Abstract:The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. The mission ended its nominal phase after 3 years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to provide a state of the art of the achievements of the SARAL/AltiKa mission in terms of quality assessment and unique characteristics of AltiKa data. It shows that the AltiKa data have similar accuracy at the centimeter level in term of absolute water level whatever the method (from local to global) and the type of water surfaces (ocean and lakes). It shows also that beyond the fact that AltiKa data quality meets the expectations and initial mission requirements, the unique characteristics of the altimeter and the Ka-band offer unique contributions in fields that were previously not fully foreseen.

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Five-parameter model (BETA5) has been used to derive significant wave height (SWH) using 20-Hz Jason-2/OSTM Ku band waveforms in the Bay of Bengal basin of the Indian Ocean near the Indian city of Visakhapatnam for the years 2010-2012. Majority of the waveforms are traditional ocean-like waveforms based on Brown model. Nevertheless, quite a few of them have shapes differing significantly from the Brown model. Historically, two standard methods have been used for waveform retracking. One is maximum likelihood estimator (MLE) with 4 parameters, i.e. MLE4 (OCEAN retracker), which uses analytical second-order Brown model and the second one using first-order Brown model is MLE with three parameters, i.e. MLE3. It has been found in the study that standard MLE retrackers are not appropriate for retracking the nontraditional waveforms. Estimated SWHs have been validated against SWH measured by a buoy located in the close proximity of Visakhapatnam. SWH from BETA5 retracker matches buoy-derived SWH quite well except for a positive bias. Also, the bias is less for SWH retrieved using BETA5 than using MLE retrackers. Quantitatively, at a distance of 8 km from the coast correlation coefficient between BETA5 retracked SWH and buoy measured SWH is 0.85 and the unbiased root mean square error (RMSE) of the retracked SWH is 0.26 m. These are better than the corresponding values for MLE3 and OCEAN retrackers.

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