风云三号A星中分辨率光谱成像仪反射太阳波段的多场地定标跟踪
Multisite Calibration Tracking for FY-3A MERSI Solar Bands
中文摘要：
搭载于中国第二代极轨气象卫星FY-3A上的中分辨率光谱成像仪（MERSI）是一个类似于MODIS的传感器，具有19个反射太阳波段和一个热红外波段。尽管MERSI配备了可见光星上定标试验设备，但是其只能用于仪器响应的衰减追踪。每年一次的敦煌场地替代定标一直是MERSI反射太阳波段在轨绝对辐射定标的主要方法。为了提高在轨定标频次，提出了多场地辐射定标跟踪方法。该方法依赖于多个稳定目标场的大气顶模拟辐射，并基于定标系数时间序列的趋势分析建立了定标日更新模型。以Aqua MODIS观测值为参考，对MERSI的定标基准进行了评估，两者的平均相对偏差在5%以内（0.4～2.1μm）。研究发现：MERSI的短波部分衰减显著，特别是412nm波段，年衰减率达9.7%；红光和近红外波段相对稳定，年衰减率在±1%以内。采用多种方法分析了MERSI定标结果的可靠性。多场地定标和敦煌场年度替代定标结果间的相对偏差小于3.8%。以Aqua MODIS为参考对定标后的MERSI数据进行了辐射质量监测，双差分析表明在辐射稳定的沙漠地区，两者的平均相对偏差约在5%之内；同时星下点观测分析也显示出良好的一致性。
中文关键词：
风云3A（FY-3A），中分辨率光谱成像仪（MERSI），多场地，反射太阳波段，替代定标
KeyWords：
FY-3A, MERSI, multi-site, reflective solar bands, vicarious calibration
Abstract：
The MEdium Resolution Spectral Imager (MERSI), onboard the second generation Chinese polar-orbit meteorological satellite FY-3A, is a MODIS-like sensor with 19 solar bands and one thermal infrared band. Although there is a visible on-board calibration device, it can only be used for tracking temporal instrument degradation. The vicarious calibration (VC) campaign at the Dunhuang site, conducted once a year, has been the main post-launch absolute radiometric calibration method for MERSI in the solar bands. To increase the in-flight calibration frequency, a multi-site radiometric calibration tracking method is presented. This method relies on simulated radiation over several stable sites, and a daily calibration updating model is built from longterm trending of calibration coefficient series. The MERSI calibration reference is evaluated against the observations of Aqua MODIS, showing mean relative biases within 5% from 0.4 to 2.1 μm. The shortwave channels of MERSI are found to experience large degradation, especially the 412 nm band with an annual degradation rate of 9.7%, while the red and near-infrared bands are relatively stable with annual degradation rates within ±1%. Several approaches have been used to analyze the reliability of MERSI calibration results. A comparison of the calibration slopes shows that the relative biases between the multi-site method and the annual Dunhuang VC campaign are below 3.8%. Aqua MODIS is used as a reference to monitor the data quality of the recalibrated MERSI. A double difference analysis shows that the mean relative biases are almost within 5% over stable deserts, and the synchronous nadir observation analysis also reveals good agreement.