Fully focused (FF) 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 article 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 for the processing of 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.

External calibration of radar altimeters by means of passive elements such as trihedral corner reflectors only became feasible after the development of Fully-Focussed SAR techniques specific for radar altimeter processors. Since then, corner reflectors have been demonstrated to be very interesting tools for instrument monitoring, with performances comparable to ones from active transponders for range and datation measurements, but also useful for power and radar cross section measurements. The corner reflector deployed by isardSAT in 2021 close to the ground tracks of Sentinel-6 MF, Sentinel-3B and CryoSat-2 has already been operative for more than two years. In this manuscript the main Sentinel-6 MF measurements are reported, showing excellent performance in terms of precision. A first analysis of correlation between the measurables and geometric and geophysical parameters is also provided.

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.

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.

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.