A MULTICHANNEL CLOUD PYRANOMETER SYSTEM FOR AIRBORNE MEASUREMENT OF SOLAR SPECTRAL REFLECTANCE BY CLOUDS

The design and performance of a spectral radiometer system are describ ed for airborne measurements of solar flux reflectance by clouds. The system consists of a pair of identical multichannel pyranometers: one installed on the top and the other on the bottom of an aircraft fusela ge to measure the downward and upward solar irradiances, respectively. This measurement scheme has an advantage in that reflectances derived from ratios between the upward and downward irradiances can avoid the need for absolute radiometric calibrations. The multichannel cloud py ranometer (MCP) system measures near-monochromatic solar irradiances a t nine discrete wavelengths between 420 and 1650 nm by using interfere nce filters with very narrow bandwidths. Included among these waveleng ths are 760 and 938 nm in the oxygen and water vapor absorption bands, respectively. Solar radiation passing through the filters is instantl y detected by a silicon photodiode for wavelength lambda < 1 mu m and by a germanium photodiode for lambda > 1 mu m. Good performance of the MCP system was confirmed through laboratory calibrations and airborne tests. The MCP system is suitable for remote sensing application to r etrieve cloud physical parameters of water clouds from airborne spectr al reflectance measurements.