Radar Altimeter in-situ monitoring at the Lake Issyk Kul observatory (Kyrgyzstan)

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.