Ocean-wave retrieval from Sentinel-6 radar altimeter cross-spectra
Authors: Hernández Sergi , Kleinherenbrink Marcel , Gibert Ferran , Guerra Michel 
Published: March 2024

Swell, as a fundamental component of wave dynamics, plays an important role in oceanographic systems. These long-period waves, generated by distant storms and weather systems, travel vast distances across open waters, impacting coastal environments and influencing a range of marine and coastal phenomena. Thus, accurate determination of swell wave parameters, such as the dominant wave period and the significant wave height, are of interest in ocean research.&#160;While swell retrieval using SAR techniques has been conducted for numerous years utilising satellites like Sentinel-1, observing swells through radar altimetry has been a pending research primarily due to the unique nadir-pointing geometry characteristic of radar altimeters and consequent cross-track ambiguity, along with the restricted spatial resolution that existed prior to the development of high-resolution algorithms. Traditionally, operational satellite radar altimeter missions such as Sentinel-3 or CryoSat-2 processed data with spatial resolution of around 300 meters with delay/Doppler processing, which was insufficient for short-period swell parameter estimation. However, recent advancements in processing techniques such as the Fully-Focused SAR (FF-SAR) backprojection algorithm and the 2D frequency algorithm have introduced significant improvements, increasing the along-track resolution to the meter-scale. In 2022, Ourania et al. demonstrated the feasibility of retrieving swell waves parameters through the analysis of intensity modulations in the waveform tail. Given that the geometry of radar altimetry diverges from that of other remote sensing systems such as conventional SAR, the interpretations and limitations associated with swell retrieval also differ significantly, ushering in new perspectives. In the context of radar altimetry, the monitoring of swells assumes a critical role in discerning the influence of wind waves and swell within the sea-state bias. Moreover, the process of swell-flagging is vital to identify potential biases present in retracker. Furthermore, swell observations derived from altimetry can help in the cross-calibration of swell observations originating from various platforms.&#160;Ongoing projects like SARWAVE by the European Space Agency (ESA)&#160; are currently involved in research efforts to provide a more detailed understanding of changes in swell intensity. This includes the cross-spectra analysis, aimed at elevating the accuracy of retrieving swell-wave parameters. In this context, we introduce a methodology and share the results related to swell parameters for the Sentinel-6 radar altimeter case, specifically swell wavelength and swell wave period. We have processed FF-SAR radar altimeter data using a sublooking technique that enabled us to generate a cross-spectra image, providing a basis for extracting valuable swell parameters. An Ordinary Least-Squares (OLS) model has been trained by using swell data from buoys and the ERA5 database as validation data. This rigorous training process aimed to refine the model's performance, ensuring its effectiveness in predicting and characterising swell characteristics.