|
<< Click to Display Table of Contents >> ScanSAR Interferometry Module - 4 - Refinement and Re-flattening |
  
|
|
<< Click to Display Table of Contents >> ScanSAR Interferometry Module - 4 - Refinement and Re-flattening |
|
Purpose
This step is crucial for a correct transformation of the unwrapped phase information into height (or displacement) values. It allows both to refine the orbits (i.e. correcting possible inaccuracies) and to calculate the phase offset (i.e. getting the absolute phase values), or remove possible phase ramps.
The execution of this step is mandatory for Digital Elevation Model generation as well as for Displacement Mapping.
To execute this step a Ground Control Point file must be previously created.
Technical Note
Depending on the specific processing parameter setting (Preferences>Flattening>Refinement and Re-flattening/Refinement Method), the polynomials and the correction factors are calculated and written in the header file of the unwrapped phase image (_upha.sml>interferometric_processing section) - as well as on a popup window when the process is not executed in batch mode - at the process completion; these correction factors/polynomials are:
| - | ORShiftOrbitInX - Orbital shift in X direction (in meters) - It is generated if the "Orbital" method is applied. |
| - | ORShiftOrbitInY - Orbital shift in Y direction (in meters) - It is generated if the "Orbital" method is applied. |
| - | ORShiftOrbitInZ - Orbital shift in Z direction (in meters) - It is generated if the "Orbital" method is applied. |
| - | ORAzShiftOrbitInX - Dependency of the shift in X direction, from the azimuth position (in meters per pixel) - It is generated if the "Orbital" method is applied. |
| - | ORAzShiftOrbitInY - Dependency of the shift in Y direction, from the azimuth position (in meters per pixel) - It is generated if the "Orbital" method is applied. |
| - | ORAzShiftOrbitInZ - Dependency of the shift in Z direction, from the azimuth position (in meters per pixel) - It is generated if the "Orbital" method is applied. |
| - | ORPhaseOffset - Absolute phase offset (in radians) - It is generated if the "Orbital" method is applied. |
| - | ORRMSError - Root mean square error calculated as the difference between the height value of the Ground Control Points and the corresponding value in the interferometric phase (in meters) - It is generated if the "Orbital" method is applied. |
| - | PhaseError - A-priori achievable root mean square error, calculated as average on the input GCPs (height in meters) - It is generated if the "Default" method flag is checked. |
| - | RPPPhasePolyDegree - Degree of the polynomial used to estimate the phase ramp - It is generated if the "Polynomial Refinement" method is applied. |
| - | RPPPhasePoly - The polynomial used to estimate the phase ramp - It is generated if the "Polynomial Refinement" method is applied. |
| - | RPPRMSE - Root mean square error calculated as the difference between the height value of the Ground Control Points and the corresponding value in the interferometric phase (in meters) - It is generated if the "Polynomial Refinement" method is applied. |
The popup window additionally provides, for each input Ground Control Point, the following values:
| - | Mean difference between SRDEM - slant range input DEM - and SAR DEM (in meters). |
| - | Mean difference between Unwrapped Phase and calculated Phase Ramp (in radians) |
| - | Standard Deviation between SRDEM and SAR DEM (meters). |
Very large "ORRMSError" or "RPPRMSE" - root mean square errors - (in the order of hundreds or thousands) eventually bring to wrong results. Care must be paid also when very small "ORRMSError" values (less than 1) are reported; as a rule of thumb errors from some units to some tens are a good preliminary indication that the Ground Control Points have been properly located.
Large "ORRMSError" values can be reported when processing pairs with very small baseline (i.e. less than about 10 meters).
Ground Control Points located on null/dummy value pixels (NaN) are discarded.
The correction parameters are calculated depending on the specific "Refinement Setting" and they are applied to rebuild the following input files:
| - | Unwrapped Phase (_upha). |
| - | Flattened Interferogram (_dint or _fint). |
| - | Synthetic Phase (_sint). |
The points ("Refinement Ground Control Point file") used to calculate the correction parameters (Refinement Setting), shall be selected on the input flattened interferogram (_dint, _fint) in order to avoid areas where topographic fringes remained "unflattened" and "moving areas". The Ground Control Points must be well distributed throughout the entire scene.
