Unlocking the Imaging Capabilities of Synthetic Aperture Radar Altimetry for Ocean Applications

Abstract

As climate change continues to drive an increase in extreme weather events, the need for reliable predictions and observational data has never been more critical. Oceans, playing a vital role in regulating the Earth’s climate, are central to understanding these changes. Specifically, accurately modeling ocean wave dynamics, i.e., how waves are generated, evolve, and interact with oceanic processes such as currents, helps track ocean circulation and predict future variations. Thanks to Earth Observation satellites, continuous global observations have been available over the past few decades. Satellite altimeters, active sensors utilizing radar’s ranging capabilities, have emerged as pioneers in space oceanography. These instruments measure critical geophysical parameters such as sea surface height, significant wave height and near-surface wind speed along satellite tracks. Recognizing the immense value of these measurements for climate studies and operational activities, continuous technological innovations are essential for optimizing the performance and use of these instruments. One major breakthrough in satellite altimetrywas the incorporation of Synthetic Aperture Radar (SAR) technology in 2010, which enhanced spatial resolution from around 7 km, provided by Low-ResolutionMode sensors, to about 300m. In 2017, the fully-focused coherent processing of pulse echoes was implemented, a concept widely used in SAR imaging, enabling meter-scale resolution. This improvement led to significant benefits in near-coast applications, improving the quality of geophysical parameters by reducing signal contamination from surrounding land features. Additionally, for the first time, offnadir signals, previously considered a nuisance, were exploited to map narrow inland water bodies and detect sea-ice leads and floes. Recognizing this imaging potential led to investigating its capabilities over open oceans. Existing challenges in conventional, or unfocused, SAR altimetry relate to the accuracy of significant wave height estimates, especially when long waves, known as swells, dominate the sea surface. Swell waves, with wavelengths exceeding 150 meters, are often too long to be fully captured within the SAR altimeter’s footprint, leading to noisy, multi-peaked waveforms. Recognizing the interference of swell signals in SAR responses, combined with the high-resolution data provided by fully-focused processing, this dissertation first investigated the feasibility of transforming what was previously considered a nuisance into valuable information about the sea surface. To achieve this, the identification of swell-induced power variations in off-nadir altimeter’s signals, representing the so-called trailing edge of the returned echo, was first confirmed. These patterns were analyzed to compute a fully-focused modulation spectrumderived from altimetry. The modulation spectrumis a commonly used Level-2 product provided by satellites designed to measure the wave field, such as Sentinel-1 and CFOSAT, and allows for the estimation of swell characteristics, including wavelength, direction and wave height. The proposed method involved normalizing the signal intensity and re-projecting the range bins to cross-track ground locations, followed by spectral analysis akin to side-looking SAR systems. The analysis revealed that fully-focused altimetrymodulation spectra display power in all four quadrants due to the inherent 180-degree SAR ambiguity, plus two additional ambiguities caused by inseparable signals received from both sides of the radar footprint. The study also identified the main modulation mechanisms, using as reference numerical and analytical models. Range bunching was found to be a dominant mechanism alongside velocity bunching, with their relative strength highly dependent on the wave propagation angle. Fully-focused altimetry modulation spectra, derived from Cryosat-2, were evaluated through comparisons with buoy-derived directional wave spectra, showing good agreement. Furthermore, applying the proposed technique to Sentinel-6A data demonstrated that exploiting its full-beamfootprint, which is partially truncated onboard for data volume efficiency, improves swell retrieval. This is particularly true for waves propagating in or near the cross-track direction, due to its extended observational window and higher resolution compared to the operational truncated data. Yet, the development of a method to invert modulation-derived spectra to real ocean wave spectra is necessary to reliably use these instruments as a new source for providing operational global swell observations. The dissertation further explored the limitations of SAR altimeters in ocean wave imaging, focusing on resolution loss. This was addressed by estimating the azimuth cutoff wavelength, which serves as a proxy for the shortest detectable waves across different sea states and wave directions. The method used to estimate this parameter involved a SAR imaging technique applied in the spatial domain, minimizing residuals between the along-track autocorrelation function of fully-focused SAR radargrams, representing successive waveforms, and a fitted Gaussian function. Sentinel-6A data were then used to evaluate the method’s performance through comparisons with model-derived values globally. The analysis revealed that the method performs well under the majority of sea states but tends to underestimate values in extreme wind wave conditions. Furthermore, sensitivity to swell presence was observed, leading to pronounced over estimations, with the magnitude of these errors influenced by the swell direction. To mitigate these errors, an alternative approach was developed in the wave number domain. Results revealed an improvement in the correlation between azimuth cutoff estimates and model-derived values by 10%. Given the strong relationship between resolution loss and sea state conditions, the azimuth cutoff was further used to derive a new sea-state parameter: the variance of wave orbital velocities. Wave orbital velocity statistics offer valuable insights into wave climate by isolating wave components associated with developing seas. Comparisons between modeled and estimated wave orbital velocity variances showed similar sensitivities to swell presence and high sea states, suggesting further refinement of the proposed methods. Despite these challenges, the ability to extract these two additional parameters from the radar signal is valuable for identifying sensors capabilities and providing a new geophysical parameter for oceanographic studies. Lastly, the dissertation assessed the impact of wave-current interactions on wave products derived from both models and satellites, focusing on the Agulhas Current region, one of the most dynamic ocean environments. In situ wave measurements, collected during the One Ocean Expedition in 2023, in which the author participated, served as a reference for this study. The study first examined ocean current products. A clear underestimation of surface current velocities exceeding 0.5 m/s was found for both the Mercator operational model and the altimetry-derived Globcurrent product, with Mercator showing greater variability. Next, wave products, both with and without these ocean current products included in their modeling, were validated. The ECMWF reanalysis v5, known as ERA5, consistently underestimated wave heights above 2.5 m, whereas the MFWAM, which is the Global Ocean Wave Analysis and Forecast system from Meteo-France, showed good agreement with in situ data. This discrepancy was attributed to the lack of ocean current forcing in ERA5, underscoring the need for refinement in areas dominated by currents. Customized MFWAM simulations, including and excluding current data, further supported this finding. MFWAM forced with Globcurrent aligned most closely with drifter measurements, outperforming the operational product that uses Mercator currents. Comparisons between satellite altimeter observations and drifters also showed good agreement in significant wave height, with clear evidence of current-induced wave height variations along satellite tracks. Additionally, a multi-mission analysis of swellinduced modulation spectra from Sentinel-1, CFOSAT and SAR altimeters demonstrated alignment with in situ data and between them, highlighting the potential for synergistic use of these instruments in operational oceanography and climate studies.