Radiometer calibrations using solar radiation

Airborne radiometric instruments are often used to collect calibrated radiance data, whether for producing remotely- sensed imagery, for use in vicarious calibration, or for atmospheric correction. Typically, these radiometers are calibrated in a laboratory environment using source whose spectral outputs are traceable to some established, man-made standard. In the field, these devices are used with a different source: solar radiation. The use of solar radiation as a calibration source should therefore be considered when calibration radiometers used to collect energy in the solar reflective region. This paper presents a novel method of calibration which makes use of scattered solar radiation as the source. This technique is particularly applicable for airborne radiometers intended to view low-reflectance surfaces, since the magnitude and spectral distribution of the collected energy is very similar to that of skylight, especially at shorter visible wavelengths. The method is applied to visible and near-IR bands of a Barnes Modular Multispectral 8-channel Radiometer. A sensitivity study was performed for the method and an associated uncertainty analysis is presented. The calibration results are compared to a second, more established solar-based method whose source is directly- transmitted solar irradiance.