基于嫦娥三号就位探测数据的月海反射率定标区建立方法研究

More new standard targets on the Moon, besides the widely used Apollo 16 area, are needed for imaging spectrometers on lunar missions to improve their reflectance calibration accuracies.Recent lunar orbital observations suggested that the surface reflectance calculated based on the Apollo 16 standard area and Apollo 16 sample laboratory measurement is significantly different from its true value, because the physical properties of the returned lunar sample, such as porosity, have been changed during the sampling operations.The Chang’E-3 VIS/NIR Imaging Spectrometer (VNIS), equipped with a white spectralon panel as reflectance calibration standard, can perform in situ multispectral observationsaround the Chang’E-3 landing site, without altering the physical and mineralogical natures of lunar soils. Therefore, it provides an opportunity to establish a new reliable standard area for in-flight reflectance calibration.Our study will focus on: a)convert the in-situ radiometrically calibrated Chang’E-3 VNIS data into reflectance, photometrically correct the reflectance data into standard illumination condition, and then estimate its uncertainties; b)select a flat, compositionally homogeneous area as standard calibration target, using the topography data derived from the Chang’E-3 Panoramic Camera observations and compositional data obtained by instruments such as the Chang’E-3 APXS spectrometer; c) compare the Apollo 16 standard area's different reflectance data measured by other lunar missions, the reflectance of which are calibrated by our calibration target, and the accuracy is evaluated.

反射率定标区的建立将会极大地提高探月光谱仪器的反射率定标精度。最近研究结果表明以APOLLO 16采样点附近高地地区作为反射率定标区会存在误差，因为该地区月壤返回样品的实验室反射率与真实月表存在较大差异。搭载在“玉兔号”月球车上的嫦娥三号红外成像光谱仪将首次对月表进行就位探测，并且该仪器携带有定标板，具备在轨定标能力，这相当于将实验室光谱测量方式搬到月表，从而为嫦娥三号就位探测区建成反射率定标区提供数据基础和保证。本研究首先利用嫦娥三号在轨辐射定标后的辐亮度数据进行反射率计算，重点研究就位探测下反射率数据的光度校正方法并进行误差分析，从而得到准确的反射率数据；其次结合嫦娥三号全景相机地形数据和粒子激发X射线谱仪等物质成分数据，选取地形平坦、物质成分均一且反射率一致的区域作为定标区。最后利用该定标区反射率对其他光谱仪在相同区域的反射率进行定标，通过对比定标前后的反射率差异来评价定标区精度。

本项目主要对嫦娥三号红外成像光谱仪四次光谱探测数据进行了反射率计算和光度校正，同时利用嫦娥二号1.5米DOM数据对着陆区附近4KM×4KM区域范围内的地形进行了分析，分析结果表明除了三个较大的撞击坑外，着陆区附近区域整体较为平坦；嫦娥三号粒子激发X射线谱仪（APXS）在两次不同探测位置的元素探测结果十分接近，表明该区域内的物质成分较为均一。除此之外，嫦娥三号红外成像光谱仪四次光谱数据光谱归一化结果表明该区域内的反射率光谱特征在450nm-1750nm范围内几乎重合，表明该区域内的反射率特性较为一致。因此我们选取了紫薇撞击坑西部400米*400米平坦区域作为反射率定标区，同时考虑避免撞击坑溅射物和着陆区羽流对反射率数据的影响，选择第三次探测数据（探测点7，N203）作为标准反射率数据，对M3及SP光谱数据进行了定标，定标结果表明同一位置两种不同载荷的光谱数据差异明显减小，同时证明了嫦娥三号着陆就位探测区作为反射率定标区的可行性。