Ocean colour sensors are designed to retrieve the spectral distribution of up-welling radiance just above the sea surface (the water-leaving radiance) that is used to estimate a number of geophysical parameters through the application of specific bio-optical algorithms. Atmospheric correction for ocean colour data is challenging, as only about 4% of the radiation measured by a satellite instrument originates from the water surface, and sensors require high Signal to Noise Ratios (SNR), particularly for the ‘blue' bands (approximately 400 nm). Ocean colour instrument design must therefore incorporate extremely sensitive and stable radiometry, dedicated on-board calibration and a large number of spectral channels.
The OLCI instrument is a programmable, medium-spectral resolution, imaging spectrometer operating in the solar reflective spectral range (400 nm to 1 040nm).
The instrument scans the Earth's surface using the push-broom method. CCD arrays provide spatial sampling in the across track direction, while the satellite's motion provides scanning in the along track direction. The OLCI swath is tilted 12.6° westwards to mitigate the negative impact of sun glint contamination. OLCI is designed to acquire data over the Earth whenever illumination conditions are suitable. The instrument's 68.5° field of view around nadir covers a swath width of 1 270 km. This wide field of view is shared between five identical optical modules arranged in a fan configuration. The Earth is imaged with a spatial resolution of 300 m (at nadir).
The scene is imaged simultaneously across the entire spectral range, through a dispersing system, onto the CCD array. Signals from the CCD pass through several processing steps to achieve the required image quality. These CCD processing steps include dumping of spectral information from unwanted bands and spectral integration to obtain the required bandwidth. On-board analogue electronics perform pre-amplification of the signal, correlated double sampling and gain adjustment before digitalisation. The on-board digital electronics system has two major functions: it completes the spectral integration and it performs offset and gain corrections in full processed mode.
Figure 1: RGB Image Over the Baltic Sea (MERIS FR sensor)