| We introduce an alternative method for the retrieval of polarisation in
the ultraviolet by the satellite spectrometer SCIAMACHY. Unlike the
operational polarisation retrieval algorithm, this method does not use the
Polarisation Measurement Devices (PMDs) onboard SCIAMACHY, but only
requires the reflectance signal. This makes the algorithm more robust and
less sensitive to calibration errors caused by either improper
characterisation of the instrument's response functions (key data) or
degradation of the optical components.
The alternative polarisation retrieval is able to retrieve the full state
of atmospheric polarisation in the wavelength range between 330 and
400 nm, which is essentially the wavelength region covered by SCIAMACHY's
PMD 1. This allows a direct comparison with the current operational
product.
When we compare the alternative polarisation algorithm with the
operational algorithm, we find in some cases agreement, but not in other
cases. The alternative algorithm compares well with an analytical model of
the polarisation of a cloud-free scene. Using the alternative algorithm
the polarisation-sensitive feature in the SCIAMACHY reflectance around
350 nm is automatically corrected for. |