<< Click to Display Table of Contents >> Navigation:  Help Content > Interferometry Module > Amplitude Tracking > Interferometry - Amplitude Tracking - 1 - Amplitude Tracking Process

Purpose

 

This functionality provides an estimate of the scene displacement possibly occurred between two temporally separated SAR acquisitions. Phase (when available) and amplitude correlation techniques are exploited in order to identify the images geometric misalignment, induced by a displacement of the scene scattering elements.

 

Technical Note

 

This estimate of the shift is performed by means of a coregistration process using the coherence first and, where the coherence is below the "SNR Threshold", the amplitude cross correlation (refer to the relevant Technical Note for details). The values of the output cross correlation vary between 0 (worst conditions) and 1 (best conditions), same as the coherence product. Note however that the coherence estimate requires a longer computational time with respect to the amplitude estimate, and is very sensitive to interferometric phase noise.

 

The relationship between the Signal to Noise Ratio (SNR) value and the coherence (γ) value is:

 

SNR = γ2/1-γ2

 

The shift is calculated with steps (in terms of number of pixels) defined by the "Azimuth Looks" and "Range Looks" factors. In order to improve processing speed we suggest using factors which are three times larger than the default ones used in the multilooking process.

 

The coregistration shift estimate is optimized by means of the input Digital Elevation Model.

 

A flattening process is executed to remove the systematic effects due to the topography. It is executed by transforming the input Digital Elevation Model into the Reference slant range image geometry. In case of precise orbits and accurately geocoded reference Digital Elevation Model, this process is run in a fully automatic way. However, in case of inaccuracy in the satellite orbits or in the Digital Elevation Model geolocation, one Ground Control Point (GCP) is required to correct the Reference acquisition of the SAR 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.

 

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.

 

The shift between Reference and Secondary data is calculated in pixel units and it is measured in the satellite viewing geometry: considering the pixel position of the Secondary respect to the Reference acquisition, positive values correspond to an increase of the sensor-to-target distance, in the satellite viewing (_dr output file) and in the satellite flight (_da output file) directions respectively. In order to transform these values from pixel to meters, they must be multiplied by the pixel sampling.

 

It is possible to geocode the displacement by means of the Phase to Displacement functionality. This process must be performed separately for the two displacement components (i.e. _dr and _da). In particular the across track component (_dr) can be re-projected into any user-defined direction.

 

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.

 

Coregistration file

A previously created vector file (either .xml or ), 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). This file is an optional output.

 

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 used to average the shift estimate in range direction.

 

Azimuth Looks        

Number of looks used to average the shift estimate in azimuth direction.

 

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.

 

Amplitude

By setting this flag to 'true', the shifts will be identified optimizing amplitude cross-correlation for those image pixels where the best coherence value is below the SNR threshold. It is important to note that the if the Coherence flag (see below) is set to ‘false’ only amplitude cross correlation will be considered to estimate the correct shift. The following parameters (relevant to amplitude processing) can also be set:

 

-CC Range Window Size

Range dimension, in pixels, of the windows where the cross correlation based shift is estimated.        

 

-CC Azimuth Window Size

Azimuth dimension, in pixels, of the windows where the cross correlation based shift is estimated.

 

-CC Oversampling

The cross-correlation based estimate is applied on over sampled data. The higher this value the longer the processing time and the accuracy. Values higher than 16 are typically not required.

 

-Cross Correlation Threshold

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

 

Coherence

By setting this flag to “true”, the shifts will be identified optimizing coherence between the two input SAR images. The following parameters (relevant to coherence processing) can also be set:

 

-COH Range Window Size

Range dimension, in pixels, of the windows where the coherence based shift is estimated.        

 

-COH Azimuth Window Size

Azimuth dimension, in pixels, of the windows where the coherence based shift is estimated.

 

-COH Oversampling

The  pixel oversampling factor, for a more accurate coherence based estimate, can be entered. The larger this value the longer the processing time and the accuracy.

 

-COH SNR Threshold

The coherence based estimate is performed only whether the Signal-to-Noise Ratio is greater than this value.

 

Coregistration with DEM        

This parameter is mandatorily set to “true” and cannot be modified by the user.

 

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.

 

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.

 

 

 

Output Files

 

Output Root Name

Name of the output root. This file is mandatory.

 

_dr

Shift measured in range direction with the associated header files (.sml, .hdr).

 

_da

Shift measured in azimuth direction with the associated header files (.sml, .hdr).

 

_rg_sint

Synthetic shift in range with the associated header files (.sml, .hdr).

 

_az_sint

Synthetic shift in azimuth with the associated header files (.sml, .hdr).

 

_coh

Coherence image with the associated header files (.sml, .hdr).

 

_cc

Cross correlation image with the associated header files (.sml, .hdr).

 

_srdem

Digital Elevation Model in slant range geometry (.sml, .hdr).

 

_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 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

 

None.