Abstract
During the last years commercial hyperspectral imaging sensors have been miniaturized and their performance has been demonstrated on Unmanned Aerial Vehicles (UAV). However currently the commercial hyperspectral systems still require minimum payload capacity of approximately 3 kg, forcing usage of rather large UAVs. In this article we present a lightweight hyperspectral mapping system (HYMSY) for rotor-based UAVs, the novel processing chain for the system, and its potential for agricultural mapping and monitoring applications. The HYMSY consists of a custom-made pushbroom spectrometer (400–950 nm, 9 nm FWHM, 25 lines/s, 328 px/line), a photogrammetric camera, and a miniature GPS-Inertial Navigation System. The weight of HYMSY in ready-to-fly configuration is only 2.0 kg and it has been constructed mostly from off-the-shelf components. The processing chain uses a photogrammetric algorithm to produce a Digital Surface Model (DSM) and provides high accuracy orientation of the system over the DSM. The pushbroom data is georectified by projecting it onto the DSM with the support of photogrammetric orientations and the GPS-INS data. Since an up-to-date DSM is produced internally, no external data are required and the processing chain is capable to georectify pushbroom data fully automatically. The system has been adopted for several experimental flights related to agricultural and habitat monitoring applications. For a typical flight, an area of 2–10 ha was mapped, producing a RGB orthomosaic at 1–5 cm resolution, a DSM at 5–10 cm resolution, and a hyperspectral datacube at 10–50 cm resolution.
Highlights
Over the past decades, hyperspectral imaging or imaging spectroscopy has been established as a remote sensing technology that allows quantitative characterization of the earth system [1]
This orientation data is acquired with a GPS-Inertial Navigation System (GPS-INS) synchronized to the pushbroom sensor exposures
As the first step of radiometric processing, the spectrometer images are converted from digital number (DN) to radiance (L) units using a pixelwise dark current and flat field calibration according to the following equation: ij ij DN ij − DNDarkCurrent ij ij DNFlatField − DNDarkCurrent where DN ij is the intensity of a pixel in the spectrometer image at cross-track index i and spectral ij dimension index j, DNDarkCurrent is the intensity of the same pixel in the dark current calibration ij image, DNFlatField is the pixel intensity in the flat field calibration image, and LFlatField is the flat field radiance at the central wavelength of the pixel
Summary
Hyperspectral imaging or imaging spectroscopy has been established as a remote sensing technology that allows quantitative characterization of the earth system [1]. Pushbroom data are commonly georeferenced using a processing method called direct georeferencing [12] where the scan lines are projected onto a Digital Surface Model (DSM) or a flat ground based solely on external orientation data (X, Y, Z, Roll, Pitch ,Yaw) This orientation data is acquired with a GPS-Inertial Navigation System (GPS-INS) synchronized to the pushbroom sensor exposures. Because of such high requirements, the GPS-INS units are often the heaviest and most expensive components in the current UAV pushbroom systems Another requirement for direct georeferencing is the availability of a DSM. By collecting aerial images simultaneously with the pushbroom data, it is possible to perform accurate and fully automated georeferencing without the need for a high quality GPS-INS system or any external. We demonstrate a case study dataset collected using HYMSY and briefly evaluate its performance in an agricultural mapping application
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