## 4 Conclusions  
Lake Issykkul has been used as a test area for multi-satellite C/V of radar altimeters, for past missions (T/P, GFO, Envisat, Jason-1, and Jason-2), present ones (Saral/Altika, Jason-3, Sentinel-3A, and Sentinel-3B), and future ones (Jason-CS, and SWOT). It presents the advantages of reducing some the sources of errors in the absolute altimeter bias computation (tides, water dynamic topography, Barometric Pressure, and SSB). Moreover, it allows the performance of a quality assessment of altimetry for the study of continental waters--cross comparing the different retracking algorithms of the waveform and calculating their respective relative biases. The results of April and October 2016 and July 2017 campaigns were presented in this article for the computation of the altimeter biases of Sentinel-3A and Jason-3, and the estimation of their accuracy. We have determined that the absolute bias of Sentinel-3A and Jason-3 were, respectively, \(-14\pm 20\) mm and \(-28\pm 40\) mm StD, with the _Ocean_ retracker. Using _Ice-1_ led to different results. For the former, the _Ice-1_ offset was \(285\pm 20\) mm and for Jason-3 it was \(206\pm 30\) mm StD. Since the first campaigns over Lake Issykkul, in 2004, a large number of field work have been carried out till July 2017, and we have converged over the last three campaigns to an experimental design for minimizing errors. In particular, for the GPS antenna height, embarked on the vessel, the new procedure allowed us to remove errors due to the velocity of the boat. Performing the GPS levelling of the water surface, along the track, exactly at the same time as the pass of the satellite, eliminated the errors due to Seiche or wave effects. In order to reduce potential errors in the bias calculation, due to the shoreline effect on the waveform, we used only measurements of 10 km before and after the exact pass of the satellite, in the middle of the lake. The choice of using atmospheric corrections directly from the in situ barometric pressure removed the errors on the DTC and the choice to use the fixed GPS station also removed the errors in the calculation of WTC. Therefore, three sources of errors remained--the altimeter range, the orbit, and the GPS positioning. Since orbit calculation and GPS PPP do not depend on the altimetry satellite, the observed differences of C/V errors (20 mm for Sentinel-3A and 40 with Jason-3A, with the _ocean_ retracking algorithm) fully reflected the quality of the altimeters, and in particular, reflected the fact that Sentinel-3A uses a SAR mode when Jason-3A still uses an LRM mode. Figure 9: Difference of Jason-2, Jason-3, and Sentinel-3A estimates (with the _Ocean_ retracker) with in situ measurements in Cholpion Ata, located in the north coast of the lake. We also evaluated the accuracy of the Jason-3 and Sentinel-3A, using the ranges derived by the retrackers, through a comparison with in situ data. We obtained a slightly higher accuracy for the Sentinel-3A with the _Ice-1_ retracker (2.4 cm), than for Jason-2 and Jason-3 (>3 cm). The results over Lake Issykkul also showed that altimetry is highly capable of detecting small water-level changes at different time scales, ranging from seasonal changes due to local climatology and seasonal water balance of the lake, to long-term changes due to other phenomena, like regional climate changes. Since these two missions belonged to the operational satellite system, it was an essential component for the long-term monitoring of lake-level variations. We have noticed that the radiometer onboard the Sentinel-3A did not provide any reliable measurement of the WTC. Same as for previous satellite missions, the recommendation for lake studies using Sentinel-3 altimetry was to use the corrections derived from the models for DTC, WTC, and IC. Our purpose, finally, was to show that Lake Issykkul is an essential C/V site for altimetry, since it presents characteristics not covered by historical oceanic C/V sites, and it can serve for all past, existing, and future altimeters.