In-flight calibration of the bidirectional reflectance distribution function of the SCIAMACHY ESM diffuser
R.C. Snel(1) and J.M. Krijger(1)
(1) SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, Netherlands
The relative bidirectional reflectance distribution function (BRDF) of the ESM diffuser of SCIAMACHY on ENVISAT was determined from in-flight calibration measurements.
SCIAMACHY measures daily solar reference spectra using a beadblasted aluminium diffuser plate mounted on the back side of the elevation scan mechanism (ESM). Spectral features originate as a result of white light interference effects on the diffuser surface, which make the BRDF extremely sensitive to the illumination conditions of the diffuser. During on-ground calibration no representative irradiance source was available, resulting in a BRDF calibration which was not suitable for all trace gas retrieval algorithms.
Using the spectral feature and intensity variations with illumination conditions as actually observed over 6 years in orbit, a new in-flight relative BRDF calibration was derived.
The solar observations were parameterised as function of elevation angle of the sun as seen by the satellite, of azimuth angle, and as function of time, providing an intensity independent model which describes the relative change of the measured solar signal to an accuracy of a few times 10^-4.
From intensity monitoring data there are indications of diffuser degradation, which is confirmed by the relative BRDF results: the BRDF in the UV becomes somewhat more specular over time, consistent with the assumed contamination build-up on the diffuser surface.
The spectral features of the diffuser shift in a predictable way in wavelength with changing elevation angle of the sun, and show the expected wavelength dependence of the shift. The features are reproducible enough to be used as confirmation of platform pitch.
Azimuth dependence of the relative BRDF shows unexpected features at one extreme of the azimuth range, suggesting vignetting and reflection effects which increase in amplitude with wavelength. These azimuth dependent effects are also visible in the intensity.
This work has shown that in-flight calibration of the relative BRDF is possible and exceeds the accuracy generally achieved in on-ground calibration. The benefit for trace gases retrieved from the spectra based on improved BRDF calibration is smaller systematic errors and reduced seasonally dependent periodic errors. The results will be implemented in the operational data processor and in this way made available to the SCIAMACHY data users in the form of an improved daily solar reference spectrum.