Evaluation of sensitivity decay of Coastal Zone Colour Scanner (CZCS) detectors by comparison with in situ near-surface radiance measurements

Abstract

Upwelling radiance and downwelling irradiance measurements were made at 0-65 nm intervals in the 400-700 nm spectral range from an aircraft flown at an altitude of 150m around 07.30, 10.06 and 11.07hours GMT. Each time the location was different and at each location the measurements were repeated three times. The NIMBUS-7 satellite overpass was at 10.04 hours GMT. The aerosol optical path length was evaluated from the in situ measured downwelling irradiance. The time dependence of the calibration slopes and intercepts of the Coastal Zone Colour Scanner (CZCS) was incorporated according to recent work by Gordon el at. The atmospheric contributions were removed from the measured radiances and from the CZCS data. The resulting water-leaving radiances were normalized so as to minimize those variations in the water-leaving radiance which arise from varying solar irradiance at the sea surface. A comparison between the normalized water-leaving radiance retrieved from airborne measurements (10.06 hours GMT) and from the CZCS data (10.04 hours GMT) indicates that (i) the calibration slopes of 443 and 520 nm spectral channels should be slightly larger than those calculated according to Gordon et al., whereas (ii) there is a converse indication for the 550 nm spectral channel. The calibration slopes of the CZCS channels 1-3 detectors were calculated by forcing the CZCS value of the normalized water-leaving radiance to be equal to the one resulting from the airborne measurements. The slopes are remarkably close to the values calculated by using the method of Gordon et al.