<< Click to Display Table of Contents >> Navigation:  Help Content > Interferometry Module > Phase Processing - Interferometry Single Steps > Interferometry - Phase processing - 1 - Interferogram Generation

Purpose

 

The distance difference between a point on the Earth and the sensor position on the two acquisitions can be measured by the phase difference (ϕ) between two complex coregistered SAR images. This is performed by multiplying one image by (the complex conjugate of) the other one, where an interferogram is formed.

 

The final output of this step is a flattened interferogram, where the constant phase (due to the acquisition geometry) and the topographic phase (if an input DEM is provided) have been removed.

 

It is important to point out that, in case ENVISAT-ERS pairs are processed, the "Coregistration with DEM” flag must be checked.

 

Technical Note

 

The interferometric phase is expressed as:

 

Phase = ATAN[Imag(I)/Real(I)]

 

Where Imag(I) and Real(I) are respectively the imaginary and real parts of the interferogram.

 

Spectral shift and common Doppler bandwidth filtering are performed during the interferogram generation. Spectral shift is needed due to the range spectra shift caused by the variable SAR viewing angle on distributed targets. The Doppler bandwidth filtering is required to compensate for different Doppler (squint angles), which produce shifted azimuth spectra. The azimuth filter applied during the interferogram generation enables to fully capture the scene's potential coherence.

 

The Interferogram Flattening is performed using an input reference Digital Elevation Model or the ellipsoidal model if the DEM is not inputted; the GCP file, if entered, is used to correct the Reference image onto the Digital Elevation Model. The better the reference Digital Elevation Model accuracy/resolution the better the result in terms of topography removal.

 

In case of ENVISAT-ERS pairs, when the reference Digital Elevation Model is very coarse and it is preferable to adopt an ellipsoidal model as reference, this step can be run without inputting the Digital Elevation Model; in such instance a Reference Height must be provided ("Cartographic System" section). In any case the "Input Reference file" must be the ASAR acquisition.

 

Coregistration with DEM

By checking this flag, the spectral shift filter is adapted to the local slope variations; this process can increase considerably the computing time. The coregistration accuracy, and consequently the coherence and interferometric phase, can be improved especially when very high resolution data are processed. This flag should  be checked 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 recommended not to check this flag when the SAR data orbital parameters are inaccurate.

 

Details about the coregistration process are provided in the relevant Technical Note. The reference flowchart provides a schematic representation of the different steps involved.

 

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. Reference acquisition of the interferometric pair) 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 Secondary data according to the Reference data. Note that the Ground Control Point must be referred to the Reference single look image (_slc).

 

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 the "Input Secondary file" 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 Secondary Orbit Correction", in the Preferences>Flattening, must be checked.

 

In case the two 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.

 

In case of baseline conditions - or topographic conditions - which cause the interferometric phase to change very fast and to eventually get lost due to an aliasing problem, it is suggested to over sample the range pixel size by entering negative values in the Range Multilooking (Preferences>Interferometry>Multilooking). As an example using ERS-ASAR interferometric tandem pairs, which are characterized by a very small ambiguity height (hence very dense/frequent interferometric fringes), the Interferometric multilooking factors shall be set to -2 and 3 (respectively in Range and Azimuth).

 

As result of this step multi-looked products are generated (refer to the "Basic module>Multilooking" for more details). It is important to know that, unlike the multi-looked intensity images generated in the "Basic module>Multilooking", these Reference and Secondary intensities cannot be radiometrically calibrated due to the spatial varying effect introduced by the spectral shift filter.

 

Input Files

 

Input Reference file

File name of the Reference data (_slc). This file is mandatory.

 

Input Secondary file

File name of the Secondary data (_slc). This file is mandatory.

 

Optional Files

 

Geometry GCP file

Either a previously created Ground Control Point file (.xml) is loaded (Load GCP File) or the interface to create a new Ground Control Point file is automatically loaded (Create GCP File, refer to the "Tools>Generate Ground Control Point" for details). This file is optional.

 

Shift Parameter file

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

 

ScanSAR-TOPSAR Spectral Diversity Avoid Area

Spectral Diversity is used to improve the coregistration along the Azimuth direction of the ScanSAR-TOPSAR mode data. Without a perfect coregistration along the azimuth direction phase slopes will appear along the same direction. Issues may arise when the spectral diversity is calculated on areas that show phase slopes along the azimuth direction related to real and strong surface displacement event (e.g. earthquakes); in such circumstances the tool could interpretate the phase ramp as a consequence of a miss-coregistration. To avoid this behavior the user can draw the area where the spectral diversity should not be performed. The shapefile of the avoid area must be geocoded (lat-long). All the ScanSAR or TOPSAR bursts which intersect this shapefile will be avoided during the Spectral Diversity estimation step. This file is optional.

 

DEM/Cartographic System

 

Digital Elevation Model file

Digital Elevation Model file name. This should be referred to the ellipsoid. In case a list of input files is entered, the DEM must cover the whole imaged area. This file is optional.

