Initial Sample of HYPERNETS Hyperspectral Water Reflectance Measurements for Satellite Validation from the LPAR site (Argentina)

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 in Rio de La Plata, LPAR, in Argentina. It is a subset of the complete data record which consists of the best quality LPAR measurements which could be used for satellite validation. 

The provided NetCDF files are the L2A hypernets products with water leaving radiance and reflectances (without NIR Similarity Correction, see Ruddick et al., 2006, DOI:10.2307/3841124). The reflectance in the L2A products is the Water Reflectance (referred to as reflectance_nosc in the file) defined as:

ρwnosc=π(Lu−ρFLd)/Ed

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).

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 LPAR 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 400-900 nm range

2. The water reflectance at 500 nm is below 0.1

HYPERNETS项目（HYPERNETS，www.hypernets.eu）的总体目标是提供高质量原位测量数据，以支持可见近红外（VNIR）/短波红外（SWIR）光学哥白尼（Copernicus）产品。为此，项目研发了一款新型自主式高光谱光谱辐射计（HYPSTAR®，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，该仪器已部署于24个站点，覆盖多种水陆类型与不同气候、后勤条件。本数据集提供了阿根廷拉普拉塔河（LPAR）站点的首批公开数据，该数据集为完整数据记录的子集，选取了可用于卫星验证的最高质量LPAR测量数据。

本次提供的NetCDF文件为L2A级hypernets产品，包含水体离水辐亮度与反射率（未进行近红外相似性校正，参见Ruddick等，2006，DOI:10.2307/3841124）。L2A产品中的反射率为水体反射率（文件中记为reflectance_nosc），其定义为：
ρwnosc=π(Lu−ρFLd)/Ed
其中Lu为上行辐亮度（天顶角40°，相对于太阳的方位角为90°或135°），Ld为下行辐亮度（天顶角140°，相对于太阳的方位角为90°或135°）。Ed为（半球形）下行辐照度，即包含直接太阳辐照度与漫射天空辐照度。

此类反射率以波长与序列为维度，每个序列为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，文件还包含为每个序列提供的采集时间、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的其他相关元数据，更多信息可参见https://hypernets-processor.readthedocs.io/。

部署于每个陆地HYPERNETS站点的HYPSTAR®-SR（标准量程）仪器包含一台可见近红外（VNIR）传感器，可在380~1000 nm波段范围内以多种观测几何条件自主采集数据，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至1000 nm范围内覆盖1330个通道，半高全宽（FWHM）为3 nm。hypernets_processor（Goyens等，2021，DOI:10.1109/IGARSS47720.2021.9553738；De Vis等，待发表）可将所有数据自动处理为多种产品，包括本文提供的L2A级地表反射率产品。当前数据集仅限定于400~900 nm波段范围，暂未包含不确定性信息。

本数据集的构建始于完整的LPAR数据记录，剔除了所有未通过hypernets_processor执行的全部质量检查的数据。此外，还开发了额外的筛选流程以提供适用于卫星验证的最高质量数据：
1. 400~900 nm波段范围内水体反射率的变异系数低于10%
2. 500 nm处的水体反射率低于0.1