Correction of aircraft pyranometer measurements for diffuse radiance and alignment errors

A method is outlined whereby the orientation of upward pointing pyranometers on an aircraft is fully characterized. The incoming solar flux is corrected by means of a method that incorporates the fact that the flux is composed of partly diffuse and partly direct radiation. After the angle correction is found, a practical method is outlined to find the pyranometer normal with respect to the aircraft normal. It uses the heading of the aircraft and the position of the sun to adjust the pyranometer to the aircraft horizontal level. While the method is general enough to be used for any aircraft, it is applied here specifically for the National Center for Atmospheric Research C‐130 aircraft in the configuration flown during the First Aerosol Characterization Experiment (ACE 1). It was found that when the difference between the sun azimuth angle and the aircraft heading is near either 0° or 180°, special care has to be taken in interpreting the incoming flux, as it is enhanced by a few percent due to reflections off elements of the aircraft frame.