Lake Issyk Kul Observatory – Validating SWOT - 2024

Principal Investigator: Tilo Schöne (GFZ Potsdam​)

Co-Investigator(s): Saskia Esselborn, Alexander Zubovich

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Any space-based altimetric mission requires and extensive program of continuous quality control, especially monitoring possible offsets and drifts which may occur during the life-time of the space mission. CAIAG (Bishkek, Kyrgyzstan) and GFZ (Potsdam, Germany) have established a ground monitoring infrastructure at the Lake Issyk Kul in Kyrgyzstan. Lake Issyk Kul, at an altitude of ~1600m, has roughly a ~170km E-W and ~60km N-S extension. The lake, which is crossed by most of the past and active altimeter missions, is characterized by moderate wind conditions with low swell. The lake itself is ice-free the year around, allowing continuous calibration activities. We operated five GNSS-controlled tide gauges around the lake and two climate stations for a large suite of environmental parameters. Over the past years, we established GNSS-profiles for all past and active radar altimetry missions, including SWOT, along the nadir pass. As the profiles are connected with the tide gauges, we are able to provide corrected in-situ measurements along the satellite passages for any given time.

The SWOT mission passes Lake Issyk Kul twice in the eastern part on an ascending and descending track. In 2024 we performed ground-truth measurements along 60km-long both passes. On regular basis we compare the 20Hz SWOT elevation measurements for all available retracker, similar to our work for Sentinel-3 (https://doi.org/10.1007/1345_2024_268). Each pass is analyzed in respect to the number of points, the RMS of the scattered elevation values and, on long-term, the drift. Additionally, we estimate the quality of the wet and dry tropospheric corrections as well the ionospheric disturbances using our continues GNSS network near the lake.