Monitoring the Bio-optical State of the Baltic Sea Ecosystem with Remote Sensing and Autonomous In Situ Techniques - The Baltic Sea Basin - page 417

Widely used ocean colour radiometers are, e.g. NASA' s 3 SeaWiFS (launched in
1997) and MODIS (1999 and 2002) as well as ESA's MERIS (2002). The com-
mon spatial resolution for ocean colour images is about 1 km, which is sufficient
for open ocean applications. MODIS has also medium-resolution bands (250 and
500 m) which were designed for land applications. These can also give valuable
information about specific features in coastal waters (Kutser et al. 2007 ) . A method
called pan-sharpening can be used to improve the resolution of MODIS multi-
spectral images from 1 km to 250 m resolution. In this method the resolution of
multispectral radiometer is increased on the basis of higher resolution bands from
the same or another radiometer of similar acquisition terms (Carper et al. 1990 ,
Chavez et al. 1991 ) .
MERIS on ENVISAT offers currently the best spectral and radiometric resolu-
tion in operational ocean colour remote sensing radiometry (Doerffer et al. 1999 ) .
MERIS has 15 spectral bands with 10 nm bandwidth each. It also has an improved
spatial resolution of 300 m and is therefore especially suitable for coastal appli-
cations. MERIS is suitable both for aquatic and for terrestrial remote sensing as
it has a wide dynamic range, capable of detecting the low signals reflected from
the dark water, as well as bright reflectance from sea ice, clouds, or land surfaces.
Thus, MERIS is notably suitable to study land-ocean interactions and Earth system
dynamics.
Figure 20.5 shows a true colour composite of Himmerfjärden, a fjord-like bay
situated 60 km south of Stockholm. The left plate shows Himmerfjärden in full
resolution (300 m), whereas the right plate shows the reduced resolution of MERIS
(1,200 m). The figure demonstrates visually that full resolution MERIS data are
suitable to analyse coastal bays, whereas the reduced resolution image only gives
very few pixels from within the bay. These pixels are also clearly influenced by the
strong reflection from land, which is called adjacency effect.
Ocean colour sensors fly on near-polar, sun-synchronous satellite orbits to obtain
high temporal coverage. The length of the resonant orbit is 35 days for MERIS and
16 days for MODIS and SeaWiFS. The sensors have different swath widths and
global coverage is provided every 72 h for MERIS, every 48 h for SeaWiFS, and
every 24 h for MODIS.
20.2.1.3 Remote Sensing Products
Products derived from ocean colour remote sensing are commonly categorized into
three levels (Bukata 2005 ) . Level 1 products are calibrated and geo-located radi-
ances, at sensor height, i.e. at the top of the atmosphere (TOA). Level 2 products
are retrieved from TOA radiances and include the previously described in-water
optical properties, i.e. the concentration of chlorophyll- a and TSM, as well as
3 NASA: National Aeronautics and Space Administration; ESA: European Space Agency; MODIS:
Moderate Resolution Imaging Spectroradiometer; SeaWiFS: Sea-viewing Wide Field-of-View
Sensor; MERIS: Medium Resolution Imaging Spectrometer; ENVISAT: European Environmental
Satellite.