“Given a cloudless view, a quiet lake surface and a widespread algal bloom, it is relatively simple to determine the water quality in a satellite picture,” says Odermatt. The difficulty lies much more in recognizing interpretable pixels in several hundred thousand-satellite pictures. He and his research colleagues thus tinkered for several years on algorithms that search for such pixels in the pictures and determine from their colour the quantity of algae, turbidity or quantity of blue algae. “Because the satellites deliver not only snapshots but also pictures taken at high resolution over a longer period, environmental changes can be more clearly recognized than with ordinary sampling,” adds Odermatt.
As of 2012, the satellite Envisat is no longer sending out pictures, as communication with the earth station ceased suddenly for unknown reasons. But since 2015 several follow-on satellites orbit the earth, and these shoot pictures even more often and with higher resolution than Envisat. Thanks to this constant satellite monitoring, it will be possible in the future to get information about the condition of lakes all over the world in real time.