Low grazing angle radar imaging experiments over the South Falls sandbank

Lamont-Smith, T.; Jackson, A. M.; Shepherd, P.W.; Hill, Robert D.

doi:10.1080/01431160512331316441

Cited by 4 publications

(3 citation statements)

References 30 publications

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“…The details of this method are described chapter 3 of reference [10] and other references contained therein. The effects of current variations are often clearly evident on radar imagery from remote sensing satellites to low grazing angle airborne radar [12].…”

Section: Variation Of Tidal Currentsmentioning

confidence: 99%

Modelling radar sea clutter in littoral environments

Ward¹,

Tough²

2008

2008 International Conference on Radar

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A model is presented for low grazing angle sea clutter in littoral environments. It is based on a simple characterisation of the clutter in terms of scattering from ripples riding on long waves (the composite model) and from breaking waves; both being subjected to obscuration and multipath illumination. In littoral environments changes to the ocean waves are caused by limited fetch, wave refraction, current variations and enhanced breaking in shallow water. Simple models for these phenomena are described along with their effect on clutter radar cross section (RCS). I. INTRODUCTIONThe operation of maritime radar systems close to the coast is affected by the characteristics of the radar reflections from the sea surface, known as sea clutter. Experimentally it has been found that the sea clutter in these littoral environments is different from that in the open ocean, and often causes degradation in radar performance. The successful method used to model sea clutter empirically in the open ocean, by taking many measurements and looking for trends in behaviour as a function of viewing direction and sea conditions, is difficult to repeat in the littoral. This is because there are too many factors affecting the behaviour, as each environment is different. To make progress it is necessary to model the behaviour of the sea surface in littoral conditions and then to calculate radar scattering from that surface. The resulting modelled behaviour of sea clutter may then be analysed for measurable trends, which can be verified empirically.In this paper we investigate such an approach applied to low radar grazing angles, typical of radar systems operating from aircraft or ships. We consider back-scattering from the sea surface but, except for local forward scattering causing interference at the surface, do not include propagation effects that are important for some types of radar. Two sections on the description of the sea surface in the open ocean are followed by an outline of the radar cross section (RCS) modelling technique and results. The four principal processes that change the character of the sea surface in the littoral environment are then described along with how to extend RCS modelling to these conditions. Initial results of the effect of limited fetch on RCS are presented.

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Section: Variation Of Tidal Currentsmentioning

confidence: 99%

Modelling radar sea clutter in littoral environments

Ward¹,

Tough²

2008

2008 International Conference on Radar

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“…Signatures of normalized radar cross section (NRCS) modulation at the ocean surface due to sea bottom topography are visible on a variety of radar images derived by shore‐ and ship‐based radar as well as by real aperture radar (RAR) and synthetic aperture radar (SAR) on board airborne and spaceborne platforms [ McLeish et al , 1981; Hennings et al , 1994; Vogelzang et al , 1997; Lamont‐Smith et al , 2005]. A first descriptive explanation of such phenomena was given by De Loor [1981] and coworkers in the Netherlands.…”

Section: Introductionmentioning

confidence: 99%

Radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions

Hennings

Herbers

2006

J. Geophys. Res.

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The investigations carried out between 2002 and 2004 during six field experiments within the Operational Radar and Optical Mapping in monitoring hydrodynamic, morphodynamic and environmental parameters for coastal management (OROMA) project aimed to improve the effectiveness of new remote sensing monitoring technologies such as shipborne imaging radars in coastal waters. The coastal monitoring radar of the GKSS Research Center, Geesthacht, Germany, is based on a Kelvin Hughes RSR 1000 X band (9.42 GHz) vertical (VV) polarized river radar and was mounted on board the research vessel Ludwig Prandtl during the experiments in the Lister Tief, a tidal inlet of the German Bight in the North Sea. The important progress realized in this investigation is the availability of calibrated X band radar data. Another central point of the study is to demonstrate the applicability of the quasi‐specular scattering theory in combination with the weak hydrodynamic interaction theory for the radar imaging mechanism of the seabed. Radar data have been taken at very low grazing angles ≤2.6° of flood and ebb tide–oriented sand wave signatures at the sea surface during ebb tidal current phases. Current speeds perpendicular to the sand wave crest ≤0.6 m s−1 have been measured at wind speeds ≤4.5 m s−1 and water depths ≤25 m. The difference between the maximum measured and simulated normalized radar cross section (NRCS) modulation of the ebb tide–oriented sand wave is 27%. For the flood tide–oriented sand wave, a difference of 21% has been calculated. The difference between the minimum measured and simulated NRCS modulation of the ebb tide–oriented sand wave is 10%, and for the flood tide–oriented sand wave, a value of 43% has been derived. Phases of measured and simulated NRCS modulations correspond to asymmetric sand wave slopes. The results of the simulated NRCS modulation show the qualitative trend but do not always quantitatively match the measured NRCS modulation profiles because the quasi‐specular scattering theory at very low grazing angle is a first‐order theory.

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“…The strain rate is a quantity of interest because it is well known to have an effect on the RCS of the sea surface (Alpers and Hennings 1984). Lamont-Smith et al (2005) have recently shown that surface strain rate can have a particularly strong effect on radar backscatter at low grazing angles and at low wind speeds. There are quantitative differences in the image produced in figure 7, depending on exactly how the current and the current gradient are calculated.…”

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confidence: 99%

The effect of tidal currents on radar backscatter from the sea around Portland Bill

Lamont-Smith

Dovey

2005

International Journal of Remote Sensing

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Low grazing angle radar data of the sea surface were collected using three different frequencies (3, 10 and 16 GHz) from a cliff-top site on the south coast of England. A number of features were observed in the radar imagery that could be related to the tidal current flow around Portland Bill. The strongest, most obvious features occurred near the time of low water, and these features had significantly reduced backscatter levels in horizontal polarization, with reductions up to 20 dB, or more, below the clutter levels around them. In vertical polarization, the reduction in the clutter level was typically somewhat smaller (10 dB or so) than the horizontally polarized backscatter. No convincing explanation of this effect has been found. The strain rate component in the radar line of sight was estimated from measurements of the current component calculated from the radar data. A comparison of range-time intensity images of the radar backscatter and the strain rate showed a number of strongly correlated features that repeated with the semi-diurnal tidal period. The maximum strain rate was around 0.0005 s 21 , which produced modulations in the radar backscatter of around 2-3 dB. On occasions a number of bright streaks with a separation of around 100 m were also observed, moving away from the radar at a few cm s 21 . A satellite image from European Remote Sensing Satellite (ERS)-2 of the Portland area suggests that the slow-moving streaks may be internal waves generated by tidal flow over raised bathymetry.

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Low grazing angle radar imaging experiments over the South Falls sandbank

Cited by 4 publications

References 30 publications

Modelling radar sea clutter in littoral environments

Modelling radar sea clutter in littoral environments

Radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions

The effect of tidal currents on radar backscatter from the sea around Portland Bill

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