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Purpose

 

It is well known that the radar backscatter depends upon the object’s roughness and its dielectric properties. A useful rule-of-thumb in analysing radar images is that the higher the backscatter, the rougher the surface being imaged. Concerning the object’s electrical properties, in dry conditions, the dielectric constant is low, hence contributing in a limited way to the backscatter response. On the contrary, wetness of soil or vegetated surfaces produce a notable increase (up to 10 times) in the radar reflectivity, thereby making the radar system attractive to infer moisture (or wetness in a qualitative way). However, if the final goal is to obtain a land cover map, this particular sensitivity has a negative impact on the classification result. In this case, it is therefore of advantage to remove the dielectric related effects – typically resulting from rain events – on the backscattering coefficient.

 

Beside dielectric constant related changes, data can be affected also by absolute and range dependent radiometric distortions, which can be related to anomalous SAR antenna behaviors. These distortions are corrected as well using the Post Calibration functionality.  

 

Technical Note

 

In a first step, i) range dependent radiometric losses and/or, ii) dielectric related effects on the radar backscatter, iii) absolute radiometric variations are derived in a statistical way from a multi-temporal geocoded, radiometrically calibrated (and optionally normalized) data set. In a subsequent step, the estimated two dimensional image dependent correction factors are applied on each image independently.

 

Range correction

It can happen that SAR images, even after radiometric calibration and normalisation, remain affected by backscatter variations in range. The correction is typically done by identifying same land cover areas (in form of a distributed shape files) in different range positions, which are used as reference in the correction process. It is also possible to draw the shape file as a single large homogeneous area instead of several small ones. In case the shape file is not provided the program will homogeneously collect the calibration samples in range direction to perform the correction.

 

Dielectric correction

Distortions related to dielectric constant temporal changes are corrected using at least one reference image not affected by these effects (best results are achieved if more reference images are available). It is worth mentioning that the resulting dielectric correction factor (which are saved in form of raster images) can be further used as qualitative wetness indicator.

 

Absolute correction

This correction should be executed on the entire data set in case a multi-temporal analysis has to be carried out. A mean value temporal equalisation is performed (among all input files) in order to remove possible radiometric mean value anomalies affecting the entire imaged area.

 

The radiometric corrections (i.e. dielectric component, range dependency and absolute variations) can be carried out in an independent or combined way; in any case the processing sequence is the following: 1) range correction, 2) dielectric correction, 3) absolute correction.

 

Input Files

 

Input file list

File list of the geocoded, radiometrically calibrated (and optionally normalized) Intensity images (_geo), which have to be post-calibrated. More than one file is required in case the absolute correction is selected. The use of this file list is mandatory.

 

Dielectric correction list

File list of images not affected by large dielectric constant variations. These are used as reference for the dielectric correction. The use of this file list (1 image is the minimum) is mandatory only if the dielectric correction is carried out.

 

Optional Files

 

Absolute correction file

Name of the shape file (.shp, .xml) defining the area to be used as reference for the mean value calculation. Homogeneous areas, not affected by temporal and/or spatial variations of the radar backscatter, are to be selected (typically densely forested areas). The use of this file is mandatory only if the absolute correction is carried out.

 

Range correction file

Name of the shape file (.shp, .xml) defining the near to far range reference. Homogeneous zones, not affected by spatial variations of the radar backscatter, are to be selected. This file is optional.

 

Parameters - Principal Parameters

 

Absolute Correction

By setting this flag, absolute radiometric variations are corrected.

 

Range Correction        

By setting this flag, range dependent radiometric variations are corrected.

 

Dielectric Correction        

By setting this flag, radiometric variations related to dielectric constant changes are corrected.  

 

Range correction threshold

The range correction is applied only to those pixels, which are below this backscatter value.

 

Range correction degree

Degree of the polynomial applied for the range correction. Using a 1st order polynomial at least two shapes are required, which are contained by any of the input images. In case of higher polynomial order it is important that the entire range image extent is covered in order to avoid anomalous trends of the corrected backscatter values.

 

Dielectric correction threshold

Minimum ratio between the input image and the reference one, for the correction to be applied.

 

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.

 

Output Files

 

Output Root Name

Output root name of the selected feature(s). This root name is mandatory.

 

Output file list

File names of the Post Calibrated Intensity images. The use of this file list is mandatory.

 

_cal        

Post calibrated data and associated header files (.hdr, .sml).

 

dielectric_mask        

Mask file corresponding to the high frequencies areas in the dielectric correction list and associated header files (.hdr, .sml). Values equal to 255 are not corrected. This file is removed by setting the Delete intermediate files flag.

 

_cal_dielectric        

Data containing the dielectric correction factors and associated header files (.hdr, .sml). These files are removed by setting the Delete intermediate files flag.

 

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

 

None.