This study aims to assess the spatial and visible/near-infrared (VNIR) colour/spectral capabilities of the 4-band Colour and Stereo Surface Imaging System (CaSSIS) aboard the ExoMars 2016 Trace Grace Orbiter (TGO). The instrument response functions for the CaSSIS imager was used to resample spectral libraries, modelled spectra and to construct spectrally (i.e., in I/F space) and spatially consistent simulated CaSSIS image cubes of various key sites of interest and for ongoing scientific investigations on Mars. Coordinated datasets from Mars Reconnaissance Orbiter (MRO) are ideal, and specifically used for simulating CaSSIS. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) provides colour information, while the Context Imager (CTX), and in a few cases the High-Resolution Imaging Science Experiment (HiRISE), provides the complementary spatial information at the resampled CaSSIS unbinned/unsummed pixel resolution (4.6 m/pixel from a 400-km altitude). The methodology used herein employs a Gram-Schmidt spectral sharpening algorithm to combine the ∼18–36 m/pixel CRISM-derived CaSSIS colours with I/F images primarily derived from oversampled CTX images. One hundred and eighty-one simulated CaSSIS 4-colour image cubes (at 18–36 m/pixel) were generated (including one of Phobos) based on CRISM data. From these, thirty-three “fully”-simulated image cubes of thirty unique locations on Mars (i.e., with 4 colour bands at 4.6 m/pixel) were made. All simulated image cubes were used to test both the colour capabilities of CaSSIS by producing standard colour RGB images, colour band ratio composites (CBRCs) and spectral parameters. Simulated CaSSIS CBRCs demonstrated that CaSSIS will be able to readily isolate signatures related to ferrous (Fe2+) iron- and ferric (Fe3+) iron-bearing deposits on the surface of Mars, ices and atmospheric phenomena. Despite the lower spatial resolution of CaSSIS when compared to HiRISE, the results of this work demonstrate that CaSSIS will not only compliment HiRISE-scale studies of various geological and seasonal phenomena, it will also enhance them by providing additional colour and geologic context through its wider and longer full-colour coverage (∼9.4×50$\sim9.4 \times 50$ km), and its increased sensitivity to iron-bearing materials from its two IR bands (RED and NIR). In a few examples, subtle surface changes that were not easily detected by HiRISE were identified in the simulated CaSSIS images. This study also demonstrates the utility of the Gram-Schmidt spectral pan-sharpening technique to extend VNIR colour/spectral capabilities from a lower spatial resolution colour/spectral dataset to a single-band or panchromatic image greyscale image with higher resolution. These higher resolution colour products (simulated CaSSIS or otherwise) are useful as means to extend both geologic context and mapping of datasets with coarser spatial resolutions. The results of this study indicate that the TGO mission objectives, as well as the instrument-specific mission objectives, will be achievable with CaSSIS.

中文翻译：

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ExoMars 微量气体轨道器上的彩色和立体表面成像系统 (CaSSIS) 的颜色和空间能力的图像模拟和评估

本研究旨在评估 ExoMars 2016 Trace Grace Orbiter (TGO) 上的 4 波段彩色和立体表面成像系统 (CaSSIS) 的空间和可见光/近红外 (VNIR) 颜色/光谱能力。CaSSIS 成像仪的仪器响应函数用于重新采样光谱库、建模光谱并构建光谱（即，在 I/F 空间中）和空间一致的各种关键关注点的模拟 CaSSIS 图像立方体，以及对火星的持续科学研究. 来自火星侦察轨道器 (MRO) 的协调数据集是理想的，并且专门用于模拟 CaSSIS。紧凑型火星侦察成像光谱仪 (CRISM) 提供颜色信息，而上下文成像仪 (CTX) 以及在少数情况下的高分辨率成像科学实验 (HiRISE)，以重新采样的 CaSSIS 未合并/未求和像素分辨率（400 公里高度为 4.6 m/像素）提供补充空间信息。本文使用的方法采用 Gram-Schmidt 光谱锐化算法，将 ~18–36 m/pixel CRISM 衍生的 CaSSIS 颜色与主要来自过采样 CTX 图像的 I/F 图像相结合。基于 CRISM 数据生成了 181 个模拟的 CaSSIS 4 色图像立方体（18-36 m/像素）（包括一个火卫一）。从这些中，制作了火星上三十个独特位置的三十三个“完全”模拟图像立方体（即，具有 4.6 m/像素的 4 个色带）。所有模拟图像立方体都用于通过生成标准彩色 RGB 图像、色带比复合材料 (CBRC) 和光谱参数来测试 CaSSIS 的颜色能力。模拟的 CaSSIS CBRC 表明，CaSSIS 将能够轻松地分离出与火星表面的含亚铁 (Fe2+) 铁和三价铁 (Fe3+) 沉积物、冰和大气现象相关的特征。尽管与 HiRISE 相比，CaSSIS 的空间分辨率较低，但这项工作的结果表明，CaSSIS 不仅将补充 HiRISE 对各种地质和季节性现象的研究，还将通过其更广泛的颜色和地质背景提供额外的颜色和地质背景来增强它们。和更长的全彩覆盖范围（~9.4×50$\sim9.4\times 50$km），以及它对来自两个红外波段（RED 和 NIR）的含铁材料的更高灵敏度。在一些示例中，在模拟的 CaSSIS 图像中识别出 HiRISE 不易检测到的细微表面变化。这项研究还展示了 Gram-Schmidt 光谱全色锐化技术的实用性，可将 VNIR 颜色/光谱功能从较低空间分辨率的颜色/光谱数据集扩展到具有更高分辨率的单波段或全色图像灰度图像。这些更高分辨率的颜色产品（模拟 CaSSIS 或其他）可用作扩展地质背景和具有较粗空间分辨率的数据集映射的手段。这项研究的结果表明，TGO 任务目标以及特定于仪器的任务目标将可以通过 CasSIS 实现。这些更高分辨率的颜色产品（模拟 CaSSIS 或其他）可用作扩展地质背景和具有较粗空间分辨率的数据集映射的手段。这项研究的结果表明，TGO 任务目标以及特定于仪器的任务目标将可以通过 CasSIS 实现。这些更高分辨率的颜色产品（模拟 CaSSIS 或其他）可用作扩展地质背景和具有较粗空间分辨率的数据集映射的手段。这项研究的结果表明，TGO 任务目标以及特定于仪器的任务目标将可以通过 CasSIS 实现。