Optical Remote Sensing


Remote sensing by satellite provides an enormous amount of data on the marine environment. A polar-orbiting sensor such as MODIS can measure the concentration of chlorophyll in the surface waters of the entire planet in one day (if cloud-free).

In comparison a typical ship can cover only a few hundred kilometers in a straight line per day. The low marginal cost of satellite remote sensing is also a big advantage since a scientist with a PC and a fast Internet connection can now download and process daily updates in near real time (within 12 hours of acquisition) for little or no charge. Of course, not everything can be measured by satellite. Research at the OD Nature focuses primarily on optical remote sensing, while radar remote sensing is also used operationally for oil slick detection.

Satellite image MERIS

Optical remote sensors onboard satellites or aircraft measure the light coming from water broken down into wavelengths. For example, the human eye, our very own portable optical remote sensor, measures light at three different wavelengths (roughly 450nm for blue, 550nm for green and 600nm for red light) and from these components builds up a colour picture. Digital cameras allow such a picture to be recorded. For more scientific work, spectroradiometers are calibrated to provide a precise quantitative measure of the light detected at each wavelength and a much larger number of wavelengths are covered allowing detection of features which cannot be distinguished by eye.

The colour of water depends on what is in the water. The purest water, without any particles or dissolved organic matter, has a deep blue colour. Water molecules strongly absorb red and, to a lesser extent, green light, and so mainly the blue light from the sun is reflected. If microscopic algae particles (“phytoplankton”) are suspended in the water these will absorb light for photosynthesis at different wavelengths, especially blue light, turning the water colour from blue to green as the concentration of algae increases. This concentration is typically measured by the concentration of the chlorophyll a pigment. Other water constituents such as suspended mineral particles (like sand but smaller) and coloured dissolved organic matter also affect water colour, or its more scientific equivalent, “spectral reflectance”. All these parameters that affect water colour can theoretically be quantified and mapped by optical remote sensing provided that suitable mathematical methods, (“retrieval algorithms”) can be designed linking the spectral reflectance to each parameter.

Recent research at our OD Nature has focused on the development and validation of such retrieval algorithms, especially for chlorophyll a concentration and total suspended matter concentration, so that maps of these parameters can be used to support marine science and environmental management. Algorithms are also under development for new parameters such as the detection of specific plankton species (Phaeocystis globosa, Noctiluca scintillans) or the quantification of euphotic depth, which is the depth at which light intensity has been reduced to 1% of its surface value and which is important in determining how much of the water column is bright enough for photosynthesis. Daily maps generated from three satellite sensors, SeaWiFS, MODIS and MERIS are available.

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