HYPERNETS Hyperspectral Surface Reflectance Measurements for Satellite Validation from the Wytham Woods site in the United Kingdom

This dataset provides quality-checked L2B surface reflectance data for the HYPERNETS Wytham Woods site in the United Kingdom (WWUK). Data is only provided for periods that are considered suitable for satellite validation (e.g. spatially homogeneous and clear sky; see https://landhypernet.org.uk/site_descriptions/WWUK for the different deployment periods and whether they are valid). Data is included until the 1st of October 2025. For near-real-time data, monthly zip files, different product levels, and querying the data, please visit the LANDHYPERNET data portal (www.landhypernet.org.uk). The data products are described in detail in https://hypernets-processor.readthedocs.io/en/latest/content/atbd/products.html and De Vis et al. (2024b). The HYPERNETS project (www.hypernets.eu; Ruddick et al. 2024) has the overall aim to ensure that high quality in situ measurements are available to support the (VNIR/SWIR) optical satellite products. Therefore, it established a new autonomous hyperspectral spectroradiometer (HYPSTAR® - www.hypstar.eu; Kuusk et al. 2024) dedicated to land and water surface reflectance validation with instrument pointing capabilities. This instrument has been deployed over various sites covering a range of water and land types and a range of climatic and logistic conditions. The HYPSTAR®-XR (eXtended Range) instruments deployed at each land HYPERNETS site consist of a VNIR and a SWIR sensor and autonomously collect data between 380-1680 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 and the SWIR sensor has 220 channels between 1000 and 1680 nm with a FWHM of 10 nm. The hypernets_processor (De Vis et al. 2024b) automatically processes all this data into various products, including the L2B quality-checked surface reflectance products provided here. All of the products have associated uncertainties (divided into random and systematic uncertainties, including error-correlation information) which were propagated using the CoMet toolkit (www.comet-toolkit.org). The Wytham Woods (WWUK) site is a deciduous broadleaf forest comprised primarily of Oak, Hazel, Ash, Sycamore and Beech. It is located approximately 5 km North-West of Oxford, UK and has an extensive history of scientific research. The site follows the typical seasonal dynamics of a temperate forest with distinctive periods of leaf-off, green up and senescence across the growing season. By UK woodland standards, the forest is dense; it features a primary canopy and short understory layer. The HYPERNETS site itself (51.777206 degrees N, 1.338494 W), is located at a height of 28 m upon a flux tower in the centre of the forest. The HYPSTAR-XR sensor was installed in October 2021. Data are collected every 30 minutes between 9am and 6pm local time between viewing zenith angles of 0 and 30 degrees. For an example using these data for satellite validation, see Morris et al. (2024). The provided NetCDF files are the L2B hypernets products with surface reflectances, their associated uncertainties and error-correlation information. The reflectance in these products is the Hemispherical-conical Reflectance Factor (HCRF) defined as: HCRF = π L / E where L is the conical upwelling radiance (with field of view of 5 degrees) and E 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 a given geometry (combination of viewing zenith and azimuth angle). 