## 3 Unique Contributions of Ka-Band Information in AltiKa Data  
The key feature of the altimetric payload has been the selection of Ka-band [27]. Using Ka-band avoids the need for a second frequency to correct for the ionosphere delay and eases the sharing of the antenna by the altimeter and the radiometer. The use of the Ka-band also allows the improvement of the range measurement accuracy in a ratio close to 2 (30 cm compared to 47 cm for Jason Ku-band) due to the use of a wider bandwidth and to a better pulse to pulse echo decorrelation. The higher pulse repetition frequency (4 KHz compared with 2 KHz on Jason-2) also permits a better along-track sampling of the surface. Moreover, the enhanced bandwidth (480 MHz compared with 320 MHz on Jason-2) enables a better resolution of the range. Finally, Ka-band antenna aperture is reduced, which limits the pollution within useful ground footprint. The known effect of rain on the Ka-band was such to put some uncertainty on the full availability of data in some regions. In fact, the larger sensitivity to small rates of rain was found to be less constraining than expected and in counterpart provides access to low rains climatology [28]. Apart from the improved accuracy and resolution for the liquid water surfaces (ocean and inland waters), the fact that AltiKa altimeter operates in Ka-band, which is higher than the previous frequencies, offer new paths of investigation. The penetration depth in snow is theoretically reduced from around 10 m in Ku-band to less than 1 m in Ka-band, such that the volume echo originates from the near subsurface. Second, the sharper antenna aperture leads to a narrower leading edge that reduces the impact of the ratio between surface and volume echoes of the height retrieval. Moreover, the volume echo in the Ka-band results from the near subsurface layer and is mostly controlled by ice grain size, unlike the Ku-band [33]. This section gives some illustrations of the unique characteristics of AltiKa instrument that offer unique contributions in fields that where not fully foreseen. Section 3.1 will give insights of the increase of resolution and data availability close to the coasts. Section 3.2 will focus on the Ka-band contribution for altimetry over snow and ice. Section 3.3 will study the rain sensitivity and finally Section 3.4 will give an original use of Ka-band measurements for studying the soil moisture. Power Spectrum Density Analysis of SSH Signal from SARAL/AltiKa and Jason-2 (1 Hz) over Bay of Bengal  
Indian coasts are vulnerable to high waves and rough sea conditions particularly during the pre- and post-monsoon seasons due to the presence of extreme atmospheric events such as cyclones. Unfortunately, wave models are still not mature enough to predict these waves due to the lack of accurate initial conditions and correct physical parameterizations. With the availability of altimeter data, assimilation of these data in numerical models is often found beneficial in improving the accuracies of the model predictions. One very significant promising development, particularly for the coastal regions, is the SARAL/AltiKa mission carrying onboard altimeter in the Ka-band and thus offering high spatial resolution suitable for coastal studies and assimilation in coastal wave models. Tracks in the full Bay of Bengal region for the concurrent period of SARAL/AltiKa and Jason-2 (March 2013-July 2016) were used and 1 Hz Sea Surface Height (SSH) spectrum of both altimeters was studied. The Figure 14 shows the SSH spectrums of both the altimeters. Difference is noteworthy at wave length less than 50 km. The higher slopes in the wave lengths below 50 km shows the spectrum for SARAL/AltiKa have lesser noise than Jason-2 in this region. The power spectrum in the mesoscale region (50 km-250 km) is also fitted with power law a*k\({}^{b}\) (k is wavenumber). The values of b for AltiKa is \(-1.96\) as compared to b = \(-1.79\) for Jason-2. Above 50 km wave length, both SARAL/AltiKa and Jason are comparable to each other.