Generating hyperspectral reference measurements for surface reflectance from the LANDHYPERNET and WATERHYPERNET networks

Abstract

The LANDHYPERNET and WATERHYPERNET networks (which together make up the HYPERNETS network) consist of a set of autonomous hyperspectral spectroradiometers (HYPSTAR®) acquiring fiducial reference measurements of surface reflectance at various sites covering a wide range of surface types (both land and water) for use in satellite Earth observation validation and remote sensing applications. This paper describes the processing algorithm for the HYPSTAR® data products. The hypernets_processor is a Python software package to process the LANDHYPERNET and WATERHYPERNET in-situ hyperspectral raw data, collected from the measurement network under the standard measurement protocols, to the designated products, through data transmission and conversion, application of calibration, evaluation of reflectance and other variables, and, archiving for distribution to users. In order to achieve fiducial reference measurement quality, uncertainties are propagated through each step of the processing chain, taking into account temporal and spectral error-covariance. Such detailed uncertainty information is unique for any satellite validation network. We also describe the HYPSTAR® products acquired until 2023–04–31, consisting of 12,190 LANDHYPERNET sequences and 55,514 WATERHYPERNET sequences (of which respectively 11,802 and 44,412 were successfully processed to surface reflectance).

HYPERNETS: a network of automated hyperspectral radiometers to validate water and land surface reflectance (380–1680 nm) from all satellite missions

“…Finally, the HYPERNETS instrument system and data processing is also relevant for vicarious calibration of satellites at top of atmosphere if the deployment site is sufficiently spatially homogeneous and well-characterised. E.g., De Vis et al (2024a) demonstrates use of the HYPSTAR ® at Gobabeb (Namibia) and the Princess Elisabeth Antarctica base for vicarious calibration of Sentinel-2, Landsat-9 and PRISMA. The HYPSTAR ® /Gobabeb site (Sinclair et al, 2023) has been submitted to the official application process to become a RadCalNet site.…”

“…Network data processing and quality control (De Vis et al, 2024a) must be centralised and automated to ensure efficient operations and reliability of data provided to users. Measurements are acquired at the validation sites every 15-30 min during daylight.…”

Validation of satellite water products based on HYPERNETS in situ data using a Match-up Database (MDB) file structure

“…The reflectance at the air-water interface is the product of the air-water interface reflectance factor, ρF (θ, Δφ, θs, ws), taken from Mobley (1999) and with ws being the wind speed, and, the downwelling radiance, L d , measured at a an angle reciprocal to the measurement of L u , i.e., θ-140 °. More details on the HYPSTAR ® data processing are available in Goyens et al, 2021, Goyens et al, 2022and De Vis et al 2024.…”

“…A new automated hyperspectral radiometer, i.e., the HYPSTAR ® (HYperspectral Pointable System for Terrestrial and Aquatic Radiometry), has been developed within the European Union's HORIZON 2020 project HYPERNETS. This radiometer provides high-quality above-water hyperspectral radiometry data in the visible to near infrared (VNIR) from 380 to 1,020 nm with a spectral resolution of 5 nm (or better), and in the shortwavelength infrared (SWIR) from 1,000 to 1700 nm with a spectral resolution of 10 nm (Goyens et al, 2022;De Vis et al, 2024). During the project, HYPSTAR ® radiometers were deployed using fixed platforms at 7 water sites and 7 land sites in order to provide measurements at high spectral and temporal resolutions and for the multi-mission validation of satellite products.…”