 

Output Projection

In case that the Digital Elevation Model is not used, it is mandatory to define the Cartographic System.

To use the same coordinate system as another dataset, click the Import from Existing Dataset button and select the source dataset.

To apply the same Coordinate System of the current selected layer

The reset icon allows to reset the coordinate system field.

 

Parameters - Principal Parameters

 

Range Looks        

Number of looks in range.

 

Azimuth Looks        

Number of looks in azimuth.

 

Grid Size for Suggested Looks

The grid size, in meters, used to tune range and azimuth looks. If the other parameters are manually set, the grid size will not imply a change in their values.

 

Compute Shift Parameters

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

 

Generate Coregistered SLC

By setting this flag, Reference and Secondary coregistered Single Look Complex data are generated among the output products. These files are over sampled of a factor 2 in range direction.

 

Coregistration with DEM        

By setting this flag, the input Digital Elevation Model is used in the coregistration process.

 

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 - Flattening

 

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

 

Make Flattening        

Setting this flag to false allow the software to reach the generation of interferogram (not flattened, _int) even if necessary information for flattening (such as orbit information) are missing.

 

Parameters - Interferogram

 

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

 

ScanSAR-TOPSAR Amplitude and Phase Offset Calibration        

A phase offset between the different overlapping bursts is estimated and removed in order to remove discontinuities. This is applied on _pwrs, _int and _dint files.

 

ScanSAR-TOPSAR Amplitude and Phase AZ Calibration                

A phase offset between the different overlapping bursts is estimated and removed in order to remove discontinuities.Only a linear component in azimut direction is estimated.This is applied on _pwrs, _int and _dint files.

 

ScanSAR-TOPSAR Amplitude and Phase RG Calibration

A phase offset between the different overlapping bursts is estimated and removed in order to remove discontinuities.Only a linear component in range direction is estimated.This is applied on _pwrs, _int and _dint files.

 

ScanSAR-TOPSAR Make Mosaic Burst Only        

If the "Delete temporary files" flag is set to FALSE, it is possible to set this parameter to true (as intermediary temporary files are still present) in order to skip all previous steps (coregistration, interferometry, ...) in order to perform only the mosaicing step.

Please note: before setting this flag to TRUE, the whole processing has to be performed at least one time with "Delete temporary files" flag to FALSE. The same output name has to be choose.

 

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.

 

Write Coregistration Shape

Creates a shapefile containing the coregistration points.

 

Write Coregistration Auxiliary xls

creates a XLS file containing the coregistration points.

 

Output Files

 

Output Root Name

Name of the output root. This file is mandatory.

 

_int

Interferogram with the associated header files (.sml, .hdr).

 

_dint

Flattened interferogram with the associated header files (.sml, .hdr).

 

_dint_ql.tiff and/or _dint_qlt.tiff

Tiff image of the flattened interferogram. The creation of these files depends on the selection of "quick look format" in the parameters. It uses a cyclic color scale ranging from -pi to +pi which is reported here below:

 

 

_sint

Synthetic phase with the associated header files (.sml, .hdr).

 

_srdem

Digital Elevation Model, in slant range geometry, with the associated header files (.sml, .hdr).

 

_slc_rsp

Coregistered Single Look Complex data. This file is generated only if the relevant flag is set.

 

_par

ASCII file containing the coregistration shift parameters in range and azimuth. This file is generated only if the shift parameters are calculated.

 

_pwr

Multi-looked Reference and Secondary image with the associated header files (.sml, .hdr).

 

_orb.sml        

Xml file containing the scene orbital parameters.

 

_orbit_off.shp

Shape file with the points used to estimate the orbit based shift. This file contains the following information:

 

The file, which is generated only when the shift parameters are calculated, is multilooked (i.e. Azimuth and Range looks) as specified in the Input Parameters.

 

_winCC_off.shp

Shape file with the points used to estimate the cross correlation based shift from the Intensity data. In addition to the information provided by the "_orbit_off.shp" file, this file contains also the cross-correlation value (CC), which is dimensionless and it can vary from 0 to 1.

 

The file, which is generated only when the shift parameters are calculated, is multilooked (i.e. Azimuth and Range looks) as specified in the Input Parameters.

 

_winCoh_off.shp

Shape file with the points used to estimate the coherence based shift from the complex data (fine shift estimate). In addition to the information provided by the "_orbit_off.shp", this file contains also the following information:

 

The file, which is generated only when the shift parameters are calculated, is multilooked (i.e. Azimuth and Range looks) as specified in the Input Parameters.

 

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.

 

See Also

 

Task, SARscapeBatch object, SARscapeBatch script example

 

References

 

 

Monti Guarnieri A., P. Guccione, P. Pasquali, and Y.L. Desnos: "Multi-mode ENVISAT ASAR interferometry: Techniques and preliminary results, IEEE Proceedings on Radar, Sonar and Navigation, Vol. 150, No. 3, June, 2003