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Lake Issyk Kul in Kyrgyzstan is a large high-altitude inland lake but ice-free the year around. Due to its E-W extension of ~170km all active radar altimetry missions cross this lake. Since 2016 CAIAG (Kyrgyzstan) and GFZ Potsdam (Germany) installed four GNSS-controlled tide gauges (N, S, E, W) and two climate monitoring stations at Lake Issyk Kul. Tide Gauge data (radar and pressure) is sampled every minute and climate data, such as wind or air pressure, every five minutes. The lake surface is neither influenced by ocean tides nor by inverse barometric effects. Seiches, which occur from time to time, are identifiable by data of the wind sensor at the east coast and the distinct variations of the lake level. The lake is calm except during strong winds detectable from our wind sensors in the East and South. Also surge effects are small. Starting in 2017 we performed repeated ship-based lake surveys with GNSS/radar and a GNSS buoy for profiling the lake surface along the satellite passages. With the physical connection to the tide gauges, we are able to reconstruct the instantaneous lake profile in the geocentric reference system ITRF2014 for any altimeter passage. For our study we analyzed the 20Hz GDR Level 2 data of Jason-3 (since 2016) and Sentinel-6 (since 2021, STC/NTC both LRM/SAR) for all passages. Along a pass stretching ~60km we tested all retracker available within the product and two additional retracker, developed for land ice applications. We estimated the individual retracker offsets and the internal accuracy. For Sentinel-6 the SAR-OCOG retracker performs best (revision F02 onward) after applying an offset of more than 33cm in respect to the GNSS reference pass. For Jason-3 (Rev.F) the OCOG also performs best with an offset of ~19cm. Our analyses show the capability of the Lake Issyk Kul observatory for the short-latency monitoring of radar altimetry missions.

Kyrgyzstan is landlocked mountainous nation of around five million people, which borders China, Kazakhstan, Tajikistan and Uzbekistan. The total area of high mountainous terrain, alpine meadows and pastures exceeds 70% of the Republic’s territory, whereas the greater part of the Kyrgyz Republic is occupied by the Tien-Shan mountains. Kyrgyzstan is a highly active seismic region and has been shaken by numerous significant earthquakes as a consequence of the ongoing collision between the Indian and Eurasian tectonic plates. In the result, the mountainous country is faced with a large variety of natural hazards (mainly earthquakes, large landslides and floods) which frequently lead to the occurrence of natural disaster (e.g., 1994: about 1,000 landslides failed and 115 human fatalities; 2008: Nura earthquake M=6.6, 74 human fatalities and 150 injured, 90 glacial lakes endangered for regularly occurring outburst floods). Under these conditions, there is high demand for efficient and spatially differentiated hazard assessment requiring an improved understanding of natural processes with high hazardous potential. Since large areas with often limited accessibility are affected, satellite remote sensing plays an important role in contributing to improved process knowledge in this region (Roessner et al., 2005). In the presented work the potential of advanced remote sensing techniques based on Synthetic Aperture Radar (SAR) satellite data is investigated for characterizing spatio-temporal surface changes related to mass movement and earthquakes. Methodological focus has been put on using Differential SAR Interferometry (InSAR) based on data from different satellites for detecting surface displacements as a consequence of slope instabilities and earthquakes in Southern Kyrgyzstan. In the presented work we focus on one study site of high landslide activity in the Osh province and on another study site which has been affected by the recent destructive Nura earthquake in 2008., Geosciences, Geodesy and Remote Sensing