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Purpose

 

The aim of this functionality is to determine, a reference residual height (i.e. with respect to the input external DEM) within the volume containing the three-dimensional scattering object. This reference height will be exploited later, during the so called second inversion step, to “flatten” each differential interferogram thus enabling a more accurate estimation of the atmospheric phase delay layers which will be performed on a subset of highly temporally coherent points. The residual height is determined optimizing, over the interval of feasible height values associated with the input parameters, the temporal interferometric coherence.

 

This step is mandatory.

 

The approach is based on the identification of a certain number of “coherent radar signal reflectors” named Persistent Scatterers (PS). The processing is then focused on the analysis of the phase history of these reliable single targets (each one represented by an image pixel). Compared to the conventional PS approaches, typically performed to estimate displacements, the adaptation to a tomographic processing requires that each target has to be completely stable along the temporal span defined by the interferometric acquisitions set at hand. Hence, the velocity of each PS is assumed to be neglectable in terms of phase variations and the coherence optimization is performed only along the height dimension.

 

Good PS candidates - like roofs, poles, bridges - are typically found in urban settlements, or other man-made structures such as green-houses, dams, metallic and concrete features (e.g. well fields surrounding structures, pipelines and dwells). Beside these artificial features, also natural targets such as well exposed outcropping rock formations are potential PS.

 

The temporal distribution of the acquisitions shall also be adequate compared with the expected dynamics of the displacements under analysis.

The unique feature of the PS technique is to take advantage of the dense distribution of scatterers to remove most of the fluctuation of the signal propagation delay, which is mostly due to variations in the troposphere; this approach is essentially the same used for a differential GPS.

 

Technical Note
 

After the interferograms generation, an offset phase is removed from all interferograms.

One or more pixels ( Reference Points) are automatically selected by the program for the calculation of the phase offset to remove.

The number of the 'Reference Points' depends on the size of the Area of Interest. As default, just one 'Reference Point' is selected for Areas within 25 sqkm, (refer to the Preferences>Persistent Scatterers>Area for Single Reference Point).

At this point, the algorithm can follows two kind of directions:

 

•Areas of analysis with size within the value specified by the 'Area for Single Reference Point' parameter are processed using just one 'Reference point' for the entire Area.

•A second approach is carried out when larger Area has to be analyzed. Then the entire area is splitted into more sub-areas taking into account the overlap percentage too, each one with size corresponded to the input  parameter. Every sub areas is processed in independent way. Finally, a mosaicing operation is carried out to merge all sub - areas and getting the whole result.

 

From this stage all the re-flattened interferograms,  together with the phase-height pair-by-pair proportionality factors (_k_factor files, which are stored in the "work/work_interferogram_stacking"), are used to estimate the residual height. These components are removed from the re-flattened interferograms before the atmosphere estimation process takes place.

 

A linear model is assumed to provide the relationship between the phase and the residual height:

 

 

•Linear Model, to estimate residual height and displacement velocity.

 

Input Files

 

Auxiliary file

Reference file generated as output of the "Connection Graph" step and possibly updated afterwards. This file is mandatory.

 

Parameters - Principal Parameters

 

Residual Height Sampling (m)

This corresponds to the sampling frequency (in meters) which is used to estimate the residual height.

 

Min Residual Height (m)

These correspond to the minimum (negative value) residual height, with respect to the reference Digital Elevation Model.

 

Max Residual Height (m)

These correspond to the maximum (positive value) residual height, with respect to the reference Digital Elevation Model.

 

SubArea For Single Reference Point (sqkm)

It refers to the maximum size for one 'Reference Point'.

 

SubArea Overlap (%)

It refers to the overlap between the sub areas.

 

Number of Candidates

For each subarea one or more pixels is/are considered as candidates (Reference Points). The analytic method to define the candidates is based on the Amplitude Dispersion Index. They are automatically analyzed for the calculation of the phase offset and just one will be selected as reference point.

Rebuild All

By setting this flag the whole PS Inversion process is started from scratch.

 

It is advisable to leave this flag unchecked in case of process interruption, so that the products already generated have not to be computed and stored again.

 

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.

 

SubArea Merging Only

This flag allows to perform only the sub-areas merging step.

This has to be set to true if the merging parameters have been changed or if the PS Compute Specific Sub-Area has been performed.

 

Coherence Threshold SubArea Merging

Threshold used by the merging algorithm in Sub-Areas overlap in order to use only high coherence pixels.

 

Min. PS Density in SubArea Overlap (%)

Minimum amount of PSs in the overlap between sub-areas. Only overlaps with a higher PS density than the one defined here are considered for merging.

 

Mu Sigma Threshold for Ref Points (%)

This Threshold is used during the preliminary candidate searching. During this step all the points above this threshold are considered for the searching of reference points.A threshold value near to 100% will consider only the very good points as Reference point Candidate, a threshold value of 0% will consider all the points. A Threshold value of 60% will consider all the points above the 60% of maximum mu sigma value. This value ranges from 0 to 100%.

 

Water Mask (db)

Value in dB used as threshold to retrieve a water body mask from the average intensity image. All the points under this threshold will be removed from the output layers. Active if smaller than 0.

 

MuSigma Mask

Pixels characterized by values lower than the specified MuSigma mask are not considered during the processing. In case this parameter is set, the speed up increases but the at the cost of loosing potential PSs. (the more speed up, the less PSs).

 

Output Files

 

first_inversion

Directory containing the following products:

 

•Height_first, corresponding to the correction (in meters) with respect to the input Digital Elevation Model.

•cc_first, corresponding to the multitemporal coherence. It shows the degree of correlation between the phase pattern obtained by considering the linear height model and the observed interferometric differential series.

 

The displacement values are reported with:

- Positive sign if the movement corresponds to a decrease of the sensor-to-target slant range distance.

- Negative sign if the movement corresponds to an increase of the sensor-to-target slant range distance.

 

After the 'Reference Points' selection two shape files are generated:

•Ref_GCP,  which refers to the GCPs selected on the image, in slant range geometry.

•Ref_GCP_geo, which refers to the GCPs selected on the image, in geographic coordinates. It is stored inside the Geocoding folder, (geocoding).

•SubAreas, which refers to the sub-areas (slant range geometry) computed according to the atmospheric parameters, (refer to the Preferences>Persistent Scatterers>Area for Single Reference Point and 'Area Overlap for SubAreas).

•SubAreas_geo, which refers to the sub-areas (Cartographic coordinates) computed according to the atmospheric parameters, (refer to the Preferences>Persistent Scatterers>Area for Single Reference Point and 'Area Overlap for SubAreas). It is stored inside the Geocoding folder, (geocoding).

 

In order to avoid loading failures it is recommended not to move any file from its original repository folder.

•Meta files allowing to load the specific processing results (_meta).

•The "slant_range_dir" subfolder, which contains all processing results, in slant geometry, which are loaded by means of the meta files.

•The "work_dir" subfolder is used to store intermediate processing results.

 

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

 

Ferretti, Alessandro, Claudio Prati, and Fabio Rocca. "Permanent scatterers in SAR interferometry." IEEE Transactions on geoscience and remote sensing 39.1 (2001): 8-20.