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, number of valid scans used, 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. To obtain this dataset, we start from the full WWUK data record and omit data that does not pass the relevant quality checks (QC). Some QC are performed during the processing up to L2A (see https://hypernets-processor.readthedocs.io/en/latest/content/atbd/processing/quality_checks.html). Then, a number of site-specific QC are performed as post-processing to produce the L2B files (see https://hypernets-processor.readthedocs.io/en/latest/content/atbd/processing/post_processing.html). These site-specific QC cover things such as removing flags in the L2A data, avoiding periods with bad deployment conditions, removing unsuitable viewing and solar angles, as well as poorly performing wavelength ranges and individual sequences. Any potential misalignment of the sensor is also corrected, affecting L1D irradiances, and L2B reflectances. These corrected data are then used in a more stringent clear sky check, and in a check that verifies the reflectances are within realistic ranges for a given angle and time of year for the given site. Any measurements that fail any of these checks are omitted from the L2B data. Unfortunately, due to a combination of many cloudy days, instrument outages, and periods of spatial heterogeneity (due to leaves-off from October to May), there was not enough data in 2024 and 2025 to do reliable QC. Thus, no L2B data have been produced for these periods and the current zenodo dataset only extends to October 2023. The next update to this Zenodo dataset is expected in February 2026 and will include all L2B data up to end of 2025.本数据集为英国怀瑟姆森林站点（Wytham Woods, WWUK）的HYPERNETS（高光谱网络项目）提供经过质量核查的L2B地表反射率数据。数据仅适用于卫星验证的适宜时段（如空间均一、晴空条件；详见https://landhypernet.org.uk/site_descriptions/WWUK 了解各部署时段及其有效性）。数据覆盖至2025年10月1日。如需获取近实时数据、月度压缩包、不同产品层级的数据或进行数据查询，请访问LANDHYPERNET数据门户（www.landhypernet.org.uk）。相关数据产品的详细说明见https://hypernets-processor.readthedocs.io/en/latest/content/atbd/products.html 及De Vis等人（2024b）的研究。 HYPERNETS（高光谱网络项目，www.hypernets.eu；Ruddick等人，2024）的整体目标是提供高质量的原位测量数据，以支撑可见近红外/短波红外（VNIR/SWIR）光学卫星产品的研发与验证。为此，项目研发了一款新型自主式高光谱辐射计（HYPSTAR®，www.hypstar.eu；Kuusk等人，2024），该设备具备仪器指向能力，专门用于陆地与水面地表反射率的验证工作。该设备已部署于涵盖多种水陆类型、气候与运维条件的多个站点。 部署于各陆地HYPERNETS站点的HYPSTAR®-XR（扩展量程，eXtended Range）仪器包含一台可见近红外（VNIR）传感器与一台短波红外（SWIR）传感器，可在380~1680 nm波段范围内自主采集数据，支持多种观测几何条件，并将数据传输至中央服务器以进行质量控制与处理。其中VNIR传感器覆盖380~1000 nm波段，包含1330个通道，光谱分辨率半高全宽（Full Width at Half Maximum, FWHM）为3 nm；SWIR传感器覆盖1000~1680 nm波段，包含220个通道，光谱分辨率半高全宽为10 nm。hypernets_processor（De Vis等人，2024b）可自动将所有采集数据处理为多种产品，包括本文提供的经过质量核查的L2B地表反射率产品。所有产品均附带相关不确定性（分为随机不确定性与系统不确定性，包含误差相关信息），此类不确定性通过CoMet工具箱（CoMet toolkit，www.comet-toolkit.org）进行传播计算。 怀瑟姆森林（WWUK）站点是一片以橡树、榛树、白蜡树、桐叶槭与山毛榉为主的落叶阔叶林，位于英国牛津西北约5 km处，拥有悠久的科学研究历史。该站点符合温带森林的典型季节动态特征，在生长季内会呈现出落叶、返青与衰老的鲜明周期。按照英国林地标准，这片森林林分茂密，包含主林冠层与低矮的林下植被层。本HYPERNETS站点（北纬51.777206°，西经1.338494°）设置在森林中央通量塔上，海拔高度为28 m。HYPSTAR®-XR传感器于2021年10月安装完成。数据采集时段为当地时间9:00至18:00，每30分钟采集一次，观测天顶角范围为0~30°。如需了解利用此类数据开展卫星验证的示例，可参考Morris等人（2024）的研究。 本次提供的NetCDF（网络通用数据格式，Network Common Data Form）文件为包含地表反射率、相关不确定性与误差相关信息的L2B级HYPERNETS产品。此类产品中的反射率为半球锥面反射因子（Hemispherical-conical Reflectance Factor, HCRF），其定义为：HCRF = π L / E，其中L为锥面向上辐射亮度（视场角为5°），E为半球形向下入射辐照度（包含直接太阳辐射与散射天空辐射）。此类反射率包含波段与序列两个维度，其中每个序列对应一组特定几何条件下的测量数据（观测天顶角与方位角的组合）。除波段与带宽相关变量外，文件还包含每个序列的采集时间、观测与太阳角度、有效扫描次数以及质量标志（本数据集提供的数据通常未设置质量标志）。此类NetCDF文件还包含作为属性存储的其他相关元数据。 本数据集的构建流程为：首先获取完整的WWUK数据记录，剔除未通过相关质量检查（QC）的数据。部分质量检查在L2A级数据处理阶段完成（详见https://hypernets-processor.readthedocs.io/en/latest/content/atbd/processing/quality_checks.html）。随后，通过一系列站点专属的后处理质量检查，生成L2B级文件（详见https://hypernets-processor.readthedocs.io/en/latest/content/atbd/processing/post_processing.html）。此类站点专属质量检查包括：剔除L2A级数据中的质量标志、规避部署状态不佳的时段、移除不适宜的观测与太阳角度、剔除性能不佳的波段范围与单个测量序列，同时校正传感器可能存在的配准偏差，该校正会影响L1D级辐照度与L2B级反射率数据。校正后的数据将用于更严格的晴空条件检查，以及验证反射率在对应站点的特定角度与季节时段内是否处于合理范围。任何未通过上述任一检查的测量数据均会被从L2B级数据中剔除。 遗憾的是，由于多云天数较多、仪器停运以及空间异质性时段（10月至次年5月叶片脱落导致）的叠加影响，2024年与2025年的可用数据不足以开展可靠的质量检查。因此，此期间未生成L2B级数据，当前的Zenodo数据集仅覆盖至2023年10月。该Zenodo数据集的下一次更新预计于2026年2月推出，届时将包含截至2025年底的全部L2B级数据。