Aerosol research at SRON focuses on the use of instruments that measure both the intensity and polarization of light reflected by the Earth's atmosphere and surface at different wavelengths and viewed under different angles. The POLDER instruments of the French Space Agency CNES have been the only satellite instruments that have provided this type of measurements of the Earth atmosphere until now. The 3MI instrument will be launched in the early 2020s on the EUMETSAT METOP-SG-A satellite.
SRON is developing retrieval algorithms to generate scientific aerosol and cloud data products from both POLDER and 3MI and to support the development and implementation of forthcoming advanced instrumentation for aerosol research like SPEX airborne and other instruments.
The first two POLDER instruments have been launched in the 1990s on the Japanese ADEOS satellites. Due to problems with the satellite platforms, both POLDER-1 and POLDER-2 only provided data for a couple of months. In 2004, the POLDER-3 instrument has been launched on the PARASOL microsatellite. POLDER-3 has been operational from 2004-2013, being part of the NASA A-Train satellite constellation till 2009. POLDER-3 has provided intensity measurements in 9 spectral bands (443, 490, 565, 670, 763, 765, 865, 910, 1020 nm) and linear polarization (Stokes parameters Q and U) in 3 spectral bands (490, 670, 865 nm), viewing a ground scene under (up to) 16 different viewing geometries.
The POLDER instrument is based on a CCD detector, wide field optics in 2 directions, and a filter wheel containing spectral and polarization filters. The filter wheel concept allows for a compact instrument design with one detector, still enabling a wide field of view. On the other hand the filter wheel is its main limitation, i.e. that different polarization directions are measured subsequently and hence are not spatially co-registered, which limits the accuracy of the Stokes parameters for inhomogeneous scenes. SRON has developed a retrieval algorithm to generate novel global aerosol products from POLDER-3/PARASOL, such as single scattering albedo (SSA), particle sphericity, and effective radius, in addition to the aerosol optical thickness (AOT).
3MI is the POLDER follow-on instrument to be launched in the early 2020s on the three consecutive METOP-SG-A satellites of EUMETSAT providing more than 20 year time series. 3MI uses the same filter wheel concept as POLDER to perform multi-angle, multi-wavelengths intensity and polarization measurements. Compared to POLDER, the spectral range is extended to the shortwave infrared enhancing the capabilities for cloud retrieval.