Less than a century ago, Alfred Wegener's theory of continental drift was the subject of hot debate.
Today, instruments like Doris let us "see" plate motions at the Earth's surface. Doris ground beacons located on the main tectonic plates measure motion and velocities, which range from a few millimeters to 15 centimeters a year. The Doris system measures variations in the absolute position of each beacon. These measurements, coupled with data from other systems (for example, GPS, laser ranging and VLBI), are used to build models of plate motion.

Combined with over nine years of observations from the Topex/Poseidon and Spot satellites (Spot 2, 3 and 4), Doris is a key aid for studying the Earth's crustal motions. The three latest satellites to be launched with a Doris instrument on board-Jason-1, Envisat and Spot 5-will ensure continuity of measurements.