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Purpose

 

Images obtained from coherent sensors such as SAR (or Laser) system are characterized by speckle, This is a spatially random multiplicative noise due to coherent superposition of multiple backscatter sources within a SAR resolution element. In other words, speckle is a statistical fluctuation associated with the radar reflectivity of each pixel a scene. A first step to reduce the speckle - at the expense of  spatial resolution - is usually performed during the multi-looking, where range and/or azimuth resolution cells are averaged.

 

Whenever two or more images of the same scene taken at different times are available, multi-temporal speckle filtering – which exploits the space-varying temporal correlation of speckle between the images – should be applied, in order to reduce this system inherent multiplicative noise.

 

Technical Note

 

De Grandi Filter

It has to be pointed out that this multi-temporal filtering is based on the assumption that the same resolution element on the ground is illuminated by the radar beam in the same way, and corresponds to the same co-ordinates in the image plane (sampled signal) in all images of the time series. In other words the SAR geometry is the same for all acquisitions. The reflectivity can of course change from one time to the next due to a change in the dielectric and geometrical properties of the elementary scatters, but should not change due to a different position of the resolution element with respect to the radar. Therefore proper spatial coregistration of the SAR images in the time series is condition sine qua non and of paramount importance.

 

For what concerns the ENL setting,  it is suggested to leave it as default (i.e. -1) for single look data; vice versa in case of multilook data it is better to estimate it on a homogeneous area of the image.

 

The Coregistration step must be performed prior the execution of this filter. Input data must be in the satellite view geometry; geocoded (_geo) data are not admitted.

 

Please Note:

In case of resampled data, make sure the option "use min and max coordinates" was set to TRUE during the Sample Selection step. It is then possible to cut the data without this option on the filtered data.

 

 

 

Input Files

 

Input file list

Input file names of coregistered (_pwr, _rsp) or geocoded (_geo) data. This file list is mandatory.

 

Parameters - Principal Parameters

 

Data Type

The type of file which will be filtered has to be chose. Single pol is for data with only one polarization. Dual Pol in case of dual pol data, in this case, the order has to be Co-pol followed by the cross pol of the same acquisition, and so on.

 

Input file list

Input file names of coregistered (_pwr, _rsp) or geocoded (_geo) data. This file list is mandatory.

 

Input file list

Input file names of coregistered (_pwr, _rsp) or geocoded (_geo) data. This file list is mandatory.

 

Parameters - Global

 

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

 

Parameters - Other Parameters

 

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

 

 

Equivalent Number of Looks (ENL)

The Equivalent Number of Looks is equivalent to the number of independent Intensity values averaged per pixel during the multi-looking process. This parameter can be easily estimated over a homogeneous (stationary) sample in the input Intensity data according to:

 

ENL =   mean2 / standard deviation2

 

In case that ENL is not set, the Number of Looks (NL) used during the multi-looking process is considered.

 

Note that, to tune the strength of speckle filtering and the level of preservation of scene details, it is preferable to adjust the value of the ENL, rather than to change the size of the processing window:

 

•To reduce the strength of speckle filtering, with the aim to preserve the thinnest details of the scene, enter a ENL value slightly higher than the calculated one;

•Inversely, to improve the filtering of the speckle (possibly at the cost of the thinnest details of the scene), enter a ENL value slightly lower than the calculated one.

 

Output Files

 

Output file list

Output file list of the filtered data. This file list is mandatory.

 

_fil        

Filtered Intensity image and associated header files (.sml, .hdr).

 

_meta        

It allows to load the processing results as a single file.

 

Details specific to the Units of Measure and Nomenclature of the output products can be found in the Data Format section.

 

General Functions

 

Exec

The processing step is executed.

 

Store Batch        

The processing step is stored in the batch list. The Batch Browser button allows to load the batch processing list.

 

Close        

The window will be closed.

 

Help

Specific help document section.

 

 

Specific Function(s)

 

None.

 

References

 

G. F. De Grandi, M. Leysen, J. S. Lee, and D. Schuler, “Radar reflectivity estimation using multiple SAR scenes of the same target: Technique and applications,” in Proc. IEEE Int. Remote Sens. Sci. Vis. Sustain. Develop. Geosci. Remote Sens. (IGARSS’97), Aug. 3–8, 1997, vol. 2, pp. 1047–1050.