The Contrast Research of the Methods of Restraining the Speckle Noise of Sar Images

There commonly exists speckle noise in the SAR images. It badly disturbs the extraction and interpretation of the information of the objects, influences the application of the SAR images. So it has important meaning to reduce the speckle noise in the SAR images. In this paper, the mechanism and mathematical model of the speckle noise is firstly analyzed. Then the methods of filtering speckle noise is introduced, especially several local region adaptive filters. Two filter models are summarized to direct the construction of the new filter. Then for the typical filter, the contrast tests are carried out. At the last, it brings out a quantitative evaluation index to direct the filtering the speckle noise and the construction of the new filter. 1. Introduce Because Synthetic Aperture Radar (SAR) can pass the clouds and mist and has the all-weather and full work ability, it has unique superiority with compare with the visible and infrared remote sensing. Therefore, it has been shown more and more bigger application latent capacity in many domains, such as surveying and mapping, geology, hydrology, ocean, ecology, environmental monitoring, glacier charting and military affairs. Synthetic Aperture Radar (SAR) images are generally corrupted by speckle noise. The presence of the speckle noise in the SAR images is the major obstacle to interpret, classify and analyze SAR images for different application purposes of remote sensing users. The speckle noise badly disturbs the extraction and interpretation of the information of the objects, influences the application of the SAR images. Therefore many speckle noise filters have been proposed for several decades. 2. The mechanism giving rise to speckle noise and mathematical model of the speckle noise Speckle noise is a physical effect, which occurs when coherent light is reflected from an optically rough surface. The characteristic speckle effect of radar images results from the destructive and constructive interference among the echoes of individual surface scatters within a resolution cell. Therefore the resultant pixel can differ extremely from its average grey value. These grey value variations between adjacent pixels lead to the typical salt and pepper appearance of SAR images. The multiplicative speckle model supposes the speckle to be fully developed : Within each resolution cell there must be a large number of scatterers whose phase and amplitude are statistically independence ;the different scatterer amplitudes must belong to the same statistical distribution ;their phases are [0,2 ] uniformly distributed . When the ground surface rough degree surpasses the radar wavelength (for example, RadarSAT is 5.6cm and ERS-1/2 is 5.6666cm), and there are many scatters within a resolution cell, the speckle is regarded as fully developed. An appropriate model for fully developed speckle is a multiplicative fading random process F (Ulaby et al .1986 a ) ) , ( * ) , ( ) , ( y x F y x R y x I = (1) where (x,y) are the spatial azimuth and slant range coordinates of the resolution cell center , I is the observed intensity (speckled observed radiance ), R is the random radar reflectivity process (unspeckled radiance). F is a second order stationary random process, statistically independent of R, with unit mean ( 1 = F M ) and whose variance ( 2 F F V σ = ) is inversely