Empirical Correction of Stray Light within the MERIS Oxygen A-Band Channel

Abstract

Spaceborne spectrometers like the Medium Resolution Imaging Spectrometer (MERIS) on board the Environmental Satellite (Envisat) are widely used for the remote sensing of atmospheric and oceanic properties and make an important contribution to the monitoring of the earth?s atmosphere system. To enable retrievals with high accuracy, the spectral and radiometric properties of the instruments have to be characterized with high precision. One of the main sources of radiometric errors is stray light caused by multiple reflections and scattering at the optical elements within the instruments. If not corrected for properly, the stray light?induced offsets of measured intensity can lead to significant errors in the derived parameters. The effect of stray light is particularly momentous in the case of measurements inside strong absorption bands like the oxygen A band at 0.76 ?m or the ?στ absorption band of water vapor around 0.9 ?m. For example, the retrieval of surface and cloud-top pressure from MERIS measurements in the O2 A band can be biased because of an insufficient correction of stray light in the operational processing chain. To correct for the residual stray light influence after the operational stray light correction in the O2 A-band channel of MERIS, an empirical stray light correction of the measured radiance at 0.76 ?m has been developed based on optimizing the coefficients of a simple brightness-dependent stray light model. The optimal model coefficients were found by adjusting the retrievals of surface and cloud-top pressure to accurate reference data for several selected scenes. To account for the limited accuracy of the MERIS spectral calibration, the center wavelength of the O2 A-band channel was additionally adjusted within a ±0.1-nm tolerance range. The correction was tested on a variety of clear and cloudy scenes at different locations by applying the surface and cloud-top pressure retrieval algorithms to data recorded over the whole lifetime of MERIS. The results indicate the potential to greatly improve the accuracy of the retrieved pressure values using the proposed correction factors.