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Purpose

 

This functionality can be used when the result of the nominal geocoding comes with geo-location inaccuracies. It allows to correct the azimuth start time and/or the slant range distance on the basis of a reference Digital Elevation Model and, optionally, an optical or SAR geocoded image.

 

It is important to outline that the reliability, in terms of absolute geo-location accuracy, of the corrected product depends on the reliability of the reference Digital Elevation Model.

 

The properly corrected product does not require any further manual correction either for the slant-to-geocoded or for the geocoded-to-slant related processes (e.g. GCP-based Geocoding, Interferogram Flattening, etc.)

 

Technical Note

The process consists of creating a SAR synthetic image, which is used for a cross-correlation based coregistration of the "Input file". This synthetic image is simulated starting from the input Digital Elevation Model and, optionally, the input "Reference Image". The latter, if available, allows to improve the correction by adopting a hybrid approach, which exploits the DEM for the morphology/slope aspect and the "Reference Image" for the identification of land features such as roads, rivers, urban infrastructures, field boundaries etc. The selected "Resampling" method is used for projecting the "Reference Image" onto the "Input file" geometry.

 

The most suitable optical images to use are: an NDVI index, a Near Infrared channel, a panchromatic image.

Any SAR image can be used as "Input file": single look complex or detected single look amplitudes, as well as multilooked products are admitted.

 

This step does not generate any output file except the parameter file (_par), where the azimuth and range shift correction are provided. However it is possible to save also the intermediate products (e.g. the simulated SAR image) by de-selecting the Delete Temporary Files flag ("Preferences>General" panel). As result of the processing the header (.sml) of the "Input file" is modified by changing the original azimuth start time and/or the slant range distance with the corrected values.

 

Whether the correction results are inaccurate, given that the reference data (Digital Elevation Model and reference image) are fully reliable both in terms of geolocation and height, the number of windows (and if necessary also their size), in range and azimuth direction, should be increased and the process reiterated.

 

It has to be mentioned that, being the process based on the topographic aspect of the input "DEM file", in case of completely flat areas the coregistration can fail.  

 

Input Files

 

Input file

Input file name of the data to correct. This file is mandatory.

 

DEM file

Input reference Digital Elevation Model. This file is mandatory.

 

Optional Files

 

Input Reference Image

Input reference SAR or Optical image. This file is optional.

 

Parameters - Principal Parameters

 

Interpolation window size

Window dimension used for the interpolation of dummy pixels in the synthetic simulated image. If zero is entered, the interpolation is not applied.

 

Range Window Number

Number of windows, in range direction, which are used for the "Input file" coregistration process.

 

Azimuth Window Number

Number of windows, in azimuth direction, which are used for the "Input file" coregistration process.

 

Range Window Size        

Range dimension, in pixels, of the coregistration windows.

 

Azimuth Window Size        

Azimuth dimension, in pixels, of the coregistration windows.

 

Cross Correlation Oversampling        

The data are oversampled in order to improve the coregistration shift estimate. The higher this value the longer the processing time. Typically it is not not necessary to oversample more than 1/4 of pixel (standard setting).

 

Cross Correlation Threshold        

If the correlation value is below this threshold, then the window is not used for the shift estimate.

 

Coregistration With DEM

If this flag is set, the input file will be coregistered with the input DEM.

 

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 - Coregistration

 

It brings to the Coregistration 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.

 

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)

 

Undo        

The original azimuth start time and/or the slant range distance are restored. Only the "Input file" is required to run this function.

 

See Also

 

Task, SARscapeBatch object, SARscapeBatch script example

 

References

 

None.