Atmospheric Correction Specification
Details of the algorithm for the atmospheric correction are provided in an ATBD, which can be found at
http://www.brockmann-consult.de/beam-wiki/display/LAKES/Home
The following gives only a brief overview.
One instrument of the ENVISAT earth observation mission of the European Space Agency ESA is
the Medium Resolution Imaging Spectrometer MERIS. This instrument is used to measure
properties of the atmosphere and land surfaces and the ocean. Prime mission is the determination
of optical properties of oceanic water and the concentrations of its constituents. For open ocean
water robust methods have been developed for the correction of the atmospheric influence as wel
as the retrieval of phytoplankton chlorophyll. This is not so the case for some types of coastal
waters as well as inland waters, where the case 1 water routines for the atmospheric correction
and the retrieval of water constituents fail. Also the standard extension of the atmospheric
correction to turbid coastal waters fail in some areas so that the case 2 water algorithm for the
retrieval cannot be applied. These areas have to be flagged and thus are lost for further use.
This ATBD describes the atmospheric correction procedure, which is used to calculate water
leaving radiance reflectances from top of atmosphere radiances. One requirement for this
procedure is to include turbid case 2 waters into the scope of the algorithm. The standard MERIS
atmospheric correction procedure for case II water, as implemented in the ground segment, is
adapted to waters with only limited concentrations of suspended solids and yellow substances and
leads under other cases to negative reflectances and artefacts. Thus, it was necessary to write a
new procedure for BEAM. This opportunity was used to develop and test a new type of
atmospheric correction method which is based on inverse modelling and its parametrization by a
neural network. It takes into account the effect of cirrus clouds, specularly reflected sun light (sun
glint) and the water leaving radiance reflectances caused by all sort of water constituents.
