基于月球观测的静止气象卫星热发射波段辐射定标与精度评估

The radiometric calibration accuracy for thermal emissive bands (TEB) of Fengyun-2 satellites, especially for the low temperature region of atmospheric-window bands and all the dynamic range of the atmospheric-absorption band, can approach 8-10K. On-orbit absolute calibration could not be realized by using a single onboard inner-blackbody. Affected by the differences in spectral response as well as spatial resolution between different payloads, the currently adopted cross-calibration method is also unsatisfied with the operational requirement. In our study, the Moon is selected as an on-orbit steady radiative source for TEB. Based on the observed radiance modeling for the pixel-scale target on lunar surface with non-isothermal, inhomogeneous and non-grey features, the parameter, mixed-pixel dual-bands equivalent emissivity ratio (MDEER) is introduced in a newly constructed mapped-radiance space, and the dual-bands combinational radiation calibration equation which is nearly unrelated with lunar surface temperature is also established. When the in-lab calibration standard has been transferred to the on-orbit condition in both direct and indirect ways, the absolute radiometric calibration with high accuracy is finally completed. Moreover, some redesigned products, e.g. sea surface temperature and omitted long-wave radiation, are utilized to evaluate the accuracy of radiometric calibration with the performances of themselves. The proposed framework model of radiometric calibration with lunar observations will be universally suitable for remote sensing instruments with different spatial resolutions in different TEBs on different platforms, and expected to become an new radiative reference standard in space application. The relevant studies have not been reported in both domestic and oversea societies, and belong to the original works.

风云二号卫星热发射波段（TEB）辐射定标精度，在大气窗区波段低温端和大气吸收波段仍有8-10K的误差。星载单个内黑体无法实现在轨绝对定标，且现有的在轨交叉定标受不同载荷在光谱响应、空间分辨率上的差异影响，亦无法完全满足业务需求。本项目研究选择月球作为TEB稳定的在轨辐射源，在解决像元尺度月表目标非同温、非均匀、非灰体特性观测辐射建模，并在新构建的辐亮度映射空间内引入混合像元双波段等效发射率比（MDEER）参数后，建立与月表温度几乎无关的双波段联合辐射定标方程。在将实验室定标基准以直接和间接两种方式传递到在轨条件下后，实现高精度的绝对辐射定标。同时，引入新设计的海表温度、射出长波辐射等产品，通过检验产品性能来评估辐射定标精度。提出的对月辐射定标框架模型，将普遍适用于不同卫星平台、不同分辨率、不同TEB遥感器，并有望成为新的空间TEB辐射参考基准。相关研究在国内外尚未见报道，属原创性工作。

辐射定标精度是影响空间对地观测数据定量化应用的核心指标，而高精度的定标结果必须依赖稳定可靠的定标参考源。本项目研究从国产风云二号气象卫星应用需求出发，主要开展如下三个方面的研究工作：其一、在突破月球表面10-100Km量级均匀区判识方法基础上，优选确定了由若干月球表面均匀稳定目标构成的红外波段月球辐射定标场，首创了与温度无关的红外波段月球辐射定标方法，建立了自主知识产权的高精度红外波段在轨绝对辐射基准（不确定度优于0.02K）；其二、发展了两种不同类型的红外波段高置信度定标精度检验方法，即基于严格晴空检测的精细化海表温度、以及基于固定大洋浮标测量和数值预报再分析资料计算等检验方法；其三、提出了在制备模拟月壤样品基础上，通过地面测量手段（即独立黑体能量法发射率测量、积分球反射计反射率和发射率测量等方法），精确获取月表光谱辐射特性参数的新方法，相对精度优于1%。自2012年7月起，上述月球定标方法陆续在风云二号全部在轨卫星中稳定运行，迄今多星累计使用超过10星年，运行成功率100%。检验表明：风云二号主用业务星(G星)红外和水汽波段年均定标偏差为0.4-0.5K（美国第二代静止气象卫星GOES-N设计指标为1K），达到国际先进水平。该项目的主要研究成果，获得2016年度中国气象学会科学技术进步成果奖一等奖。