Research On Transfer Chain For Si-Traceable Absolute Spectral Radiance Calibration On-Orbit With High Accuracy

Remote sensing technology is the main means for human to get access to information on the Earth’s environment, and the precision of remote sensing data directly determines the depth of our recognition to the survival environment. Accurate radiometric calibration for remote sensors is the basis of its quantification and the accuracy of radiometric calibration determines the quality of the data acquired by remote sensors. At present, remote sensors radiometric calibration is based-on on-board standard source and solar diffuser plate or intercalibration method. Due to the influence of launch state and rigorous environment in space, the calibration transfer chain fracture and traceability is not good. Thus the calibration accuracy can only reach to 10-2 scale but not meet the requirements of the accuracy up to 10-3 scale for climate monitoring field. In order to acquire high precision data, this paper proposes a method for absolute spectral radiance calibration transfer traceable to SI reference benchmark on-orbit with high precision. The ISTR system consisted of a fiber coupled monochromator(FBM) and an integral sphere transfer radiometer(ISTR) is designed. This paper builds on the high-accuracy calibration transfer chain through setting the sun as the calibration source, the cryogenic radiometer on-orbit as the benchmark and ISTR system as the working standard, thus realizing conversing the power benchmark to the radiance standard and shortening the calibration transfer process. Ultimately, it makes the uncertainty of measuring the radiance reflected from the solar diffuser plate(SDP) on-orbit reach to 0.5% level, which can complete the on-orbit SI-traceable spectral radiometric calibration of optical loads like imaging spectrometers with high precision through the intercalibration method.In order to meet the requirements of achieving the calibration transfer of ISTR system with high precision, FBM adopts the dispersing structure of dual-channel with double gratings in the same light path and completes covering the Reflected Solar Bands(RSB: 0.3~2.5μm) by using off-axis parabolic mirrors cooperated with primary and secondary spectrum. It achieves the performance index of high wavelength accuracy and the bandwidth of 1~2nm through rotating gratings with high precision which is controlled by a linear encoder and a stepper motor. Through the parameters design of the front parabolic mirror and the entrance slit size, the output spectral power ranges from 1uW to 50 uW with the stray light level of 10-6 realized by the addition of the transitional slit, which meet the cryogenic radiometer measuring range and the spectral power scale of the SDP. This paper derived a novel analytic former to calculate accurately the export spectrum changes only by the exit slit height and system focal length, which is more suitable in spectral radiation calibration. Cooperated with the coordinate transformation vector equation, a vector tracing model for calculating the export spectrum changes of several typical light paths is build. Through this model, an innovation method to estimate the accuracy of the FBM’s output central wavelength and the bandwidth, which is caused by the alignment error and grating rotation bias, is proposed. Finally combined with the simulation results of Zemax and Tracepro software, it is concluded that the actual wavelength accuracy and bandwidth accuracy of FBM is about 0.04%, which meets the requirements put forward by the ISTR system with high precision.ISTR adopts integrated design and combines the traceability of the trap detector secondary standard to the cryogenic radiometer benchmark with the process of the filter radiometer(FR) spectral work standards calibrated by the secondary standards in the same calibration light path depending on Lambertian characteristics of the integrating sphere. It shortens the overall calibration transfer chain and reduces the error sources of the measurement uncertainty. ISTR system choose 15 typical spectral channels in the RSB for calibration, and realizes the high-accuracy measurement of extreme weak solar spectral signal to achieve the conversion between the power benchmark and the radiance standard through an optical chopping light modulator and digital phase sensitive detection technique. This paper proposes the accurate analysis mathematic models for the measurement equations corresponding to each calibration links in the whole traceability transfer process of the ISTR system, including secondary standard tracing to cryogenic radiometer benchmark, secondary standard calibrating FR spectral standard and the mismatch drift of different sources along FR measuring SDP. Then the type A and type B uncertainty sources of the calibration transfer were acquired accurately by theory analysis or experimental measurements and the influence factors include standard conversion, the FBM export power transmission, the gain and response non=uniformity and nonlinearity of detectors, weak signal detection accuracy, the effects of spectrum stability and the bandwidth matching on the responsivity calibration, the spatial uniformity of the integrating sphere and the alignment match error among each device. Using the response of the effective wavelength to substitute the channel average response, the influences caused by the size and mismatch of spectral bandwidth on the calibration accuracy of secondary standard and FR standard were analyzed exactly, and the method for evaluating the measuring accuracy drift caused by the difference between the calibration source and the measured source was carried out. Eventually cooperating with the high-precision calibration data obtained by the discrete laser sources and the statistical fitting corrected algorithm, the calibration uncertainty of the spectral responsivity for each standard in ISTR system is 0.25% and the overall measurement uncertainty of the radiance is 0.45%, which meet the expected requirements.Finally, this paper realized the algorithm for the SNO-x method based-on the low latitude desert used to the intercalibration among Leo satellite remote sensors, and for the first time achieved the intercalibration for the responsivity of the RSB channels in FY3C/MERSI taken TERRA/MODIS as the reference instrument. Through using SGP4 orbit algorithm to select the calibration area, pixel matching analysis to set condition thresholds, special method to match the correlative spectral channels, the filtering of substellar point environment uniformity and abnormal response, and the correction fitting for the two satellites’ matching sample data, it realizes on-orbit calibration transfer for remote sensors with high accuracy. Through the comparison among the intercalibration results of different corresponding channel groups with different bandwidth matching forms between MERSI and MODIS, it explained that the highest precision is acquired when MODIS channel bandwidth contains MERSI channel bandwidth and that the final accuracy is mainly affected by the precision of the reference instrument itself. The SNO-x method is proved to achieve the feasibility of making the SI-traceable spectral radiance calibration on-orbit for remote sensors reach 1% scale through taking ISTR system as the reference instrument.The original method for the on-orbit radiation calibration, which takes the ISTR system as the core and investigated in this paper, is the only way recognized internationally to realize the SI-traceable spectral radiance calibration on-orbit for satellite remote sensors. It has the characteristics of high spectral resolution and high precision standard transfer, which can meet the needs of SI-traceable absolute radiometric calibration for most satellite remote sensing payloads. It eventually laid the theoretical and technical basis for acquiring the high-quality remote sensing data on the Earth observation and atmospheric monitoring field for China in the future.