Alexander Zubovich (Central-Asian Institute for Applied Geosciences (CAIAG), Kyrgyzstan); Azamat Sharshebaev (Central-Asian Institute for Applied Geosciences (CAIAG), Kyrgyzstan); Saskia Esselborn (GFZ Helmholtz Centre for Geosciences, Germany); Nico Stolarczuk (GFZ Helmholtz Centre for Geosciences, Germany)

Lake Issyk Kul in Kyrgyzstan is a large, high-altitude, ice-free lake. Due to its east-west extension of ~170 km, all active radar altimetry missions cross this lake. Since 2016, CAIAG (Kyrgyzstan) and GFZ (Germany) have installed five GNSS-controlled tide gauges and two ROMPS climate monitoring stations around the lake. From 2017 onwards, we conducted repeated ship-based lake surveys using GNSS/radar and a GNSS buoy to profile the lake surface along the satellite passages. All tide gauges are regularly levelled to nearby GNSS sites. By combining the tide gauge readings with the ship-based profiles, we can reconstruct the instantaneous lake profile in the geocentric reference system (ITRF) for any altimeter passage. The lake's surface is not affected by ocean tides or inverse barometric effects. Seiches, which occur from time to time, can be identified using data from the wind sensor on the east coast and the distinct variations in the lake level. Tide gauge data is sampled every minute, while climate data such as wind speed and air pressure is sampled every five minutes. The lake is usually calm, except during strong winds, which are detectable from our wind sensors in the east and south. Surge effects are also small.

For our study, we analyzed and compared the 20 Hz GDR Level 2 STC data for the LR and HR of Sentinel-6B and -6MF during their passage over Lake Issyk Kul. Both missions performed similarly, showing comparable scatter along the lake ground-truth passage. All retrackers show an offset with OCOG being the largest in HR/LR. The RMS relative to the ground-truth passage is approximately 8 cm for the Ocean/Adaptive Retracker and 6 cm for the OCOG Retracker. The Numerical Retracker RMS is the largest, at >9 cm.