Abstract
This study describes the development of a small hyperspectral Unmanned Aircraft System (HyUAS) for measuring Visible and Near-Infrared (VNIR) surface reflectance and sun-induced fluorescence, co-registered with high-resolution RGB imagery, to support field spectroscopy surveys and calibration and validation of remote sensing products. The system, namely HyUAS, is based on a multirotor platform equipped with a cost-effective payload composed of a VNIR non-imaging spectrometer and an RGB camera. The spectrometer is connected to a custom entrance optics receptor developed to tune the instrument field-of-view and to obtain systematic measurements of instrument dark-current. The geometric, radiometric and spectral characteristics of the instruments were characterized and calibrated through dedicated laboratory tests. The overall accuracy of HyUAS data was evaluated during a flight campaign in which surface reflectance was compared with ground-based reference measurements. HyUAS data were used to estimate spectral indices and far-red fluorescence for different land covers. RGB images were processed as a high-resolution 3D surface model using structure from motion algorithms. The spectral measurements were accurately geo-located and projected on the digital surface model. The overall results show that: (i) rigorous calibration enabled radiance and reflectance spectra from HyUAS with RRMSE < 10% compared with ground measurements; (ii) the low-flying UAS setup allows retrieving fluorescence in absolute units; (iii) the accurate geo-location of spectra on the digital surface model greatly improves the overall interpretation of reflectance and fluorescence data. In general, the HyUAS was demonstrated to be a reliable system for supporting high-resolution field spectroscopy surveys allowing one to collect systematic measurements at very detailed spatial resolution with a valuable potential for vegetation monitoring studies. Furthermore, it can be considered a useful tool for collecting spatially-distributed observations of reflectance and fluorescence that can be further used for calibration and validation activities of airborne and satellite optical images in the context of the upcoming FLEX mission and the VNIR spectral bands of optical Earth observation missions (i.e., Landsat, Sentinel-2 and Sentinel-3).
Highlights
Field spectroscopy is an essential technique to gain valuable insights on Earth’s surface optical properties
The statistical indices considered in the comparison were the Root Mean Square Error (RMSE), which quantifies the differences between retrieved values (i.e., hyperspectral Unmanned Aircraft System (HyUAS) data) and assumed ground-truth (i.e., FieldSpec HH data), and the Relative RMSE (RRMSE%), which represents the percentage of error
The standard deviations observed translate to an uncertainty of the footprint size and center lower center lower than 10−2 m, considering a flight altitude equal to 10 m
Summary
Field spectroscopy is an essential technique to gain valuable insights on Earth’s surface optical properties. Sensor miniaturization often entails lower data quality in terms of signal-to-noise ratio and spatial/spectral resolving power For these reasons, applications that make use of small UAS are mostly based on RGB or VNIR multispectral cameras [41,45]. Particular, Zarco-Tejada processing techniquesand are natural more complex and technically expensive.InHowever, a few pioneering et al. UAS and imaging spectrometer collect reflectance signature, great potential of spectral data with high spatial to and temporal resolution, for example Photochemical in monitoring agricultural natural. The dimension of the area sampled at the ground can vary between 0.5 and 12 m considering a flight altitude ranging from 5 to 50 m This optical setup, commonly referred to as Gershun tube, has a two-fold advantage of reducing the spectrometer acceptance angle and providing a more uniform spot measurement.
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