Help Content > Basic Module > Intensity Processing

Purpose

When multiple images cover the same region and, in particular, a speckle filtering based on time-series will be performed, or image ratio (or similar operations) are required in slant (alternatively ground) range geometry, SAR images must be coregistered. This requires spatial registration and potentially resampling (in cases where pixel sizes differ) to correct for relative translational shift, rotational and scale differences. Note that coregistration is simply the process of superimposing, in the slant range geometry, two or more SAR images having same acquisition geometry. This process must not to be confused with geocoding, which is the process of converting each pixel from the slant range geometry to a cartographic reference system.

It is important to point out that, in case of ENVISAT-ERS data sets, it is mandatory to enter the Digital Elevation Model as input to the process.

Technical Note

This step is performed in an automatic way, according to the following procedure:

-A local non-parametric shift estimate is computed on the basis of the orbital data and the Digital Elevation Model (if provided in input). In case of inaccurate orbits a large central window (Cross-correlation Central Window) is used instead.

-A set of windows (Cross-correlation Grid) is found on the reference image.

-The input data cross-correlation function is computed for each window.

-The maximum of the cross-correlation function indicates the proper shift for the selected location.

-The residual parametric shift, which is summed to the original local non-parametric estimate, is calculated by a polynomial depending on the azimuth and range pixel position.

-In case the input SAR data are represented by SLC products, the residual parametric shift is further refined by computing "mini-interferograms" on small windows (Fine Shift Parameters) distributed throughout the image. If the coherence is too low, the number of points for the fine shift estimate can be not sufficient to optimize the coregistration process. In such case the coregistration will be carried out using the local non-parametric shift and an improved cross correlation based fit (Fine Shift Parameters>Cross Correlation Oversampling), which allow a coregistration with sub-pixel accuracy. The output coregistered data are Single Look Complex.

-In case the input is represented by SAR Amplitude or Intensity (not SLC products) data, the improved cross correlation based fit (Fine Shift Parameters>Cross Correlation Oversampling), which allow a coregistration with sub-pixel accuracy, is always performed.

The reference flowchart provides a schematic representation of the coregistration process.

It is worthwhile to note that:

-	It is possible to perform the coregistration in a manual way, by manually locating the center of the windows (i.e. Cross-correlation Grid and Fine Shift Parameters) in the Input reference file (see input "Coregistration file" below).

-	In case the images are very much different in terms of areal coverage, the smallest one should be used as "Input reference file" in order to avoid the coregistration windows to be located in areas with null pixel values, which can eventually cause the coregistration process to fail.

The use of the Digital Elevation Model in input (optional), enables to improve the data coregistration accuracy especially working with high resolution data and in mountainous or hilly areas. The use of this input can be considered important in the following cases: i) Data long stripes (i.e. segments of orbit instead of single frames); ii) Data acquired at high latitudes; iii) Non zero-Doppler annotated data (especially in case of long wavelength such as ALOS PALSAR).

It is suggested not to use the DEM when the orbital parameters of the input SAR data are inaccurate.

It must be noted that the use of an input Digital Elevation Model considerably increases the processing time. In general, with or without an input DEM, the amount of computation resources and the processing time increase linearly with the image size.

In case of inaccuracy in the satellite orbits or in the Digital Elevation Model geolocation, a Ground Control Point (GCP file) is required to correct the SAR data (i.e. Input reference file) with respect to the reference Digital Elevation Model. In this case the shift calculated in the coregistration process is combined with the Ground Control Point shift in order to correct the data of the Input file list according to the Input reference file.

It is important to note that:

❖In case the Input reference file has already been corrected with the manual or the automatic procedure the "GCP file" is not needed.

❖In case the Input reference file is correct (i.e. the nominally geocoded image fits with the DEM), while the orbital parameters of any of the Input files are not accurate (i.e. the nominally geocoded image does not fit with the DEM), the "GCP file" is not needed but the flag "Automatic Slave Orbit Correction", in the Preferences>Flattening, must be checked.

It has to be outlined that:

-	The coregistration can be performed when all input data (i.e reference and input files) are acquired with the same incidence angle and same swath. In addition, the data type of all the input file has to be the same

-	ERS and ENVISAT ASAR data can be coregistered only when the last ones are acquired, from the same satellite track and in IS2 mode. The "Input reference file" must be an ASAR image.

-	Inaccurate coregistration results may be expected when reference and input files are acquired with co- and cross-polarization (e.g. HH and HV).

Input Files

Input reference file

File name of the reference image to which the other input files will be coregistered. The input reference file can be a slant or ground range Intensity or Complex image (e.g. _pwr, _pri, _sgf, _slc). This file list is mandatory.

Input file list

Input file name(s) of all file(s) to be coregistered to the reference image. The Input file list must be one (or more) slant or ground range Intensity image (e.g. _pwr, _pri, _sgf, _slc). This file list is mandatory.

Optional Files

Digital Elevation Model file

Name of the Digital Elevation Model file. This should be referred to the ellipsoid. This file is optional. In case it is omitted, the coregistration is carried out without considering the topography.

Coregistration file

A previously created Ground Control Point file (.xml), with the points used for the manual coregistration (.xml), is automatically loaded. These points represent the center of the coregistration windows. This file is optional.

Shift Parameter file

Name of the file with the shift parameters used for the coregistration (_par). If the Compute shift parameter flag is set, it is generated as output.

Geometry GCP file

Either a previously created Ground Control Point file (.xml) can be loaded or the interface to create a new Ground Control Point file is automatically loaded (refer to the "Tools>Generate Ground Control Point" for details). This file is optional; it can be entered only whether the "Digital Elevation Model file" is used in input.

Parameters - Principal Parameters

Always compute shift

By setting this flag it is assumed that all the images in the input file list are not already coregistered each other. Unset this flag if the images in the input file list are already coregistered each other, but they are not coregistered to the reference file; in such case the first image in the input file list will be considered for the calculation of the coregistration parameters.

Compute Shift Parameters

By setting this flag, the coregistration shifts between Reference and Secondary image are calculated and saved into the _par file.

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 - Flattening

It brings to the flattening 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 reference file

Output reference image to which the other input files will be coregistered (_rsp). This file list is mandatory.

Output file list

Output file name(s) of all coregistered file(s). The number of output files must be equal to the number of input files. This file list is mandatory.

_rsp

coregistered slant or ground range detected images and associated header files (.sml, .hdr). In case of _slc files in input, the output will consist in 1x1 multilooked _pwr.

_rsp_par.sml

Xml file containing temporary processing parameters.

_meta

This file allows to load the specific processing results together with the input reference file.

_orb.sml

Xml file containing the scene orbital parameters. This file is generated only if an input Digital Elevation Model is entered.

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.

The window will be closed.

Help

Specific help document section.

Specific Function(s)

None.

Basic Module - Intensity Processing - Coregistration

Basic Module - Intensity Processing - Coregistration