AVTIS uses its antenna to beam a tightly focussed radar signal at the mountain. This beam can be used to measure the distance to the volcano. You can think of the beam as a mm-wave searchlight which illuminates a small spot on the mountainside. The radar beam is half a degree wide so the spot is only a few metres across even at several kilometres range.
When the radar signal is reflected from the
spot on the volcano AVTIS 'sees' the reflection in a measurement
called a radar range spectrum: