occur frequently,for example Global warming,earthquakes,tsunamis,etc.Spatial remote sensing technology can be used to measure the earth's environment and the distribution of resources accurately.The accuracy of remote sensing data directly determines the level of the cognition for the earth's environment.Accurate radiometric calibration of space remote sensing instrument is an important guarantee for obtaining accurate remote sensing data.At present,there are two main methods of absolute radiation calibration for remote sensing instruments.The first one is to realize absolute radiation calibration of space remote sensing instruments by using standard lamp.The second one is to realize absolute radiation calibration of space remote sensing instruments by combining the sunlight and the uniform diffuse reflector,for example the solar diffuse reflector and luner.The main problem of the above two method is that the calibration device will be affected by an unexpected effect during the process of launching and operation on orbit,which can lead the break of the calibration transfer chain and the different instruments can not be traced to a unified standard.At present,the accuracy of space radiation calibration can only reach 10-2 order of magnitude,which can not meet the precision requirements of the 10-3 order of magnitude of remote sensing data in the field of climate monitoring.Meanwhile,the requirement for the resolution of the remote sensing data is getting higher and higher on the scientific research field.So space remote sensing instruments have higher and higher spatial resolution and spectral resolution.That is,there are more and more spectral channels.Using the traditional “lamp+filter”system,a huge error will be introduced due to spectral mismatch.And it is difficult to measure the spatial attenuation of the filter.At present,it is recognized that the best performance of the on-orbit calibration in the world is MODIS,which can reach 2%.It uses the solar diffuser plate as the calibration source,and it is a medium resolution instrument.However,according to the data of past years,about 70% of its uncertainty is caused by the error of bidirectional reflectance calibration.It is a big reason that the incident angle of the sunlight is difficult to measure.So the calibration accuracy of hyperspectral remote sensing instruments is often less than 5%.Therefore,a on-orbit SI-traceable absolute spectral radiance calibration method with high accuracy is proposed in this paper.The calibration method use the laser diode as the calibration source during the spectral radiance calibration and the tungsten halogen lamp as the calibration source during the full spectrum calibration,and the transfer radiometer is used as the secondary standard,the space cryogenic absolute radiometer is used as the primary standard.The transfer chain is divided into two parts: the first part uses the laser diode as the radiance calibration source.The Gauss beam emitted by the laser diode is converted into a collimated beam with a very small divergence angle by the integral sphere and the off-axis parabolic mirror.The collimated beam illuminates the solar diffuse plate with an angle of 90 degrees.It can produce a area source with uniform radiance by the solar diffuse plate.The several spectral radiance calibration of remote sensing instruments are realized by means of that the transfer radiometer and the remote sensing instrument observe the same small source on the diffuse plate at the angle of45 degrees.The accuracy of radiance calibration is improved by using the monochromatic property of laser diode.The second part is to measure the spectral radiance value of the halogen lamp by the remote sensing instrument at the spectral channels which are calibrated at first part.Finally,the spectral curves are fitted by the measured results at the full spectrum.The wavelength of the laser diode is chose by the Genetic Algorithm.The fitting accuracy of spectral radiance at all wavelengths can be guaranteed because of the smoothness of the spectral curve.Therefore,the high accuracy radiance calibration of the remote sensing instrument can be realized by the fitting data at full spectrum.The above two part are carried out under the same light path,which can reduce error.At the same time,the laser diode is used as the spectral calibration source,which reduces the effect of spectral mismatch.The transfer radiometer does not need a filter which can reduce the error source.The main innovative content of this article are as follows:First,the existing space remote sensing radiometric calibration devices in the world are studied,and their advantages and main problem are summarized.The calibration transfer chain for the hyperspectral remote sensing instrument is designed.The laser diode is used as the calibration source.The space cryogenic absolute radiometer is used as the primary standard.The transfer radiometer is used as the secondary standard.The several spectral calibration of the hyperspectral remote sensing can be realized with high accuracy.Finally,the several spectral radiance of the tungsten halogen lamp are measured by the use of the hyperspectral remote sensing instrument.Second,the key of the space spectral radiance calibration is to require a stable monochromatic source.Therefore,the paper proposes that the radiometric calibration use laser diode as the calibration source.The calibration source device is consist of the integrating sphere and the off-axis parabolic mirrors.The integrating sphere transforms the gauss beam emitted by the laser diode to the uniform radiance source.The off-axis parabolic mirror transforms the uniform radiance to a uniform collimated beam.The plane mirror changes the direction of light and increases the compactness of the system.The result shows that maximum divergence angle of the source system is 7.5mrad,and the irradiance uniformity is 99.31%.Third,the key part of the calibration chain is to obtain the hyperspectral data from the multispectral data.This paper uses Genetic Algorithm as an optimize choice method of the wavelength of laser.The spectral curve of the halogen lamp in the500-2500 nm spectrum can be reconstructed by using the spectral data of 12 channels,and the high accuracy calibration of the hyperspectral remote sensor can be completed.The attenuation curve of halogen tungsten lamp is measured by spectrometers and detector.The Genetic Algorithm is wrote to select channels.The remaining spectral data are fitted by the Three Spline Interpolation method.Inversion accuracy is decided by the Root Mean Squared Error between the true data and the fitting data.The result shows the inversion accuracy is less than 1% at all wavelength,the high accuracy calibration for the hyperspectral remote sensor is realized.Fourth,the structure parameters of the transfer radiometer is designed accordingto the performance requirement of the calibration system.Uncertainty of calibration can be deceased by using laser as calibration source.The transfer radiometer measure weak signal by Phase-locked amplification.The calibration process of hyperspectral remote sensor is deeply analyzed.The error source which have significant impact on the calibration accuracy are analyzed and measured.Summary,This paper design the calibration transfer chain for space hyperspectral remote sensor.It select laser as the calibration source when calibration process are carry out at several wavelength and tungsten halogen lamp is used as calibration source during the all wavelength calibration.The cryogen absolute radiometer is use the first standard which can trace to the SI.The high accuracy fitting hyperspectral data can calibrate space hyperspectral remote sensor at the reflected solar spectrum. |
PDF Full Text Request