Validity assessment of SAMOSA retracking for fully-focused SAR altimeter waveforms

Ehlers, F. E.; Schlembach, Florian; Kleinherenbrink, Marcel; Slobbe, Cornelis

doi:10.1016/j.asr.2022.11.034

Cited by 9 publications

(16 citation statements)

References 28 publications

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“…The difference in offsets between FF-SAR and UF-SAR is as expected and due to the fitting of the FF-SAR-processed waveforms against the SAMOSA2 zero-Doppler beam that was generated with a so-called unambiguous PTR approximation (European Space Agency, 2021). The used α p LUT was thus generated under the assumption of an ideal PTR without considering any grating lobes (Ehlers et al, 2022), which might cause parts of the additional offset. Another part of the offset might be caused by the fitting of the SAMOSA2 zero-Doppler beam itself and the stronger dependence of FF-SAR on vertical wave velocities (Buchhaupt et al, 2021).…”

Section: Discussionmentioning

confidence: 99%

“…This is established using a multi-mission FF-SAR processor implementation originally developed for CS2 by Kleinherenbrink et al (2020), which applies a back-projection algorithm as presented in Egido and Smith (2017). The extension to the S3 and S6-MF missions is described in detail in Ehlers et al (2022). The FF-SAR processing includes the range cell migration correction (RCMC), the residual video phase (RVP) correction, and the compensation for additional phase jumps and other mission-specific settings (Ehlers et al, 2022).…”

Section: Level-1b Processingmentioning

confidence: 99%

“…The extension to the S3 and S6-MF missions is described in detail in Ehlers et al (2022). The FF-SAR processing includes the range cell migration correction (RCMC), the residual video phase (RVP) correction, and the compensation for additional phase jumps and other mission-specific settings (Ehlers et al, 2022).…”

Section: Level-1b Processingmentioning

confidence: 99%

“…That is, only minor spurious grating lobes (or: target replicas) are expected in the along-track direction at multiples of ∼300 m (Ehlers et al, 2022), as compared to the CS2 or S3 missions that exhibit more frequent and stronger grating lobes at ∼90 m (Egido and Smith, 2017;Guccione et al, 2018) due to the lacunar sampling/closed-burst operation mode.…”

Section: Introductionmentioning

confidence: 99%

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Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Schlembach

Ehlers

Kleinherenbrink

et al. 2022

Preprint

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Estimating the three geophysical variables significant wave height (SWH), sea surface height, and wind speed from satellite altimetry continues to be challenging in the coastal zone because the received radar echoes exhibit significant interference from strongly reflective targets such as mud banks, sheltered bays, ships etc. Fully focused SAR (FF-SAR) processing exhibits a theoretical along-track resolution of up to less than half a metre. This suggests that the application of FF-SAR altimetry might give potential gains over unfocused SAR (UF-SAR) altimetry to resolve and mitigate small-scale interferers in the along-track direction to improve the accuracy and precision of the geophysical estimates. The objective of this study is to assess the applicability of FF-SAR-processed Sentinel-6 Michael Freilich (S6-MF) coastal altimetry data to obtain SWH estimates as close as possible to the coast. We have developed a multi-mission FF-SAR processor and applied the coastal retracking algorithm CORALv2 to estimate SWH. We assess different FF-SAR and UF-SAR processing configurations, as well as the baseline Level-2 product from EUMETSAT, by comparison with the coastal, high-resolution SWAN-Kuststrook wave model from the Deltares RWsOS North Sea operational forecasting system. This includes the evaluation of the correlation, the median offset, and the percentage of cycles with high correlation as a function of distance to the nearest coastline. Moreover, we analyse the number of valid records and the L2 noise of the records. The case study comprises five coastal crossings of S6-MF that are located along the Dutch coast and the German coast along the East Frisian Islands in the North Sea. We find that the FF-SAR-processed dataset with a Level-1b posting rate of 140 Hz shows the greatest similarity with the wave model. We achieve a correlation of ˜0.8 at 80% of valid records and a gain in precision of up to 29% of FF-SAR vs UF-SAR for 1-3 km from the coast. FF-SAR shows, for all cycles, a high correlation of greater than or equal to 0.8 for 1-3 km from the coast. We estimate the decay of SWH from offshore at 30 km to up to 1 km from the coast to amount to 26.4%+-3.1%.

