Initial Sample of HYPERNETS Hyperspectral Water Reflectance Measurements for Satellite Validation from the VEIT site (Italy)
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The HYPERNETS project (www.hypernets.eu) has the overall aim to ensure that high quality in situ measurements are available to support the (VNIR/SWIR) optical Copernicus products. Therefore, it established a new autonomous hyperspectral spectroradiometer (HYPSTAR® - www.hypstar.eu) dedicated to land and water surface reflectance validation with instrument pointing capabilities. In the prototype phase, the instrument is being deployed at 24 sites covering a range of water and land types and a range of climatic and logistic conditions. This dataset provides the first published data for the HYPERNETS site at Aqua Alta, Venice in Italy (VEIT). It is a subset of the complete data record which consists of the best quality VEIT measurements which could be used for satellite validation. The provided NetCDF files are the L2A hypernets products with water leaving radiance and reflectances, with and without NIR Similarity Correction (see Ruddick et al., 2006, DOI:10.2307/3841124). The reflectance in the L2A products is the Water Reflectance without NIR Similarity Correction (referred to as reflectance_nosc in the file) defined as: \(\rho_wnosc=\pi (Lu-\rho_FLd)/E_d\) where Lu is the upwelling radiance (at 40° zenith angle, and, 90° or 135° azimuth angle relative to the sun), Ld is the downwelling radiance (at 140° zenith angle, and, 90° or 135° azimuth angle relative to the sun). Ed is the (hemispherical) downwelling irradiance (i.e. including both direct solar and diffuse sky irradiance). For the VEIT site, the reflectance corrected for the NIR Similarity correction (epsilon, see Ruddick et al., 2006) is also provided: \(\rho_w=\pi (Lu-\rho_FLd)/E_d-\epsilon\) These reflectances have dimensions of wavelength and series, where each series is a set of measurements for the computation of a water reflectance measurement. In addition to variables for wavelength and bandwidth, the files also contain variables that provide for each series the acquisition time, viewing and solar angles, and quality flags (typically no flags are set in the data provided in this dataset). These NetCDF files also contain further relevant metadata as attributes. See https://hypernets-processor.readthedocs.io/ for further info. The HYPSTAR®-SR (Standard Range) instruments deployed at each land HYPERNETS site consist of a VNIR sensor and autonomously collect data between 380-1000 nm at various viewing geometries and send it to a central server for quality control and processing. The VNIR sensor spans 1330 channels between 380 and 1000 nm with a FWHM of 3 nm. The hypernets_processor (Goyens et al. 2021, DOI: 10.1109/IGARSS47720.2021.9553738; De Vis et al. in prep.) automatically processes all this data into various products, including the L2A surface reflectance product provided here. The current dataset is limited to the 400-900 nm range. Uncertainties are not yet included. To obtain this dataset, we start from the full VEIT data record and omit all the data that do not pass all of the quality checks performed as part of the hypernets_processor. In addition, an additional screening procedure was developed to supply the best quality data suitable for satellite validation: 1. The coefficient of variation in water reflectance is below 10% in the 580-600 nm range 2. The water reflectance (after correction for the NIR similarity) above 800 nm is below 0.01
HYPERNETS项目（官网：www.hypernets.eu）的总体目标是提供高质量的原位测量数据，以支持可见光近红外/短波红外（VNIR/SWIR）光学哥白尼计划（Copernicus）产品。为此，该项目研发了一款新型自主式高光谱分光辐射计（hyperspectral spectroradiometer，HYPSTAR®——官网：www.hypstar.eu），专门用于陆地与水面地表反射率验证，具备仪器指向功能。在原型阶段，该仪器已部署在24个站点，覆盖多种水陆地表类型与多样的气候、后勤保障条件。本数据集为意大利威尼斯阿夸阿尔塔（Aqua Alta）HYPERNETS站点（编号VEIT）的首批公开数据，是完整数据记录的子集，选取了可用于卫星验证的高质量VEIT测量数据。 本次提供的网络通用数据格式（NetCDF）文件为L2A级HYPERNETS产品，包含水体离水辐射亮度与反射率，分别对应是否进行近红外相似性校正（NIR Similarity Correction，详见Ruddick等，2006，DOI:10.2307/3841124）。L2A产品中的反射率为未进行近红外相似性校正的水体反射率（文件中记为reflectance_nosc），其定义为： $$ ho_{wnosc}=pi (L_u- ho_F L_d)/E_d$$ 其中，$L_u$为上行辐射亮度（观测天顶角40°，相对太阳的方位角为90°或135°）；$L_d$为下行辐射亮度（观测天顶角140°，相对太阳的方位角为90°或135°）；$E_d$为（半球形）下行辐照度（包含直接太阳辐射与漫射天空辐射）。 针对VEIT站点，本次还提供了经近红外相似性校正（ε，详见Ruddick等，2006）后的水体反射率： $$ ho_w=pi (L_u- ho_F L_d)/E_d-epsilon$$ 上述反射率以波长与序列为维度，其中每个序列代表一组用于计算水体反射率的测量数据。除波长与带宽相关变量外，文件还包含每个序列对应的采集时间、观测与太阳角度，以及质量标记（本数据集提供的数据中通常未设置标记）。这些NetCDF文件还包含作为属性存储的其他相关元数据。更多信息可参见https://hypernets-processor.readthedocs.io/。 部署在每个陆地HYPERNETS站点的HYPSTAR®-SR（标准量程，Standard Range）仪器搭载可见光近红外（VNIR）传感器，可自主采集380~1000 nm波段范围内的多观测几何数据，并将数据传输至中央服务器进行质量控制与处理。该VNIR传感器在380~1000 nm范围内包含1330个通道，半高全宽（FWHM）为3 nm。HYPERNETS处理器（hypernets_processor，详见Goyens等，2021，DOI:10.1109/IGARSS47720.2021.9553738；De Vis等，待发表）可自动将所有数据处理为多种产品，包括本次提供的L2A级地表反射率产品。当前数据集限定在400~900 nm波段范围内，暂未包含不确定性信息。 本数据集的构建流程为：从完整的VEIT数据记录中剔除所有未通过hypernets_processor内置质量检查的数据。此外，还开发了额外的筛选流程，以获取适用于卫星验证的最高质量数据，具体筛选标准如下： 1. 580~600 nm波段范围内的水体反射率变异系数低于10% 2. 经近红外相似性校正后的800 nm以上波段水体反射率低于0.01