An indication about the Ground Control Points quality can be obtained by inspecting the "_refinement.shp" (see "Output" product description below).
If the Interferogram Flattening has been performed using a reference Digital Elevation Model, it is not necessary to specify the co-ordinates of each Ground Control Point; in this case the cartographic co-ordinates (easting, northing and height on the reference DEM) of each GCP are written in the log file at the end of this processing step.
Areas with good coherence should be preferred for the Ground Control Points location. In any case the importance of each GCP is weighted by the program on the basis of its coherence value.
If errors exist in the unwrapped phase image, they must be corrected before running this step. Otherwise wrongly unwrapped areas (disconnected phase "islands") have to be discarded for the Ground Control Points location.
Tandem-X Data
When these data are acquired in bistatic mode, the unwrapped phase (_upha) is automatically reflattened (_reflat_upha). Due to this the Refinement and Re-flattening step has not to be performed.
Input Files
Coherence file
Name of the coherence image (_cc). This file is mandatory if a "Refinement Method" is selected.
Input Reference file
File name of the multi-looked Reference Intensity data (_pwr). This file is mandatory.
Input Secondary file
File name of the multi-looked coregistered Secondary Intensity data (_pwr). This file is mandatory.
Unwrapped Phase file
Name of the unwrapped phase (_upha). This file is mandatory.
Synthetic Phase file
Name of the synthetic phase (_sint). This file is mandatory.
Slant Range Digital Elevation Model file
Name of the Digital Elevation Model in slant range geometry (_srdem).
Refinement 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 mandatory if a "Refinement Method" is selected.
Optional Files
Interferogram file
Name of the flattened phase (_fint, _dint). 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
Refinement method
The choice is given between the following refinement methods:
•Automatic Refinement;
•Polynomial Refinement;
•Orbital Refinement.
Refer to the flattening section of the Preferences for further information about these methods.
Refinement Residual Phase Polynomial Degree
Degree of the polynomial used to estimate the phase ramp, which will be removed from the input unwrapped phase during the Re-flattening operation. In case this value is higher that the number of input Ground Control Points, it will be automatically decreased. The default values of 3 means that a phase ramp in range and azimuth direction plus a constant phase offset will be corrected. In case only the phase offset correction is needed, the polynomial degree will be set to 1.
Coregistration With DEM
This parameter can not be defined 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 - Refinement
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 Root Name
Name of the output root. This file is optional.
_reflat_dint/fint
Re-flattened interferogram with the associated header files (.sml, .hdr).
_reflat_sint
Re-flattened synthetic phase with the associated header files (.sml, .hdr). This file is generated only if a "Refinement Method" was selected.
_reflat_upha
Re-flattened unwrapped phase with the associated header files (.sml, .hdr).
_reflat_srdem
Digital Elevation Model, in slant range geometry, with the associated header files (.sml, .hdr). This file is generated only if a "Refinement Method" was selected.
_reflat.txt
Text file with the orbital correction parameters resulting from the refinement.
_refinement.shp
Shape file containing those points retained (valid GCPs) form the input "Refinement GCP file". The following information is provided if the "Orbital" method is applied:
| - | Height value (in meters) from the input DEM in slant range - "ReadHeight". |
| - | Absolute - "AbsHgtDiff" - and relative - "HeightDiff" - difference (in meters) between the real height (input DEM in slant range) and the height value derived from the corrected orbits. |
| - | Standard deviation (in meters) of the input "Refinement GCP file" - "SigmaMt" - It is based on interferometric coherence and orbital configuration. |
| - | Standard deviation (in radians) of the input "Refinement GCP file" - "SigmaRad" - It is based on the interferometric coherence. |
The following information is provided if the "Polynomial Refinement" method is applied.
| - | Unwrapped phase value (in radians) - "ReadPhase". |
| - | Absolute - "AbsPhDiff" - and relative - "PhaseDiff" - difference (in radians) between the real phase and its fitted value based on GCPs. |
| - | Standard deviation (in meters) of the input "Refinement GCP file" - "SigmaMt" - It is based on interferometric coherence and orbital configuration. |
| - | Standard deviation (in radians) of the input "Refinement GCP file" - "SigmaRad" - It is based on the interferometric coherence. |
_refinement_geo.shp
Shape file containing the geocoded location of the valid GCPs used in the refinement process.
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
None.