## 1 Introduction
and local scales (geodetic systematicerrors, land contamination for the instruments (e.g., the radiometer), land contamination for the radar echoes (e.g., tracking/retracking concerns), and geophysical corrections). In addition, we enter a new era of altimetry with SAR and wide swath altimetry and our group is taking a particular focus on the new measurement systems and their reliability with the past ones (LRM). Although not being specifically designed for hydrology, the SAR mode is expected to have substantial benefits for continental waters monitoring. In SAR mode, the large circular footprint is sliced into bands in a cross-track direction, allowing an optimal selection of the water body. Compared to the LRM measurements, the contamination of the echo from the surrounding terrain is also reduced. In the case of Lake Issykkul, the coastline is closely oriented, in a direction perpendicular to the along-track direction of Sentinel-3A. In this context of using satellite altimetry over continental waters, the C/V over lakes and rivers took a central place over the last decade. Since the mid-nineties, more and more scientific studies have used satellite altimetry to monitor inland waters ([PERSON], 2014; [PERSON], 2014). However, as for ocean studies, linking time series from different missions requires accurate monitoring of the biases and drifts of each parameter, contributing to the final estimate of the surface water height. Moreover, the calibration of satellite altimetry over ocean does not apply directly to inland seas (e.g., corrections, retracking, and geographical effects). Comparisons using results over Lake Ontario and Lake Issykkul have shown that lakes' sites allow performing C/V of radar altimeter with a quality similar to the ones obtained using sites over the oceans ([PERSON], 2014; [PERSON], 2014; [PERSON], 2014; [PERSON], 2014). Fifteen campaigns with GPS receivers have been conducted on Lake Issykkul from 2004 until 2017, with receivers installed on a boat, and receivers on the shore. Cruise with GPS data were conducted along the ground track of each satellite. They allowed estimating absolute bias of each altimeter, and relative bias between them. They also allowed assessing if C/V over continental surface leads to different results than those obtained over the historical C/V on the ocean. Moreover, C/V over lake surfaces has interesting characteristics--tides are low, inverse barometric effect is fully negligible, and dynamic variability is much smaller. SSB is also very low and not accounted for over the inland waters. Calculating multi-mission absolute altimeter bias allows for the continuity with further missions on the same orbit (the Sentinel-3 and Jason-series). For hydrology, in particular, it is essential to calculate long-term (multi-decadal) water-level time-series. Continuity can be obtained if during a certain period of time (few months) the satellites are in the same orbit as the so-called tandem phase. It has been done for T/P with Jason-1 (9 months in 2002), then Jason-1 with Jason-2 (8 months in 2008), and Jason-2 with Jason-3 (8 months in 2016). In other cases, like with the Envisat and the Saral/AltiKa missions, the continuity, unfortunately, has not been possible, and at least a one and half years separated the last repeat cycle of the Envisat, with the first one of the Saral/AltiKa. C/V operations were performed over Lake Issykkul with these two satellites allowing the absolute bias of each satellite to be calculated, therefore, estimating the relative bias between them ([PERSON], 2014; [PERSON], 2014). The main objective of this work is to determine the absolute bias of the two altimeters onboard Jason-3 and Sentinel-3A, using the two retracking algorithms (_Ocean_ and _Ice-1_). We also quantify the final accuracy of these two altimeters in the specific context of the lake's water-level measure.