Sergi Hernández

Fully focused synthetic aperture radar (SAR) back-projection algorithms for radar altimetry have gained attention in the altimetry community due to their improved azimuth resolution compared to traditional methods, such as the delay/Doppler algorithm. However, the significant computational cost of back-projection has limited its use in operational applications. This paper presents an accelerated back-projection algorithm that reduces the runtime of the classic back-projection by a factor of 28 on a CPU-based architecture. When implemented on a GPU-based architecture, the accelerated back-projection achieves a speedup ranging from 8 to 13 times compared to the classic back-projection on the same GPU. In comparison to the classic back-projection running on a CPU, the combination of the accelerated back-projection and GPU processing results in an overall improvement of up to 1570 times. The accelerated algorithm maintains the accuracy of classic time-domain algorithms, and allows to process data in the order of the sensing time under a GPU-based architecture. The algorithm has been extensively validated using transponder and open ocean data. These results indicate that operational back-projection algorithms are feasible for current radar altimetry missions, such as Sentinel-6, and upcoming missions like CRISTAL and Sentinel-3 Next Generation.

Until recently, intensity modulations in synthetic aperture radar (SAR) altimetry waveform tails have been considered a nuisance for geophysical-parameter retrieval. These modulations are actually predictable and might be exploited using a spectral analysis of the waveform tails. After Altiparmaki et al. (2022), a more elaborated analysis is performed to improve the interpretation of these SAR altimeter spectra. A fast numerical model is developed to explain the modulation mechanisms in focused SAR altimetry waveform tails. Using numerical solutions, standard analytical closed-form solutions, are demonstrated to be invalid to retrieve ocean-wave-spectra retrievals from nadir altimeters. Although not valid, a closed-form derivation provides intuitive insights about the information contained in a SAR altimetry cross-spectrum. Under moderate environmental conditions (significant wave heights of ~2 m), a closed-form solution might still be useful to infer swell-wave spectra from swath-altimetry SAR spectra at incident angles of ~4°. Comparable to side-looking SAR ocean processing, the cross-spectral analysis for nadir signals reduces noise and might remove the 180-degree ambiguity of the wave direction. Since the synthetic aperture length of nadir altimeters is larger than sidelooking imaging SARs (e.g. Sentinel-1, RadarSat, Gaofen-3), sublook processing can be performed to compute multiple cross-spectra for the same scene. With a slightly changing observation geometry, the cross-spectra reveal slightly different parts of the ocean-wave spectrum. The resulting stack of cross-spectra can thus be used to improve the retrieval of ocean-wave parameters. Retrieved ocean-wave parameters shall then enhance the sampling of the global wave field, but also serve to advance more consistent sea-state-bias corrections.

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. 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. Ongoing projects like SARWAVE by the European Space Agency (ESA) 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.

The two-dimensional frequency-based omega-K method is known to be a suitable algorithm for Fully-Focused SAR (FF-SAR) radar altimeter processors, as its computational efficiency is much higher than equivalent time-based alternatives without much performance degradation. In this paper we provide a closed-form description of a two-dimensional frequency domain omega-K algorithm specific for instruments such as Poseidon-4 onboard Sentinel-6. The processor is validated with real data from point targets and over open ocean. Applications such as ocean swell retrieval and lead detection are demonstrated, showing the potentiality of the processor for future operational global-scale products.

The Copernicus Sentinel-6 mission was planned to keep studying the sea-surface height and ocean state measurements and since April 2022 its first satellite, the Sentinel-6 Michael Freilich (S6MF), has become the current reference altimetry mission.One of the key design aspects of the S6MF altimeter is the “interleaved” chronogram pattern, which increases the number of received pulses, reducing the ambiguities in the along-track dimension, and increasing the energy obtained from the surface. Thanks to the continuous surface illumination, the echoes from the same target can be coherently integrated making a significant improvement in the along-track resolution leading to more detailed understanding of the ocean, the polar zones and inland and coastal waters dynamics. Measurement parameters such as swell state, lead and iceberg detection or inland water level changes can take advantage of these advances.Nowadays, unfocused azimuth steering methods, such as Delay Doppler, provide along-track resolutions around 300 meters. However Fully-Focused SAR (FF-SAR) algorithms can improve it to the order of sub-meter. In 2017, Alejandro Egido and Walter H.F. Smith published the FF-SAR method description, based on the backprojection approach. Compared to the unfocused steering, it requires more computational time, making it difficult to be implemented operationally, keeping the data generation rate in the same order of magnitude as the unfocused chain products. In 2018, Pietro Guccione et. al. published the FF-SAR 2D Frequency Domain algorithm, based on the Omega-K (WK) algorithm from SAR radar. In this paper, it is shown that the two dimensional frequency domain can be used to decrease the number of operations needed to focus the data, under some circumstances that depend on the type of orbit and the emitted signal. FF-SAR WK algorithm achieves similar results in terms of along-track resolution for the CryoSat-2 mission, but notably improving the computational efficiency. Although Omega-K algorithm intrinsic assumptions can impact negatively in the accuracy estimation of parameters such as the sea surface height, there are applications like sea ice related activities that could benefit of faster execution times. In this presentation, an adapted and redesigned Omega-K algorithm for Sentinel-6 is presented. FF-SAR Omega-K and Backprojection have been used to process Sentinel-6 data over the Crete transponder, evidencing that both FF-SAR methods are capable to achieve the expected theoretical along-track resolution. Moreover, open ocean data from Sentinel-6 has been processed for both algorithms and results have been compared.

A trihedral corner reflector has been used to evaluate the capability of passive reflectors to calibrate radar altimeters such as the Poseidon-4 altimeter on board Sentinel-6A. The reflector location, placed on the top of a mountain ridge and about 4-km off-nadir of the Sentinel-6A subsatellite track, allows capturing echoed signals with a Signal-to-Clutter Ratio around 40 dB when processing the received data with a Fully-Focussed SAR algorithm. Results obtained show a range bias of 33.9 mm with 8.5 mm of standard deviation and datation bias of -2.3 μs with standard deviation of 1.8 μs for the measurement campaign between September 2021 and April 2022. Such values are comparable to what is currently achieved by means of active transponders and therefore it is demonstrated that passive reflectors may be of interest to support radar altimeter regular calibration.