## 3 Unique Contributions of Ka-Band Information in AltiKa Data
### Complementarity between Ku and Ka Measurements over Ice Sheet Surfaces
As for SARAL/AltiKa, Sentinel-3A SAR mode mean echo over Lake Vostok presents a leading edge not impacted by penetration effects. Figure 16: Mean SARAL/AltiKa (**left**) and Sentinel-3A (**right**) waveforms over ocean (blue) and Lake Vostok (red). This analysis illustrates the complementarity between Ku and Ka measurements over ice sheet surfaces and the potential relevant information that could be retrieved by combining them. Of particular interest is the snow thickness that could be derived from the exploitation of the Ku and Ka waveform shapes, in particular their trailing edges. Analysis of SARAL/AltiKa measurements over ice sheet have not only brought a new insight of the radar measurements of the snowpack but also encourage the development of new algorithms able to derive new parameters such as snow or ice pack properties. Scientific applications based on Ka-band unique characteristics and complementarity with Ku-band over ice sheet, icebergs and sea ice are developed in [3]. Impact of Rain on SARAL/AltiKa Measurements, a New Opportunity to Observe and Study the Rainfall Climatology  
Since the first developments of the SARAL/AltiKa mission, one of the major concerns has been the Ka-band sensitivity to atmospheric conditions, and especially to atmospheric liquid water (both rain and clouds). Hence, the instrument power and gain have been optimized in order to maximize the Signal to Noise Ratio (SNR) and to minimize atmospheric attenuation issues [27]. As a result, the data loss over ocean is lower than anticipated (<0.1%) and the SNR is higher than the preflight expected value (14 dB) thanks to margins taken in the altimeter link budget (see details in [27]). An example of an AltiKa SNR map over ocean is provided in Figure 18 for cycle number 110, showing values varying from 18 to 32 dB. The altimeter link budget is thus excellent and allows a full data coverage over ocean. Despite the very low number of lost data, the high sensitivity of the Ka-band to atmospheric liquid water (ten times larger than in Ku-band) impacts the altimeter waveform shape. AltiKa echoes are attenuated and distorted by rain events of heavy clouds [38; 39; 40], as shown in Figure 19a. The waveform amplitude is not only strongly attenuated but its shape is distorted, especially on the trailing edge part of the waveform. The example shown in Figure 19a is collocated to WindSat and SSMIS-F16/F17 rainfall rate measurements indicating a rain rate of about 4.5 mm/h during this event. Figure 17: **Left**: Mean SARAL/AltiKa (blue) and Sentinel-3A (SAR in red, P-LRM in green) waveforms at a cross-over between SARAL/AltiKa and Sentinel-3A over Lake Vostok. **Right**: MODIS image of Lake Vostok with SARAL/AltiKa and Sentinel-3A tracks locations. Even if measurements are not lost during a rain event, the resulting waveform corruption may degrade the retracker performances and the geophysical estimates [42]. Indeed, operational ocean retrackers implemented in the ground segment are not designed to correctly process rain waveforms: these retrackers consider that the altimeter waveform footprint contains homogeneous backscattering properties. This is clearly not the case during rain events. As a result, the noise on each geophysical estimate increases when SARAL/AltiKa is overflying a rain cell. Impact of rain pollution on the range noise is illustrated in Figure 20. The collocation between SARAL/AltiKa and the WindSat/SSMIS measurements has been performed accounting for a maximum time delay of 1 hour. A clear rain  
Figure 19: (**a**) AltiKa waveforms corrected from Automatic Gain Control during a rain event, pass 121 cycle 5; (**b**) collocated rainfall rate from SSMI and WindSat measurement in grey and Matching Pursuit flag in black [41]. Figure 18: Map of Signal to Noise Ratio computed from standard MLE-4 estimates over ocean for cycle number 110 (from 2017/06/19 to 2017/07/24). rate dependency of the altimeter range noise (about 0.35 cm/mm/h) can be observed.