Image interpretation: Speckle filters

We can consider that an extended area is characterised by:
- the radar backscattering coefficient (radiometric information)
- spatial variability (textural information)

The presence of speckle reduces the separability of the various land use classes, based on radiometry and texture. It is thus important to treat speckle so as to improve the possibility of separation, but with minimal loss of information.

If we take the example of homogeneous areas such as agricultural zones with a defined field pattern, the filters to be used must preserve the average backscattering value, and maintain sharp edges between adjacent fields.

In the case of textured areas, such as forest for example, the filters should also preserve the spatial variability (textural information) relating to the scene.

The figure provides an example of simple filter. On the left part of the screen is displayed the speckled SAR image, and on the right part a filtered version of
the same image. The filter applied in this case is the Mean filter which substitutes the central pixel with the mean value within the window. It results in a less noisy, but blurred, image.

More complex filters try to make some hypotheses on the noise statistic. The Frost filter models the signal as a slow varying function representing the texture of the
scene multiplied by a gaussian distributed noise. The result of the filtering is shown on the right part of the image, while on the left part it is possible to observe the original SAR image. Filtering with this technique results in a less noisy image, better preserving the actual texture of the scene.

A more refined technique of speckle filtering is performed with the MAP Gamma filter . This filter models the distribution of the texture as a Gamma distribution
and applies a maximum likelihood estimation to retrieve the detected intensity.
A separate routine allows to detect edges and lines. The result of the filtering is showed on the right part of the figure, together with the unfiltered SAR image (left side), and it is particularly useful for classification purposes.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry