Airborne sampling of the reflectivity by the hyperspectral line scanner in a visible and near infrared wavelengths

摘要

Hyperspectral remote sensing is and was the focus of attention of the EARSeL. In the paper we present use of the novel type V9 (ImSpector) of the hyperspectral line scanner (HSLS) in visible and near infrared wavelengths aimed to sample reflectivity of the ground surface by airborne remote sensing. The airborne system was developed for the aerial sampling of hyper spectral data, while the platform was the helicopter Bell-206. The system was used in 2003 and first data were collected above minefields and suspected areas in two regions that have very different terrain characteristics, climate and other factors. Spectral resolution of the used HSLS V9 is up to 100 channels in the wavelengths from 430 nm to 900 nm, while the spatial resolution is determined by focal length of the optical objective, dimensions of the chip, width of the slit and distance h to the ground. The HSLS collects up to 24 spectral samples per second in a form of a gray level image. This image shows intensity of the reflected waves, and pixels at the right side of the image contain information about incident waves, collected by diffuse collector. Ratio of reflected and incident value at the wavelengthλ_i is coefficient of reflection at this wavelength. Waves reflected by ground surface are collected at nadir, by optical objective, incident waves are collected by diffuse collector oriented to the Sun. HSLS measures at height h above ground, this produces difficulties to the radiometric calibration. The values E_(inc) and E_(refl) measured by HSLS enable calculation of the reflection coefficient r according to the relation r = [G_(refl)E_(refl)/F(α)]/[G_(inc)E_(inc)/F(θ)]. While the gains of the channels G_(refl) and G_(inc) were not determined yet, instead of coefficient of reflection r, obtained value is measure of reflectivity r(G_(inc)/G_(refl)) and has maximum values greater than one. While for navigation of helicopter Bell-206 was not available inertial measuring unit, and for this purpose was used GPS and moving map with pilot in the loop, the spatial accuracy of hyper spectral samples is mainly limited by the accuracy of GPS data and undesired movements of the platform (heading, pitch, yaw). The common effect of the undesired variation ΔW of the width of the strip and of its displacement dy, can be conservatively estimated by max (|ΔW| + |dy|) < 0.1 W, W is width of the line sample. By use of described system were collected hyperspectral samples of the reflectivity above the minefields together with digital images in visible and near infrared wavelengths. Radiometric calibration and increase of spatial accuracy by use of inertial navigation measuring system will be realized in next phase of the system development.