Background

Despite great improvements in aquatic remote sensing over the last decades, remote sensing of optically complex waters, such as lakes and the coastal zone (so-called optical Case-2 waters) is still a challenge. Several specific problems of aquatic remote sensing are characteristic for the Nordic and the Baltic Sea countries:

1. the low water reflectance compared to the atmospheric reflectance makes removing the atmospheric signal (i.e. atmospheric correction) difficult
2. complex interactions between coastal/inland waters (turbid and/or humic) and the atmosphere make it difficult to apply a good atmospheric correction scheme;
3. clouds are often present;
4. specific aerosol properties over the Nordic region are not well defined in common models;
5. water pixels may be influenced by environmental effects from adjacent areas, such as coastal areas, ice covers or patches of cyanobacteria blooms, which tend to be highly reflective;reflection from land is especially a problem in narrow fjords and bays.

With its improved spectral and radiometric resolution, MERIS, the European Space Agency’s sensor on the ENVISAT-1 satellite, is the first ocean colour sensor that is adequate for coastal remote sensing. It is currently in its validation and calibration phase, and several remote sensing groups of our Nordic Network are part of the MERIS validation team (MVT).