Study On On-orbit Calibration Of Ultraviolet Hyperspectral Ozone Spectrometer

With further development of space remote sensing and constant improvement ofrequirement on quantitative application of remote sensing data, high precisioncalibration of space-borne sensor is increasingly more necessary and important. Thecalibration is the premise of remote sensing products quantitative application.Reliability，breadth and depth of application of remote sensing data depend largely onaccuracy of calibration. The calibration is to establish a quantitative relationshipbetween the sensor output digital signal and the corresponding radiation of targets.space-borne sensor calibration includes laboratory calibration and on-orbit calibration.Although laboratory calibration were carried out before space-borne sensor’slaunching, the calibrated performance of the instrument would be certain changesover time because of the impact of harsh space environment and conditionsinconsistency between on-orbit operation and laboratory calibration. In order tocorrect and calibrate the impact,on-orbit calibration is required for space-borne sensor.For high-precision imaging spectrometer, the spectral and polarization properties aredirectly related to its radiometric calibration accuracy. So, on the basis of theoreticalanalysis and development studies on-orbit calibration of both domestic and foreignspace-based sensor, for FY-3(02) ultraviolet hyperspectral ozone spectrometer characteristics, the research on on-orbit spectral calibration, on-orbit polarizationcorrection and on-orbit radiometric calibration were carried out.The first part of this paper is about on-orbit spectral calibration. On the basis ofsummarizing the current spectral calibration methods, a new high accuracy on-orbitspectral calibration method has been introduced. According to the principles ofspectrometer slit function effect on target spectrum detection，a high resolution targetspectrum was used to convolve with instrument’s slit function. The convolved resultswere matched with corrected spectrum measured by instrument. The spectral variationwas determined by minimizing matching error, then, the on-orbit spectral calibrationwas finished. By this method, on-orbit spectral calibration of FY-3B ultraviolet ozonevertical spectrometer was made. And the result was validated by standard spectrum ofon-board mercury lamp. Under the condition of elimination the influence ofinstrument itself, the calibration accuracy could reach±0.01nm, which meet therequirements of ultraviolet hyperspectral ozone spectrometer. The feasibility thehigh-accuracy on-orbit spectral calibration was verified.The second part of this paper is about on-orbit polarization correction. Firstly,polarization properties of atmospheric scattering spectra were introduced andpolarization sensitivity of space-borne spectrometers was presented using Muellermatrix calculus. The effect of polarization on measurement data of space-bornespectrometers was analyzed. Two methods were introduced to minimize the effect,depolarizer and polarization correction. But, for ultraviolet hyperspectral ozonespectrometer, the depolarizer scheme could not meet the requirements. So,polarization correction was employed. According to the polarization correction theory,ground polarization calibration of space-borne spectrometers and on-orbit polarizationmeasurement of incident light were made to complete polarization correction. Interms of ground polarization calibration of space-borne spectrometers, a highprecision method (overdetermined equation method) was proposed. And according tothe method, a measuring system of instrument polarization characteristics wasestablished in the laboratory. The main error sources of overdetermined equationmethod and polarization measurement system were analyzed and correction method was given. Polarization properties of limb ultraviolet imaging spectrometer werecalibrated with the overdetermined equation method and polarization measurementsystem. According to uncertainty analysis, the calibration accuracy was up to1.53%.In terms of on-orbit polarization measurement of incident light, the basic scheme andsome improvement program were set for ultraviolet hyperspectral ozone spectrometer.And the related principle experiment was carried out by joint use of two earlyprototypes in XX project. The results showed that the accuracy of the polarizationmeasurement scheme could be up to2.0%. Finally, the effect of the groundpolarization errors and on-orbit polarization measurement errors on the polarizationcorrection accuracy was analyzed. It was showed that polarization effect on ultraviolethyperspectral ozone spectrometer by use of the on-orbit polarization correction wasreduced to less than2%, which meet the requirements of the instrument on-orbitpolarization index.The last part of this paper is about on-orbit radiometric calibration. On the basisof summarizing the current radiometric calibration methods, a combination calibrationplan of "sun+diffuser" absolute calibration and fixed moon’s observation relativecalibration was applied for ultraviolet hyperspectral ozone spectrometer. The specificimplementation and algorithms of the calibration plan were described. Finally, theaccuracy of on-orbit radiometric calibration scheme was analyzed, and it was showedthat the calibration uncertainty was about4.2%, which meet the on-orbit requirements.The reliability and accuracy of remote sensing data were guaranteed, and a usefulreference was provided for subsequent space-borne sensors on-orbit calibration.