Adaptive Speckle Filtering in Radar Imagery

Abstract

" . Since , the team led by Dr. "rmand Lopès at the Centre d'Etude Spatiale des Rayonnements in Toulouse France , has carried out and then inspired the development of the most efficient speckle filters existing today. Since , with speckle filters having reached a satisfactory level of performance, no significant advances have been made. Nevertheless, in this period, the use of speckle filters in a wide range of applications using S"R imagery has become generalized." radar wave can be considered, with a good approximation, as plane, coherent and monochromatic. It is emitted by an antenna towards a target. The target backscatters partially the radar wave in the direction of a receiving antenna. In the vast majority of spaceborne Synthetic "perture Radars S"R , a single antenna assumes the two functions of emission and reception monostatic radar .The complete radar measurement is the combination of the horizontally H and vertically V linearly polarised radar waves, at emission and at reception after backscattering by the observed target. "fter signal calibration, this measurement, affected by noise, enables to restitute for each resolution cell a polarimetric backscattering matrix S This detected signal intensity I is proportional in average to the radar "backscattering coefficient" σ°. The backscattering coefficient σ°= .π.|S pq | is the average radar cross-section per surface unit [ ]. σ°, expressed in m /m , is a dimensionless physical quantity. It is a physical property of the sensed surface, which depends principally on its roughness, its dielectric properties, its geometry, and the arrangement of its individual scatterers.Carrying the radiometric information with regard to the sensed target, σ° is a function of the frequency of the radar wave, of its angle of incidence upon the target, and of the configuration of polarisation. In terms of physical meaning, the radar backscattering coefficient is analogous to the bidirectional reflectance in the domain of optical wavelengths σ° # cos θ, where is the incidence angle of illumination on the sensed target.Nevertheless, detected radar images look visually very noisy, exhibiting a very characteristic salt-and-pepper appearance with strong tonal variations from a pixel to the next. Indeed, since radar imaging systems are time-coherent, radar measurements over random rough surfaces are corrupted by "speckle" noise due to the random modulation of waves reflected by the elementary scatterers randomly located in the resolution cell. Then, coherent summation of the phases of elementary scatterers within the resolution cell results in a random phase of the complex pixel value.This speckle "noise" makes both photo-interpretation and the estimation of σ° extremely difficult. "ctually, speckle is a physical phenomenon, which is inherent to all coherent imaging systems radar, lidar, sonar, echography . In most remote sensing applications using radar/S"R imagery, speckle is generally considered a very strong noise that must be energically filtered to obtain an image on which cl...

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“…The unwanted spatial variations of brightness across the images in Figure 4a,c are caused by speckle; this is a result of constructive and destructive interference from the coherent source of radiation and it is most noticeable at limits close to the spatial resolution [49]. Speckle noise is an undesirable effect and it is a system phenomenon, i.e., radar and SAR, and it is not the result of spatial variation of average reflectivity of the radar illuminated surface [50].…”

“…Sensors 2020, 20, x FOR PEER REVIEW 11 of 18 radiation and it is most noticeable at limits close to the spatial resolution [49]. Speckle noise is an undesirable effect and it is a system phenomenon, i.e., radar and SAR, and it is not the result of spatial variation of average reflectivity of the radar illuminated surface [50].…”

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