AIRBORNE SPECTROMETER MEASUREMENTS FMOM BOREAL SNOW-COVERED LANDSCAPE

Sampling: The airborne hyperspectral data was acquired with an AisaDUAL imaging spectrometer manufactured by Spectral Imaging Ltd (SPECIM). The airborne spectral measurements were made from helicopter at an altitude of 800 m producing a spatial resolution of 0.8 m and swath of 240 m. The flight lines were several kilometers long. All measurements were carried out in direct illumination (i.e. clear sky: 0/8 to 2/8 cloud cover). The Oxford Technical Solutions RT4000 GPS/INS was utilized to enable high accuracy measurements with low drift rates. The instrument foreoptics unit was set to look at nadir (0 degrees) direction and the field of view (FOV) was 17 degrees. Processing: The AisaDUAL spectrometer data was radiometrically and geometrically corrected by using CaliGeo tool by SPECIM in the ENVI software by Bestcomp 97. Kft. The original spectral resolution for the VNIR bands was 5 nm and for SWIR bands 6 nm totaling to 359 spectral bands. Airborne radiances were calibrated by comparing them to concurrent radiances measured from 30 meter high mast from forest site and determining calibration coefficients to airborne data by using least squares fitting technique. The original spatial resolution of the data is 80 cm x 80 cm. From this data the bands 555 nm, 645 nm, 858.5 nm and 1640 nm were extracted from the spectra by using the band specific FWHM criterion (Full width at half maximum) corresponding to MODIS bands. For these bands the data was filtered with mean filter using 12x12 window corresponding to pixel size of 10 metre. Reflectances were calculated by using simultaneously measured radiance from a white Spectralon panel at a particular wavelength to describe incoming radiation. Spectralon measurements were done with a well calibrated spectrometer installed on 30 m high mast. The utilized instrument of the mast was Field Spec Pro JR from Analytical Spectral Devices Inc (ASD).