Data and methods

What do satellites measure?

The optical sensors on board the satellites whose data we use measure the intensity of the light that reaches them on various wavelengths. Of course, these are not selected by chance when the sensor is designed, but are linked to certain physical phenomena that occur in the ocean. With optical remote sensing, the work covers the field of visible wavelengths (blue to red) and infrared wavelengths. We therefore commonly talk about studying the 'colour of the ocean'.

The past few years have seen a rapid increase in the number of satellites carrying sensors that can be used (among other things) to make oceanographic observations. The sensors of the greatest use to us are, for example, NOAA AVHRR, SeaWiFS, MERIS or MODIS. The hyperspectral CHRIS sensor was recently put into orbit. New generation sensors such as MERIS (launched on board of Envisat on March 1, 2002) will enable substantial progress to be made in numerous areas. MUMM plays an active role in the use of the scientific data from these missions.

The following figure, a small picture of the Belgian coast, provides a simple illustration of what the SeaWiFS sensor sees from its orbit, at an altitude of approximately 700 km. There are six measurement bands in the visible range and two in the infrared range. The intensity of the light for each point in the image is represented by a degree of grey: black or dark grey for low intensities and clear or white for strong intensities. Other examples of imagery are also available.

Light intensity measured by the SeaWiFS sensor on different wave lengths for the same view of the Belgian coast. Pass the mouse across the measurement wavelengths to see what is measured there.

How do we work with these data?

The light that reaches the orbiting sensor unfortunately does not come entirely from the sea — far from it. In fact, most of the light intensity measured (approximately 90%) comes from the atmosphere. For oceanographers wishing to obtain information on the concentration of suspended sediment or chlorophyll, this constitutes a 'noise' which absolutely must be eliminated, as accurately as possible. This work, which is known as atmospheric correction, is one of the most important and complex parts of the processing of satellite imagery. Once the marine component has been extracted from the signal, a colour model of the ocean has to be used to calculate concentrations of suspended sediment and chlorophyll. This is advanced research in which MUMM is involved. Significant progress is expected over the next few years thanks to the more sophisticated sensors which are to be put into orbit and to progress in the theoretical knowledge of oceanic optics and data processing.

Great importance is attached to validating the models developed at MUMM. This consists of comparing the results of calculations with observations made at sea as a satellite passes over. Several campaigns are organised every year on board the Belgica. Numerous samples are collected and then analysed by our laboratory.

 Examples of derived products