Accurate analyses of satellite-detected data require calibration to ground-surface reflectance to remove effects of atmospheric scatter and absorption. Calibration requires pairing satellite data with reflectance measured near the ground. A nonimaging spectrometer flown aboard a light aircraft is a solution that enables application of the empirical line calibration technique. Collecting spectra by aircraft requires that the data contain geoposition information with the aircraft flown sufficiently low to pair the spectrometer data with the size of the satellite-generated data pixels while minimizing the atmospheric path length between the sensor and the ground—hence, generating quasi-ground level reflectance. Several additional factors must be solved for accurate spectrometer application. These include (i) changes to the aircraft to enable nadir view of the ground, accomplished with simple engineering and installation through the plane’s belly, (ii) on-the-fly calibration for changing light quality during the flight, solved by an uplooking Teflon membrane, accessed by fiber optic and a port through the cabin and upper wing section; (iii) spectrometer aiming, a factor of aircraft attitude during data collection that can be solved empirically or mathematically, and (iv) spatial uncertainty for the spectrometer aim point due to uncontrolled aircraft pitch and roll, solved by identification of areas of low variability on the satellite image to be paired with spectrometer data. Although measurement of quasi-ground-level reflectance using this system may be performed at times other than satellite overpass, a crucial consideration is the ephemeral nature of the phenomena that are being investigated.
