Analysis of Projection Effects in OSIRIS-REx Spectral Mapping Methods

We search for an optimized mapping protocol and explore the characteristics of mapping remote sensing observations from a point spectrometer onto a shape model comprised of triangular facets - in the context of NASA's Asteroid Sample Return Mission: OSIRIS-REx. The shape model used was created from ground based radar observations by \citet{nolan2013shape}. Our study is conducted before the spacecraft arrives at Bennu and we use observational sequence plans of the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS). We explore 6 methods of mapping data to the shape model facets, and using 3 different shape model spatial resolutions. We also attempt to increase map fidelity by increasing the observational coverage of the surface. We find that: (A) increasing shape model resolution increases mapping quality. However, once the shape model's mean facet edge length is smaller than about two fifths of the diameter of the instrument's field of view (FOV - or ``spot" size), the increase in quality tapers off and may not be worth the additional computation time. (B) The 6 mapping methods we have conceived can be broken into two categories: 1) selecting or 2) averaging observations when assigning values to each facet of the shape model. The quality differences between similar methods that average values from different spots together are insignificant, i.e. averaging, taking the median, or using the weighted average. Selecting the nearest observation to a facet best preserves an enclosed outcrop shape and signal, but averaging spots is more conservative against errors in spot positioning or signal. (C) A completely enclosed outcrop border expands into the surrounding region by 0.8-1.5 radii of the instrument's FOV after mapping. (D) Regions smaller than the instrument's FOV are present (detectable) in resulting maps, however their signal strength is reduced as a function of their size relative to the instrument FOV. We expected the sub-FOV detectability to depend on whether the contrast between the signal of the sub-FOV region and the surrounding area is greater than the instrument uncertainty. Lastly, (E) we have demonstrated that mapping quality can be improved by increasing the number of observations that cover the surface. For OSIRIS-REx, increasing the observation count requires adding together observations from the 3 dayside surveys that were obtained at different phase angles. Uncertainties due to photometric correction of these observations to a standard viewing and illumination geometry can result in systematic differences that result in map artifacts. We suggest that if combining data sets from different phase angles while mapping, averaging the spots together can help to de-emphasize any small systematic differences.

本研究以美国国家航空航天局（National Aeronautics and Space Administration, NASA）的小行星样本返回任务OSIRIS-REx为背景，旨在探索优化的映射协议，并研究将点式光谱仪（point spectrometer）的遥感观测数据映射至由三角面元（triangular facets）构成的形状模型（shape model）的相关特性。本研究使用的形状模型由Nolan等（2013）基于地基雷达观测数据构建而成。本研究在航天器抵达贝努小行星（Bennu）之前开展，采用了OSIRIS-REx可见红外光谱仪（OSIRIS-REx Visible and Infrared Spectrometer, OVIRS）的观测序列计划。我们探索了6种将数据映射至形状模型面元的方法，并使用了3种不同的形状模型空间分辨率，同时尝试通过提升地表观测覆盖度来提升映射保真度（map fidelity）。

我们得到如下研究结论：
（A）提升形状模型分辨率可改善映射质量，但当形状模型的平均面元边长小于仪器视场（field of view, FOV，即"光斑"尺寸（spot size））直径的约2/5时，质量提升的幅度会趋于平缓，额外投入的计算时间可能不再值得。
（B）我们提出的6种映射方法可分为两类：在为形状模型各面元赋值时，1）选取观测数据，或2）对观测数据取平均。将不同光斑的数值进行平均的同类方法之间，质量差异并不显著，例如直接平均、取中位数或采用加权平均。选取与面元距离最近的观测数据，能最佳保留封闭露头（outcrop）的形态与信号，但对光斑定位或信号误差而言，取光斑平均的方法则更为稳健。
（C）经映射后，完全封闭的露头边界会向周边区域扩展约仪器视场半径的0.8至1.5倍。
（D）在最终生成的映射图中，存在小于仪器视场的区域（可被探测到），但其信号强度会随其相对于仪器视场的尺寸大小而衰减。我们此前推测，亚视场区域的可探测性取决于该区域信号与周边环境的对比度是否高于仪器的不确定度。
（E）最后，我们证实了提升地表观测覆盖数量可改善映射质量。对于OSIRIS-REx任务而言，增加观测数量需要将3次不同相位角（phase angle）下获取的日照面观测数据进行合并。将这些观测数据校正至标准观测与照明几何时产生的不确定度，会引发系统性差异，进而导致映射伪影（map artifacts）。我们建议，若在映射过程中合并不同相位角的数据集，对光斑取平均的方法有助于弱化微小的系统性差异。

搜集汇总

背景与挑战

背景概述

该数据集聚焦于NASA OSIRIS-REx任务中光谱映射方法的投影效应分析，旨在优化将点光谱仪观测数据映射到小行星形状模型的协议。研究探索了6种映射方法和3种形状模型分辨率，发现提高分辨率可改善映射质量但存在收益递减点，同时比较了选择与平均映射方法的优劣，并指出增加观测覆盖能提升地图保真度，但需注意光学校正可能引入的伪影。