Automatic ground-based station for vicarious calibration

Vicarious calibration generally requires field activities in order to characterise surface reflectances and atmosphere to unable us the prediction of the radiance at the satellite level. To limit human presence on the field, an automatic ground-based station was defined as well as the required protocol to achieve satellite vicarious calibration. The solar irradiance measurements are self calibrated using the Langley technique. The instrument was designed so that, firstly, the same gun measures both the solar irradiance and the radiance (sky or ground) and, secondly, that the field of view is constant over the spectral range. These two conditions offer an intercalibration opportunity between radiance and irradiance as well as the field of view is well defined. Experimental determination of the field of view is possible in U.V. region based on the Rayleigh scattering. We, then, describe how to derive the TOA signal from measurements. Two approaches have been developed according the energetic characteristics we want to estimate (reflectance or radiance). Preliminary results of a field campaign in June 1997 are reported.