Image interpretation: Speckle

A detailed analysis of the radar image shows that even for a single surface type, important grey level variations may occur between adjacent resolution cells. These variations create a grainy texture, characteristic of radar images. This effect, caused by the coherent radiation used by radar systems, is called speckle. It happens because each resolution cell associated with an extended target contains several scattering centers whose elementary returns, by positive or negative interference, originate light or dark image brightness.
This creates a "salt and pepper" appearance.

An example of speckle is shown in the figure.
The SAR scene acquired on 21/4/1994 over Tiber Valley (I) shows some agricultural fields located along the Tiber River north to Rome, in the central
part of Italy.

The homogeneous patches representing the fields have high variability in backscattering due to the speckle noise. This results in a grainy image, which renders difficult the interpretation of the main features of the surface imaged by the SAR..

Speckle is a system phenomenon and is not the result of spatial variation of average reflectivity of the radar illuminated surface. For a high resolution
radar, there may be useful scene texture which differs from the speckle.

This is the case for example in forested zones where the combined effects of radar illumination and tree shadowing create a rougher texture granularity than
the speckle. In this case, there exists a spatial variability of the physical reflectivity of the illuminated zone. In a radar image we may find:
- zones where the only image texture is related to speckle that we may call regions "without texture" (extended homogeneous target),
- zones "with texture" that have spatial variations in scene reflectivity in addition to speckle.

Thus, in the case of "no texture" zones, it becomes possible to study the statistical distribution of the backscattered radar signal, which helps to estimate certain radar characteristics.

Speckle can be reduced by two methods:

SAR image multi-look processing
Independent measurements of the same target can be averaged in order to smooth out the speckle. Actually, it is obtained by splitting the synthetic aperture
into smaller sub-apertures, the so called "looks", each separately processed and then averaged.

The different looks are averaged to reduce the grey level random variations provoked by speckle. For N statistically independent (non-overlapping) data
sectors, the speckle variance is reduced by a factor of N. Likewise, the resolution is degraded by a factor of N.

In such a way, we can for example have 8-look images. A compromise has to be found between desired spatial resolution and an acceptable level of speckle.

Filtering techniques
When the finest resolution is required, moving window filters are used. A moving window filter changes the intensity of the central pixel depending on
the intensities of all the pixels within the window. Different algorithms have been proposed to properly shape the impulse response of the filter within the
window.

In both cases speckle is reduced at the expenses of the spatial resolution..

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