Abstract
Synthetic aperture radar (SAR) images map Earth’s surface at high resolution, regardless of the weather conditions or sunshine phenomena. Therefore, SAR images have applications in various fields. Speckle noise, which has the characteristic of multiplicative noise, degrades the image quality of SAR images, which causes information loss. This study proposes a speckle noise reduction algorithm while using the speckle reducing anisotropic diffusion (SRAD) filter, discrete wavelet transform (DWT), soft threshold, improved guided filter (IGF), and guided filter (GF), with the aim of removing speckle noise. First, the SRAD filter is applied to the SAR images, and a logarithmic transform is used to convert multiplicative noise in the resulting SRAD image into additive noise. A two-level DWT is used to divide the resulting SRAD image into one low-frequency and six high-frequency sub-band images. To remove the additive noise and preserve edge information, horizontal and vertical sub-band images employ the soft threshold; the diagonal sub-band images employ the IGF; while, the low- frequency sub-band image removes additive noise using the GF. The experiments used both standard and real SAR images. The experimental results reveal that the proposed method, in comparison to state-of-the art methods, obtains excellent speckle noise removal, while preserving the edges and maintaining low computational complexity.
Highlights
Synthetic Aperture Radar (SAR) images employ active sensors that detect microwave radiation, which has longer wavelength than visible light that is detected in passive sensors, such as the optical sensor
We proposed a novel algorithm that is based on statistical characteristics of speckle noise and the discrete wavelet transform (DWT) to remove speckle noise from the SAR images
The speckle reducing anisotropic diffusion (SRAD) filtering method, which can be directly applied to the SAR image, is used as a preprocessing filter
Summary
Synthetic Aperture Radar (SAR) images employ active sensors that detect microwave radiation, which has longer wavelength than visible light that is detected in passive sensors, such as the optical sensor. In contrast with the optical sensor, the active sensor of the SAR is accompanied by speckle noise that arises from the coherent imaging mechanism. Speckle noise in SAR images is generated by the random interference of many elementary reflectors within one resolution cell [10]. This noise has different features from the noise observed in images that were obtained by passive sensors, such as the optical sensor. Speckle noise degrades SAR images and interferes with the transfer of image information to the observer. The development of effective filtering methods in the reduction of speckle noise is critical for the analysis of information that is contained in various SAR images
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