Abstract: The present invention is an apparatus and method for calibrating a downward viewing image acquisition system. The apparatus comprises a calibration panel with calibrative material of known reflectivity. The calibrative material coats the panel surface or is pulled across its surface or pulled across its frame so as to maintain a consistent reflectivity and/or emissivity. A housing is provided which protects the calibration panel from the deteriorative effects of natural elements. The housing alternately exposes the calibration panel to the downward viewing image acquisition system as a calibration exposure and covers the calibration panel after the calibration exposure.