The ground based radar presented here is a new tool for volcano deformation monitoring. With respect to other systems it has various advantages: operational by any weather (rain, fog, aerosols), high frequency sampling capability (10 a few tens of seconds), possibility of monitoring surfaces not equipped with reflectors. It can be used to monitor unstable and dangerous parts of volcanoes (craters, lava domes, …). The all weather capability and the high frequency sampling rate are crucial on volcanoes where activity can change within a few hours or less. We present the system and show an application for range measurements on corner reflectors. Then, we present the results obtained in the Pyr¿n¿es mountains (France) on a natural surface not equipped with reflectors. We analyze the evolution of the coherence of the reflected signal as a function of the nature of the terrain and elapsed time. ¿ 2001 American Geophysical Union