Assessment of the water-leaving reflectances of the oceanic and coastal waters using MERIS satellite products in Sagres off the southwest coast of Portugal

Cristina, Sónia; Goela,  Priscila; Icely, John; Newton, Alice; Fragoso, Bruno

http://hdl.handle.net/10400.1/1106

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Optical properties of oceanic waters have provided an important tool for observing large scale fluctuations in oceanic conditions by remote sensing. However, there are still problems with calibrating the algorithms that link remote sensing data with in situ measurements, particularly, in coastal areas where there are adjacency affects from land on the reflectance data observed by the satellite sensor. As part of a contract with the European Space Agency to validate the marine products of the MERIS (Medium Resolution Imaging Spectrometer) sensor on the ENVISAT satellite, this study is taking regular measurements with a Satlantic hyperspectral radiometer ranging from inshore to offshore sites, covering both Case-1 and Case-2 waters off the south west coast of Portugal. Water-leaving reflectances (rw) were estimated from downwelling and upwelling spectral irradiances,and compared with MERIS rw for a period between September and November 2008. The results of the “matchup” analysis showed that there was reasonable agreement between the data sets over the period of study at the offshore site where the land adjacency affect on the satellite data was reduced. The greatest deviation of in situ data from satellite values was at the inshore site and at the lower wavelengths of the visible spectrum, where the rws for the blue band were most affected by atmospheric corrections. This study suggests that calibration algorithms of MERIS products with in situ data from the south west coast of Portugal could be improved by using, for example, ICOL ( Improved Contrast between Ocean and Land) processing.