The glaciated Tianshan mountain chains of Kyrgyzstan represent the main reservoir of water supply of large parts of Central Asia. The regression of glaciation in this area, observed since the last two decades, caused changes in the dynamics of the glaciers. Hence, especially at the largest glacier lake of Central Asia - the Merzbacher Lake - the number of dam failures raised dramatically and causes massive damage of the infrastructure of the regions situated below the glacier outflow every year. During the Inylshik 2005 expedition (July 22 and August 13, 2005) - conducted by GFZ-Potsdam and CAIAG, Bishkek - a GPS altimetry/reflectometry experiment was installed at the border of the Merzbacher Lake at 42.196N, 79.847E and a height of 3271 m above sea level in order to monitor changes of the lake level. A single unmodified RHCP GPS patch antenna was mounted at a secure iceberg-free location in a height of 43 m above the glacier lake and was tilted 45 toward the lake surface. The 12-channel L1 frequency OpenGPS receiver allows tracking of up to 8 direct GPS signals for navigation and open-loop tracking of up to 4 independent reflected GPS signals simultaneously for altimetry. Although the lake surface was fully covered with ice floes and icebergs, the OpenGPS receiver collected several height profiles every day and monitored the temporal change of the lakes water level during 15 days, between July 27 and August 10, 2005. The replenishment of the lake could be fixed from the OpenGPS measurements to a rise rate of 4cm/h, until the dam broke and the estimated water level decreased by an estimated value of 29cm/h. Single measurements state a water level drop of 13.75 m between July 31 and August 3. The standard deviation (STD) of the derived height profiles - used as an estimate of the surface roughness - significantly increases and the power of the reflected signal decreases by a factor of 10.

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Helm, A.; Wetzel, H.-U.; Michajljow, W.; Beyerle, G.; Reigber, C.; Rothacher, M. (2006): Natural hazard monitoring with L1 carrier-phase-delay observations of reflectedGPS signals: The 2005 dam failure event of Merzbacher glacier lake. GNSSR’06 - Workshop on GNSS Reflections (Noordwijk, The Netherlands 2006), 1-8.