## 3 Unique Contributions of Ka-Band Information in AltiKa Data
### Relationship between Surface Soil Moisture and Radar Altimetry Backscattering Coefficients  
Spatial and temporal variations of radar altimetry backscattering coefficients were related to the dynamics of surface properties. In semi-arid areas, soil moisture drives several surface processes including soil organic matter mineralization [49], vegetation productivity [50], land surface fluxes [51] and land surface atmosphere interactions [52]. A first inversion of the surface soil moisture (SSM) from the Ice-1 derived backscattering coefficients at Ku-band from Envisat RA-2 data was performed over Sahelian savannahs in the Gourma region of Mali [53]. Due to the lower sensitivity of nadir-looking altimeters to vegetation cover compared with side-looking SAR and scatterometers [54], higher correlation was found between in-situ SSM measurements and altimetry (R = 0.88, [53]) than with SAR (R = 0.85, [55]) and scatterometer (R = 0.63, [56]) backscattering coefficients over the sandy sites of the same semi-arid study area. Using the 16 first available cycles of the SARAL/AltiKa mission, the correlation is 0.88 between radar altimetry Ice-1 derived backscattering coefficients at Ka-band and level-3 SSM products derived from the Soil Moisture and Ocean Salinity satellite (SMOS) passive microwave observations over Sahelian savannahs [57]. The results obtained were generally better than the ones obtained using 3.5 times more numerous backscattering coefficients at Ku and C-bands from Jason-2 during the same observation period or the ones obtained using backscattering coefficients at Ku and S-bands from Envisat during its whole observation period (and comparing them to level-3 SSM products derived from AMSR-E) or passive microwave observations [57]. Time series of altimetry backscattering coefficients (obtained using Ice-1 retracking algorithm) at Ka-band from SARAL/AltiKa and Ku and C bands from Jason-2 are presented in Figure 22 from February 2013 to May 2016 over Sudano-Sahelian savanahs. This period corresponds to the 35 cycles of SARAL/AltiKa on its nominal orbit (and to cycles 169 to 291 of Jason-2). The backscattering level during the dry season decreases as the frequency increases--around 22.5 dB at C-band, 17 dB at Ku-band, and between 5 dB and 6 dB at Ka-band--whereas the amplitude of variations increases with the frequency--up to 10 dB at C-band, 20 dB at Ku-band and 30 dB at Ka-band. The larger dynamic of the backscattering coefficient at Ka-band during the rainy season is a strong advantage to accurately monitor the time-variations of SSM in semi-arid areas. Correlation coefficients between backscattering coefficients and SSM from SMOS (derived using the approach from [58]) were computed on the common period of availability of the two datasets, from February 2013 to May 2015. Due to the rapid response of SSM to rainfall--strong increase after a rainfall event followed by a fast decrease due to evaporation--in semi-arid environments,  
Figure 21: Percentage of 1 Hz flagged samples using the operational MP algorithm for which the ILWC >0.1 kg/m\({}^{2}\) for 1 year (cycles 100 to 110). correlation were re-estimated applying a three-day smoothing window to limit the effect of the difference of acquisition time between altimeters and SMOS. Very similar results are obtained for both ascending and descending tracks:  
1. R = 0.80 and 0.79 at Ka-band for SARAL/AltiKa tracks 0001 and 0846 respectively,
2. R = 0.58 and 0.54 at Ku-band for Jason-2 tracks 046 and 161 respectively,
3. R = 0.64 and 0.65 at C-band for Jason-2 tracks 046 and 161 respectively. As the backscattering coefficient is linearly related to SSM over sand, better estimates of SSM can be expected from the inversion of altimetry backscattering coefficient at Ka-band than at other frequency bands commonly used in altimetry.