In recent decades, numerous scientific field-instrument networks have been developed in different contexts and for several purposes. SpecNet, Aeronet, FluxNet, or similar are just a few examples. One promising area of research involves the use of automatic field spectrometers, operating in the visible and near-infrared spectral domain, for the purpose of long-term and unattended monitoring of their targets. The applications of field spectroscopy are manifold: from vegetation studies (plant phenology, plant physiology, and phenotyping) to the study of water quality, the atmosphere, and the optical properties of snow/glaciers.
In recent years, JB Hyperspectral Devices, with the support of the scientific community, has developed autonomous instruments, capable of operating continuously in the field in a wide range of harsh climatic conditions (from the equator to the poles) along with dedicated open-source software to process and manage the collected data.
These instruments acquire punctual, high-temporal-resolution hyperspectral data. Thanks to a robust and standardized data processing chain (based on R: A language and environment for statistical computing), they return time series of radiometric parameters as well as optical properties related to the monitored targets. JB devices are made to synchronously acquire upwelling and downwelling radiance, optimizing the acquisition speed according to light conditions and acquiring the dark current signal at each measurement cycle. For example, the FloX (Fluorescence BoX), features a high-performing spectrometer (FWHM: 0.3 nm, SSI 0.15, SNR 1000) and allows continuous measurement of solar-induced chlorophyll fluorescence (SIF) emission. Furthermore, continuous measurements of spectral down-welling and up-welling radiance, using an additional spectrometer in the VIS-NIR range, allows for the computation of reflectance and different spectral indices. Another example of JB’s instruments is the RoX (Reflectance BoX), which is a simplified version of the FloX. It contains only the VIS-NIR spectrometer and therefore it is designed to measure reflectance and spectral indices.
To further utilize the data collected with these devices, JB is currently in the process of making these instruments compatible with the latest flux data collection and processing systems operated in typical flux stations (e.g. SmartFlux from LI-COR). This approach is aimed to help with accurate synchronization and improved compatibility of various types of data collected by different devices at flux stations. Moreover, a collaboration with existing consolidated flux networks (e.g. FluxNet) is ongoing to explore better coupling between flux, optical, and remote sensing measurements. The possibility to integrate JB devices into flux networks, via a standardized procedure of configuration, set up, data processing, and different data product levels are further evaluated.
In the view of satellite cal/val activities, the need of multiple instruments distributed around the globe is furthermore introduced. A specific study, conducted on a subset of nine RoX and FloX field spectrometer systems, deployed across the northern hemisphere in Europe, US, Africa and China was conducted, with the purpose being to validate ESA’s Seninel-2 time-series. The instruments were installed above agricultural fields, broadleaf forest, savannah and alpine pasture for timeframes ranging from several months to up to three years, continuously measuring hyperspectral reflectance data in the visual/NIR range between 350nm and 900nm. Spatially temporal clustered time-series were validated using the ground-truth for clear-sky conditions. Our results suggest a good agreement with R² larger than 0.5 between field spectrometers and satellite bands. Vegetation indices agree very well with R² larger than 0.7, e.g. NDVI with R² =0.89 between Sentinel and field spectrometer subsets across all sites. Our results suggest that RoX and FloX monitoring field spectrometers are a valid ground truth for known vegetation targets. In terms of reflectance bands, they are in good agreement with the Sentinel-2 satellite time-series and in terms of Vegetation Indices, they are in very good agreement.
In this contribution, we present an overview of the state-of-the-art JB hyperspectral devices with applications ranging from SIF, water, atmosphere and snow studies. Finally, point field spectrometers are put forth to promote the potential for a future ground-based network of devices with multiple purposes: from investigating ecosystem properties to the validation of satellite products. In this way, field spectrometers can contribute to guiding the application of ground-based remote sensing practices towards an improved insight into vegetation, water and snow across scales, as well as bridging information between ground and satellite.