## 4 Conclusions  
Altimetry has been dominated by the Ku-band since the beginning and strong fears about a possible degradation by using Ka-band had limited the studies about launching a mission embarking on such technology. [PERSON] et al. [36] demonstrated the feasibility of such an altimeter but also importantly the need of higher resolution altimetry to address one of the most problematic features of ocean circulation, the mesoscale eddies and fronts. These features are essential to understanding the dynamics of ocean circulation on all space and time scales and theoretical studies of the cascade of energy over scales from 100-1000 km have always suffered from the lack of high-resolution observations. From [36] a Ka-band altimeter technology looked to be able to provide higher resolution to help answering these key issues. Selecting the Ka-band allows a larger bandwidth (480 MHz), which provides a vertical resolution of 0.3 m instead of 0.5 m in Ku-band. With such a resolution, the altimeter is close to a beam limited one: there is no 'plateau' in the echo as in Ku-band, since it strongly attenuates shortly after the leading edge, due to the small antenna aperture. This greatly  
Figure 22: Time-series of backscattering coefficient (dB) for (**a**) SARAL/AltiKa (Ka-band) 0001 (blue) and 0846 (green) tracks and (**b**) Jason-2 (Ku and C-bands) 046 (blue and black) and 161 (light green and red), of volumetric soil moisture (m3.m-3), from SMOS (blue) at SARAL/AltiKa (**c**) and Jason-2 (**d**) sites. reduces the pollution of 'land gates' into 'ocean gates', when considering land-sea transition areas. Moreover, due to the smaller antenna beamwidth, the Brown echo has a sharper shape in Ka-band than what is obtained with conventional Ku-band altimeters. Finally, the shorter decorrelation time of sea echoes at Ka-band also enables to double the number of independent echoes per second compared with Ku-band altimeters, allowing high frequency measurements (40 Hz, 20 Hz for Ku-band). SARAL/AltiKa mission is the realization of the [36] study and in this paper we show that the quality of its measurements meets and generally exceeds the expectation of the visionary designers of this mission. Firstly, one of the main result is probably the fact that the AltiKa data quality and accuracy are met at a similar level whatever the water surfaces and whatever the scale (Section 2). This is demonstrated using global, regional or local methods and based on comparisons with various external data (in situ, other altimetric missions). The precision is at least at the same level or even better than for Ku-band for the open-occans but improves substantially the along-track resolution (e.g., Figure 13 or Figure 14). These results will continue to improve with the reprocessing in 2018 with upgraded standards (GDR-E). Moreover, the characteristics of the AltiKa instrument allows to observe smaller water surfaces with equivalent precision and are of very great interest for coastal and inland waters studies [3]. The benefits of this increase of precision and resolution are highlighted through Science Applications of SARAL/Altika data [3]. Apart of these great achievements in term of precision and resolution, the characteristics of the altimeter and the Ka-band offer unique contributions in fields that where not fully foreseen. Before launch, concerns were raised about the sensitivity of the Ka-band to rain events, leading to missing and invalid measurements. However, in practice, the SARAL/AltiKa data return is remarkably high with few missing data (e.g., Figures 11 and 18). In fact, the sensitivity of Ka-band measurements to rain may also lead to a way of estimating very light rainfall over the oceans, for which we dramatically lack of information and which could lead to a great improvement of our knowledge of the oceanic rain climatology. Another specificity of the Ka-band is that electromagnetic wave penetration effects are minimized, which is particularly important over continental ice surfaces.