Last June a team of French researchers showed that they could measure the shape of a volcano to within a fraction of an inch and perhaps help predict an eruption--from space. The researchers used images made by the European satellite ERS-1, which sweeps a radar beam over the ground and collects the echoes as it orbits Earth. If a feature on the ground is stationary, one radar picture of it looks like the next. But if the feature has moved even slightly--as a volcano does, for instance, when it inflates with magma just before erupting--the second set of echoes is different. Combining both sets of echoes produces an image called an interferogram, like the one shown here of Mount Etna in Sicily, that reveals with extraordinary sensitivity just how much the ground has moved.
The French team, led by radar image specialist Didier Massonnet of the National Center of Space ...
