## 4 Conclusions
From Ku- to Ka- bands, the scattering coefficient is increased by a factor 55: volume scattering is then being clearly dominant over surface scattering and gives information on snow or ice pack properties. Inversely, the radar wave extinction is also increased, leading to a penetration depth over the snow surface between 0.1 m and 0.3 m (versus 2 to 10 m in Ku-band). The altimetric observation and height restitution thus correspond to a thin subsurface layer and give complementary information than the one given historically by the Ku-band (see Section 3.2 and [3]). Finally, the spatial and temporal variations of Ka-band backscattering coefficients that are related to the dynamics of surface properties, show valuable information for studying snow and ice state but also surface soil moisture (e.g., Section 3.4). This last field of studies is at his infancy but results presented in this paper are clearly encouraging. With the upcoming Surface Water & Ocean Topography (SWOT) mission embarking on a Ku-band nadir altimeter and a Ka-band interferometer (KaRIN) and the phase 0 of a new cryosphere satellite mission (bi-frequency altimeter in Ku- and Ka-band) that is currently studied by CNES and ESA, it is more than likely that SARAL/AltiKa Ka-band altimetric mission will continue to help preparing these missions but also help to understand Ku-band better. The SARAL/AltiKa mission is an achievement of a very fruitful cooperation between ISRO and CNES. The contribution of EUMETSAT to data distribution is also strongly appreciated. We acknowledge the support of all the Investigators, PIs and Co-Is, of the SARAL/AltiKa Mission. Most of the studies performed by the French authors have been conducted and financed thanks to Centre National d'Etudes Spatiales (CNES), Centre National de la Recherche Scientifique (CNRS), and French Ministry of Research. The altimetry data used in Section 3.4 were made available by \"Centre de Topographie des Oceans et de l'Hydrosphere\" (CTOH)--[[https://cdn.lego.obs-mip.fr/](https://cdn.lego.obs-mip.fr/)]([https://cdn.lego.obs-mip.fr/](https://cdn.lego.obs-mip.fr/)). The SMOS data were obtained from the \"Centre Aval de Traitement des Donnees SMOS\" (CATDS), operated for the \"Centre National d'Etudes Spatiales\" (CNES, France) by IFREMER (Brest, France)--[[https://www.catds.fr/sipad/](https://www.catds.fr/sipad/)]([https://www.catds.fr/sipad/](https://www.catds.fr/sipad/)). [PERSON] and [PERSON] coordinated the whole paper. [PERSON] designed the study, analyzed the data and wrote the Section 2.1.1. [PERSON] and [PERSON] designed the study, analyzed the data and wrote the Section 2.1.2. [PERSON] and [PERSON] designed the study, analyzed the data and wrote the Section 2.1.3. [PERSON] designed the study, analyzed the data and wrote the Section 2.2 while [PERSON], [PERSON], [PERSON] and [PERSON] provided the data and contributed to the writing and analysis. [PERSON] and [PERSON] designed the study, analyzed the data and wrote the Section 2.3. [PERSON] and [PERSON] designed the study, analyzed the data and wrote the Section 3.1. [PERSON] and [PERSON] designed the study, analyzed the data and wrote the Section 3.2. [PERSON], [PERSON] and [PERSON] designed the study, analyzed the data and wrote the Section 3.3. [PERSON] designed the study, analyzed the data and wrote the Section 3.4. All the other co-authors helped in the analysis and paper writing. The authors declare no conflict of interest.