The spectrum show how much of the radar signal is reflected at each 85cm step in distance away from the radar so it is very sensitive. You can see here how the reflection from the volcano shows up as a sharp spike rising out of the background 'noise'.

So the basic measurement that AVTIS makes is the distance to a single illuminated spot on the side of the volcano. To build up a larger picture of the lava dome, the antenna is rotated sideways very slightly and a new range spectrum is recorded. This process is repeated and with each new measurement a different peak in reflection shows the distance to different parts of the volcano.