FY-3B卫星中分辨率光谱成像仪可见光星上定标系统辐射性能稳定性分析

Stability Analysis of Radiometric Properties in Visible On-Board Calibration System of FY-3B Satellite Medium-Resolution Spectral Imager

风云三号B星(FY-3B)中分辨率光谱成像仪(MERSI)可见光星上定标(VOC)系统辐射性能随时间衰减变化,使其难以用于MERSI的在轨绝对辐射定标。本文提出一种确定VOC辐射性能衰减变化的新方法,该方法在探究太阳光定标时机与太阳天顶角和方位角依赖关系的基础上,分析MERSI相对辐射响应衰减变化的趋势;利用高质量的MODIS数据构建大气层顶双向反射分布函数模型,对MERSI约10年的在轨有效数据进行交叉定标处理,获取MERSI绝对辐射响应衰减变化趋势;将二者归一化后的差值作为VOC系统辐射性能的衰减变化值。结果表明:VOC系统辐射性能在发射初期衰减率较大,后期衰减趋势趋于平稳,且衰减率与波长相关;在发射初期,短波波长(<500 nm)处的最大年衰减率达到了49.51%,长波波长(800~1000 nm)处的衰减率相对较小,最大年衰减率均小于26%;在发射后期,除412 nm外(衰减率为1.91%),其余波长处年衰减率均小于0.64%。自仪器发射以来,基于本方法获得的VOC衰减数据与VOC监视数据之间的百分误差均小于15%。本文提出的VOC系统稳定性分析方法可为实现MERSI星上绝对辐射定标提供重要的参考。

Objective The output of the FY-3B satellite's medium resolution spectral imager(MERSI)visible on-board calibration(VOC)degrades with time,which causes concerns regarding its reliability in absolute radiometric calibration.The users must distinguish between the uncertainties determined by the VOC system's radiometric output and the MERSI detectors since this will lead to a detailed temporal evolution comprehension of the MERSI system and VOC radiometric characteristics.Additionally,this can ensure the remote sensing data are fully calibrated and utilized in the observed target study.We aim to investigate the output variation of the MERSI VOC system and have made special efforts to extract the information variations of VOC radiometric performance.The annual degradation rates which are defined as the percentage difference between the results of the first and last measurements of each year are employed to evaluate the VOC radiometric performance.The results are evaluated against the trap detector monitoring to further validate the proposed proceeding approach.Methods Based on the characteristics of the satellite orbit and the structures of the MERSI VOC,we introduce a novel methodology to assess changes in the VOC system's radiometric output,with a particular focus on analyzing the relationship between the sunlight calibration opportunities and the angles of solar zenith and solar azimuth.Then we screen out the sunlight-based calibration data from multiple light sources(interior lamps,sunlight,space view background)calibration data.The analysis is to provide perspectives on the comparative radiometric performance of MERSI.Meanwhile,the majority band response seems to follow a somewhat downward trend.Subsequently,the performed relative response characterization step employs an exponential function created via least-squares fitting of the VOC data.High-quality MODIS data are leveraged to develop a top-of-atmosphere(TOA)bidirectional reflectance distribution function(BRDF)model and thus enhance the study precision.The time series of TOA reflectance acquired by BRDF model fitting is compared with that measured by MODIS,with the time series of BRDF modeling residuals analyzed.This model is consequently utilized in cross-calibration processing with nearly 10 years'worth of on-orbit data from MERSI.Cross-calibration processes include spectral matching between the two distinct sensors,viewing geometry correcting,and spectral interpolation.Additionally,the TOA reflectance is further converted to calibration coefficients using a calibration equation with zenith angle,azimuth angle,digital counts,and earth-sun distance.This is to comprehensively evaluate MERSI's absolute radiometric performance,and the relative and absolute radiometric characteristics of MERSI are standardized based on the initial regression point.This standardization treats the normalized difference as an indicator of the decay in VOC radiometric performance.Results and Discussions Recent studies using analysis of the MERSI sensor response to the Libya4 pseudo-invariant site and cross-calibration with MODIS show that the FY-3B MERSI has not deteriorated as much as the sunlight-based calibration trend has suggested.The comparison of the above lifetime trends and the relative and absolute radiometric characteristics of MERSI produce a distinction estimate in the calibrations of consecutive FY-3B MERSI pairs.We conclude that the degradation effects of the VOC radiometric performance can explain the observed differences.The results illustrate that the degradation rates of VOC radiometric performance are wavelength-dependent,with an initially higher rate gradually decreasing over the years and eventually stabilizing.Notably,in the early mission stages,the shortwave outputs(below 500 nm)exhibit a substantial degradation,reaching up to 49.51%.Conversely,the decay rates at longer wavelengths(800-1000 nm)are relatively modest,remaining within 26%.In the later stages of the satellite's mission life,the decay rates for most wavelengths are approximately 0.64%,except for 412 nm,which experiences a higher rate at approximately 1.91%.For further validating the employed proceeding approach,we make a comparison of the decay in VOC radiometric performance calculated by us with that monitored by the trap detector.Since we cannot determine how the data amount that passes through the filter changes while in orbit,the radiometric performance of VOC is all normalized by the first measurement value.The results indicate that the maximum percent differences observed throughout the instrument's lifetime remain below 15%at 470 nm and 14%at 65 nm.Conclusions A general procedure is developed and implemented to provide users with the ability to characterize the decay rate in the VOC system's radiometric output.The results demonstrate that the maximum annual decay rates(ADRs)of the short-wave output(<500 nm)range from 46%-50%,while the longer wavelengths(800-10000 nm)reveal relatively smaller changes of approximately 26%.The current procedure implementation leads to further comprehension of changes in the VOC system output.The adopted novel methodology serves as a valuable reference for extending analogous endeavors that aim at conducting on-orbit absolute radiometric calibration for other sensors.