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Section: Discussionmentioning

confidence: 99%

Section: Level-1b Processingmentioning

confidence: 99%

Section: Level-1b Processingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Schlembach

Ehlers

Kleinherenbrink

et al. 2022

Preprint

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“…For S6-MF, this results in an approximate A value of −33 dB and a position d of around 300 m. Recent research [14,15] has highlighted the multifaceted nature of replicas, affecting both the along-track and across-track dimensions. These replicas can contaminate different range gates, resulting in a smearing effect.…”

Section: Replicas Mitigationmentioning

confidence: 99%

Optimal Configuration of Omega-Kappa FF-SAR Processing for Specular and Non-Specular Targets in Altimetric Data: The Sentinel-6 Michael Freilich Study Case

Amraoui,

Guccione,

Moreau

et al. 2024

Remote Sensing

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In this study, the full-focusing (FF) algorithm is reviewed with the objective of optimizing it for processing data from different types of surfaces probed in altimetry. In particular, this work aims to provide a set of optimal FF processing parameters for the Sentinel-6 Michael Freilich (S6-MF) mission. The S6-MF satellite carries an advanced radar altimeter offering a wide range of potential FF-based applications which are just beginning to be explored and require prior optimization of this processing. In S6-MF, the Synthetic Aperture Radar (SAR) altimeter acquisitions are known to be aliased in the along-track direction. Depending on the target, aliasing can be tolerated or may be a severe impairment to provide the level of performance expected from FF processing. Another key aspect to consider in this optimization study is the unprecedented resolution of the FF processing, which results in a higher posting rate than the standard SAR processing. This work investigates the relationship between posting rate and noise levels and provides recommendations for optimal algorithm configurations in various scenarios, including transponder, open ocean, and specular targets like sea-ice and inland water scenes. The Omega–Kappa (WK) algorithm, which has demonstrated superior CPU efficiency compared to the back-projection (BP) algorithm, is considered for this study. But, unlike BP, it operates in the Doppler frequency domain, necessitating further precise spectral and time domain settings. Based on the results of this work, real case studies using S6-MF acquisitions are presented. We first compare S6-MF FF radargrams with Sentinel-1 (S1) images to showcase the potential of optimally configured FF processing. For highly specular surfaces such as sea-ice, distinct techniques are employed for lead signature identification. S1 relies on image-based lineic reconstruction, while S6-MF utilizes phase coherency of focalized pulses for lead detection. The study also delves into two-dimensional wave spectra derived from the amplitude modulation of image/radargrams, with a focus on a coastal example. This case is especially intriguing, as it vividly illustrates different sea states characterized by varying spectral peak positions over time.

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Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Schlembach

Ehlers²,

Kleinherenbrink³

et al. 2023

Preprint

View full text Add to dashboard Cite

Estimating the three geophysical variables significant wave height (SWH), sea surface height, and wind speed from satellite altimetry continues to be challenging in the coastal zone because the received radar echoes exhibit significant interference from strongly reflective targets such as mud banks, sheltered bays, ships etc. Fully focused SAR (FF-SAR) processing exhibits a theoretical along-track resolution of up to less than half a metre. This suggests that the application of FF-SAR altimetry might give potential gains over unfocused SAR (UF-SAR) altimetry to resolve and mitigate small-scale interferers in the along-track direction to improve the accuracy and precision of the geophysical estimates. The objective of this study is to assess the applicability of FF-SAR-processed Sentinel-6 Michael Freilich (S6-MF) coastal altimetry data to obtain SWH estimates as close as possible to the coast. We have developed a multi-mission FF-SAR processor and applied the coastal retracking algorithm CORALv2 to estimate SWH. We assess different FF-SAR and UF-SAR processing configurations, as well as the baseline Level-2 product from EUMETSAT, by comparison with the coastal, high-resolution SWAN-Kuststrook wave model from the Deltares RWsOS North Sea operational forecasting system. This includes the evaluation of the correlation, the median offset, and the percentage of cycles with high correlation as a function of distance to the nearest coastline. Moreover, we analyse the number of valid records and the L2 noise of the records. The case study comprises five coastal crossings of S6-MF that are located along the Dutch coast and the German coast along the East Frisian Islands in the North Sea. We find that the FF-SAR-processed dataset with a Level-1b posting rate of 140 Hz shows the greatest similarity with the wave model. We achieve a correlation of ~0.8 at 80% of valid records and a gain in precision of up to 29% of FF-SAR vs UF-SAR for 1-3 km from the coast. FF-SAR shows, for all cycles, a high correlation of greater than or equal to 0.8 for 1-3 km from the coast. We estimate the decay of SWH from offshore at 30 km to up to 1 km from the coast to amount to 26.4% +- 3.1%.

show abstract

Validity assessment of SAMOSA retracking for fully-focused SAR altimeter waveforms

Cited by 9 publications

References 28 publications

Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Optimal Configuration of Omega-Kappa FF-SAR Processing for Specular and Non-Specular Targets in Altimetric Data: The Sentinel-6 Michael Freilich Study Case

Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

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