DOI: 10.5281/zenodo.7962556
title: HYPERNETS Hyperspectral Surface Reflectance Measurements for Satellite Validation from the Wytham Woods site in the United Kingdom

description: 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.