## 2 Quality Assessment of Water Level through Calibration and Validation
#### 2.1.3 Absolute Calibration, Validation of SARAL/AltiKa over the Lake Issykkul  
SARAL/AltiKa is used over the continents to measure the water height changes of lakes and rivers [1]. In order to determine the performances of altimeters over inland waters, a dedicated site  
Figure 5: Sea Surface Height (SSH) bias time series for SARAL/AltiKa over Corsica for both direct (GPS-zodiac, red circles) and indirect (tide gauges, blue crosses) approaches. The bold symbols correspond to situations where the satellite was not overlying the calibration sites in a nominal configuration. of calibration/validation has been setup in 2004 over the lake Issykkul. Lake Issykkul (42-43\({}^{\circ}\)N, 76-79\({}^{\circ}\)E) is a large lake (6000 km\({}^{2}\) of area extent) located in Kyrgyzstan in Central Asia. Initially used as experimental site, thanks to promising first results obtained with TOPEX/Poseidon [13], the Lake Issykkul became permanent cal/val site for several altimeters in orbit. It has served for Jason-1, Jason-2, and Envisat with results coherent with other cal/val sites over the oceans [14; 15]. The interest of having cal/val sites over a lake is to benefit from specific conditions of inland water (no tides, no inverse barometer) and to quantify the accuracy of altimeters to measure their water level changes. Meanwhile, some corrections like the wet or dry tropospheric delay are not measured as accurately as over the ocean. Using ground instrumentation over the lake Issykkul therefore helps quantifying the error budget of altimeters over lakes in general. Since 2004, not less than 15 field campaigns have been done over the Lake Issykkul. The full experimental design is given in [16] and is not described here in details. In few words, it simply consists in measuring the water level changes along the track of the satellite, due to the geoid's gradient (which over the Lake Issykkul reaches several meters over long distance) using a GPS kinematic survey on a boat following the satellite tracks. It allows us to calculate the absolute bias of the altimeter. First of all, the water level changes of the Lake Issykkul from 2013 to 2016 are calculated every 35 days using the altimetry data from the SARAL/AltiKa instrument. It is compared to the water level changes measured at an historical floating tide gauge located on the north coast of the lake (providing daily water level). It is also compared to hourly water level changes of the Lake measured by a radar installed on the south east lake shoreline. These comparisons allow us to quantify the accuracy of SARAL/AltiKa over the lake Issykkul in particular, which gives an order of magnitude of accuracy of this instrument for large lakes in general. It confirms results obtained in [17] that SARAL/AltiKa allows us to measure water level changes at very high accuracy (3.5 cm for lake Issykkul: Figure 6). Figure 6: Water level of the lake Issykkul measured by SARAL/AltiKa, a radar and a tide gauge located on respectively east and north coast of the lake, delivering hourly and daily data. RMS of differences between SARAL/AltiKa and the in situ daily data is 3.5 cm. For specific calculation of altimeter bias of AltiKa, a GPS survey of the track number 554 has been performed during a campaign done on 1 July 2014. The GPS vertical profile along this track is compared to individual measurements of the altimeters over the first 15 cycles using in situ measurements for all corrections:  
* Wet and dry tropospheric delay using a permanent GPS and weather stations
* The GPS height antenna using a radar installed just below the GPS on the boat: see ([PERSON] et al., 2016) for details. * The in situ daily level changes for the correction of the hydrological signal from cycle 1 to cycle 15. 1 July 2014 is when the GPS survey was done and corresponds to cycle 14. * The GINS ([PERSON] et al., 2016) software was used for the calculation of the GPS 3D coordinates in PPP (Precise Point Positioning) mode.