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Merapi Volcano, Indonesia, is one of the most active dome building volcanoes worldwide. Instabilities at the growing dome cause rockfalls and hot block and ash flows, which can reach run-out distances of several kilometers. Therefore, Merapi has been monitored extensively for many years. However, direct visual observation of the dome is often impossible due to cloud coverage of the summit. In October 2001, a first prototype Doppler radar system was installed to overcome this shortcoming. The system is able to penetrate clouds and observe material movements at the dome, giving valuable information about dynamic processes in the dome area. The system also allows detection of rainfall in several distance intervals. For precise positioning of the beam, the system was equipped with an electromechanical mounting in 2003. A charge-coupled device (CCD) camera attached to the radar mirror documents the radar beam position and provides visual observation of the dome. Recorded data, camera images, and status information are telemetered to the Merapi Volcano Observatory, where they can be processed and interpreted. Status information is also sent via short message service via a global system for mobile communications (GSM) modem. By processing the Doppler radar data, we are able to discriminate between three different types of instability events: sliding dome material, dome material gravitationally breaking off the dome, and explosive outbursts of dome material due to expansion of volcanic gas. In order to independently verify our observations, we compared rockfall events detected by the radar system to seismic recordings and found a good correlation. [ABSTRACT FROM PUBLISHER]