Real aperture radar imaging of ocean waves during SAXON‐FPN: A case of azimuth‐traveling waves

Askari, F.; Keller, William C.

doi:10.1029/93jc01903

Cited by 4 publications

(2 citation statements)

References 21 publications

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“…All these parameters can be obtained reliably from directional in situ instruments measuring surface displacements or bottom pressure and velocity [16]. As reported, SAR-derived spectra have a tendency to overestimate the wave heights by 10-70% in images acquired at lower incidence angles (19)(20)(21)(22)(23)(24)(25)(26). According to the inversion methodology, this is likely a result of an underestimation of the tilt MTF.…”

Section: Validation Of Sar-inverted Spectra In Coastal Areasmentioning

confidence: 88%

“…It should be noted that waves propagating in the radar azimuth direction were observed, with wavelength down to 60 m. This is a very unusual situation due to a low wind speed and small wave slopes for which the cutoff is very weak. The imaging process for these waves is thus likely related to a RAR modulation mechanism such as described by [20] and not explicitly represented in the inversion algorithm used here.…”

Section: Discussionmentioning

confidence: 99%

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Extraction of Coastal Ocean Wave Fields From SAR Images

Collard

Ardhuin

Chapron

2005

IEEE J. Oceanic Eng.

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Wave spectra derived from synthetic aperture radar (SAR) images acquired by ENVISATs are compared to in situ measurements by seven sensors, deployed in a field experiment carried out on the French coast of La Manche (English Channel). The wave spectra inversion scheme is adapted for shallow water from the European Space Agency (ESA)'s operational processing techniques used for level 2 ocean wave products. Under the low to moderate wind speed observed conditions, overall good agreement is found between in situ and SAR observations over a wide range of wave heights and directions, including waves propagating in the radar azimuth direction and incidence angles different from the standard imagette products. Index Terms-Ocean waves, synthetic aperture radar. I. INTRODUCTION K NOWLEDGE of wave conditions, either as climatology or as short-term forecast, is critical for all human activities at sea, including shipping, fishing, oil extraction, and naval operations. The development of wave models has been very fruitful over the past few decades, and wave forecasts are now quite reliable in the open ocean. Because wave models compute the wave field from surface winds, often provided by atmospheric models, this progress was made possible by advances in weather forecasting and remote sensing of surface winds over the oceans [1]. This reliability of wave models in forecasting significant wave heights has also been established thanks to wave height measurements from space-borne range altimeters as well as in situ observation from wave buoys. Current efforts to improve global wave forecasting are now limited by the poor availability of spectral wave measurements that may validate choices in the model parameterizations that are tuned to give similar wave heights on average, but often result in widely different spectral shapes. In particular, it is widely recognized that long period waves unrelated to local winds (swells) are still poorly predicted [2]. Synthetic aperture radar (SAR) "imagettes" covering small areas of the ocean surface (10 5 km) have now been acquired routinely since 1992 by ERS-1 and-2 and now EN-VISAT. These data are still little used because of limitations in the conditions under which a wave spectrum can be easily retrieved from the radar image and the difficulty of assimilating Manuscript

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Section: Validation Of Sar-inverted Spectra In Coastal Areasmentioning

confidence: 88%

Section: Discussionmentioning

confidence: 99%

Extraction of Coastal Ocean Wave Fields From SAR Images

Collard

Ardhuin

Chapron

2005

IEEE J. Oceanic Eng.

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Radar imaging of sand waves on the continental shelf east of Cape Hatteras, NC, U.S.A.

Donato

Askari

Marmorino

et al. 1997

Continental Shelf Research

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Combined analysis of the radar cross‐section modulation due to the long ocean waves around 14° and 34° incidence: Implication for the hydrodynamic modulation

Hauser¹,

Caudal²

1996

J. Geophys. Res.

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The analysis of synthetic aperture radar observations over the ocean to derive the directional spectra of the waves is based upon a complex transfer function which is the sum of three terms: tilt modulation, hydrodynamic modulation, and velocity bunching effect. Both the hydrodynamic and the velocity bunching terms are still poorly known. Here we focus on the hydrodynamic part of the transfer function, from an experimental point of view. In this paper a new method is proposed to estimate the hydrodynamic modulation. The approach consists in analyzing observations obtained with an airborne real‐aperture radar (called RESSAC). This radar (C band, HH polarized, broad beam of 14° × 3°) was used during the SEMAPHORE experiment, in two different modes. From the first mode (incidence angles from 7° to 21°) the directional spectra of the long waves are deduced under the assumption that the hydrodynamic modulation can be neglected (small incidence angles) and validated against in situ measurements. From the second mode (incidence angle from 27° to 41°) the amplitude and phase of the hydrodynamic modulation are deduced by combining the measured signal modulation spectrum at a mean incidence angle of 34° and the directional wave spectrum obtained from the first mode. The results, obtained in four different wind‐wave cases of the SEMAPHORE experiment, show that the modulus of the hydrodynamic modulation is larger than that of the tilt modulation. Furthermore, we find that the modulus of the hydrodynamic transfer function is several times larger (by a factor 2–12) than the theoretical value proposed in previous works and 1.5–2.5 larger than experimental values reported in recent papers. The phase of the hydrodynamic modulation is found to be close to zero for waves propagating at an angle from the wind direction and between −20° and −40° for waves propagating along the wind direction. This indicates a significant influence of the wind‐wave angle on the phase of the hydrodynamic modulation, in agreement with experimental results reported in recent papers.

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Real aperture radar imaging of ocean waves during SAXON‐FPN: A case of azimuth‐traveling waves

Cited by 4 publications

References 21 publications

Extraction of Coastal Ocean Wave Fields From SAR Images

Extraction of Coastal Ocean Wave Fields From SAR Images

Radar imaging of sand waves on the continental shelf east of Cape Hatteras, NC, U.S.A.

Combined analysis of the radar cross‐section modulation due to the long ocean waves around 14° and 34° incidence: Implication for the hydrodynamic modulation

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