Help Content > Interferometric Stacking > PS & E-PS > 3

Purpose

This functionality implements the first model inversion to derive the residual height and the displacement velocity. They are used to flatten the complex interferograms.This step is mandatory.

The approach is based on the identification of a certain number of “coherent radar signal reflectors” (Persistent Scatterers). The processing is then focused on the analysis of the phase history of these reliable single targets (each one represented by an image pixel), as opposed to the conventional approaches that process the input scene as a whole. A Persistent Scatterer is subject to two main constraints: it has to be stable (fluctuations lower than a millimeter) and it has to be properly oriented in order to be detectable from the SAR antenna.

It is important to know that:

•Constant displacements, which affect all Persistent Scatterers in the observed "Geographical Region", are not detected.

•The system is designed to estimate displacements characterized by a linear trend, which means that displacement rate variations over time are not properly represented.

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.

In case the E-PS approach is computed, the filtered interferograms, obtained during the Adaptive Filtering step, are considered for the inversion.

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 and the displacement related information (i.e. velocity), which are known as low pass components. These components are removed from the re-flattened interferograms before the atmosphere estimation process takes place.

Unlike the SBAS tool, just one model is implemented:

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

The model can be synthesized as follows:

Disp = V*(t-t0)

where Disp is the displacement at time t; V is the 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

Displacement Sampling (mm/year)

This corresponds to the sampling frequency (in mm/sec) which is used to estimate the displacement velocity.

Min Displacement Velocity (mm/year)

This corresponds to the value expected (in mm/year) as the minimum displacement velocity.

Max Displacement Velocity (mm/year)

This corresponds to the value expected (in mm/year) as the maximum displacement velocity.

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 - Cluster

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

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.

•Velocity_first, corresponding to the mean displacement velocity (in mm/year). This product is not generated when the "No Displacement Model" is selected.

•cc_first, corresponding to the multitemporal coherence. It shows how much the displacement trend fits with the selected model.

•mu_sigma, amplitude dispersion index. It is computed as ratio (SAR intensity average/Standard Deviation). This file can be found in the interferogram_stacking folder.

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).

•super_reference_gcp, which refers to the best GCP selected on the image, in slant range geometry.

•super_reference_gcp_geo, which refers to the best GCP 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.

The window will be closed.

Help

Specific help document section.

Specific Function(s)

None.

Interferometric Stacking - Persistent Scatterers - 3 - Inversion: First Step

Interferometric Stacking - Persistent Scatterers - 3 - Inversion: First Step