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Overview

Area of application

Process flowsheets

Geo processor

Interferometric processor

Stereo processor

Image processing tools

Oil slicks detection processor

Ship detection processor

Sea waves analysis software tool

Polarimetric processor

Coherent change detection

Coherent co-registration

Quality estimation software tools

Detailed specifications

Examples

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Interferometric processor

InSAR/DInSAR processor has been developed as tool for DEM generation and surface shift estimation using of phase information extracted from complex spaceborne SAR data

As the inputs for InSAR processor the SIR-C/X, RADARSAT, ERS-1/2, and ENVISAT imagery presented in single look complex (SLC) CEOS format should be used. A pairs of SAR images covering the same area of the Earth that is taken from slightly different orbits is suitable for interferometric and differential interferometric processing.

The inter-orbital distance causes some phase difference on images. This information is combined with precise orbital information to determine the elevation and shift of each pixel. In order to obtaining of absolute DEM at least one three-dimensional ground control point is required.

The processes of interferometric terrain extraction and surface shift estimation are quite complicated however InSAR/DInSAR processor uses sophisticated techniques, such self-automatic images co-registration, adjustment of orbital parameters, interferogram generation, phase filtering and phase unwrapping which are designed in manner to simplify their usage.

The output DEM or surface shift map are referenced to WGS84 ellipsoid and allocated in geographic projection.

Review of processing flow

Import of data and auxiliary information
Data file reading.
Parameters reading from CEOS format.
Processing parameters formation.
   
   

Image coregistration
Search of identical points on the master and slave images.
Transformation of slave image to the master image geometry.
Overlapping area determination.

Slave image transformed to geometry of master image
   
Preliminary processing
Subset selection.
Multilook parameters selection.
   
   

Interferogram and coherence calculation
Calculation of flattening coefficients for range and azimuth directions.
Complex multiplication of master and transformed slave images.

Interferogram and coherence matrixes
   
Interferogram filtration
Local estimation of phase noise parameters.
Calculation of filtered phase values.
Matrix of filtered interferogram
   

Phase unwrapping
Conversion of wrapped [0, 2Pi] phase values to absolute ones.

Absolute phase matrix
   

Absolute phase to height recalculation
Transformation of absolute phase to height.
Georeferencing of height matrix.

Matrix of heights in ground coordinate system
   

Geocoding of height matrix
Geocoding of height matrix.
Orthorectification of height matrix.

Matrix of absolute relief heights in geographical projection

Coregistration

Accurate coregistration of two radar images to 1/8 pixel is necessary for precise interferogram calculation. The main coregistration aim is to transform slave image in master image geometry with help of affine transformation. It is assumed that satellite orbits is parallel and affine transformation is enough for slave image geometry adjustment.

Interferogram and coherence calculation

Interferogram is a result of complex multiplication of master image and complex conjugate to slave image.
In result phase differences image of a surface is formed. Besides of this, simultaneously flat Earth phase correction operation is executed. Flat phase correction includes operations of phase correction in range and azimuth directions. Flat phase correction in range is necessary because the phase difference image contains the information both about height of object and about distance up to object. Flat phase correction in azimuth is necessary because the satellite orbits is not parallel during master and slave images obtaining.
The choice of a mode "Precise" means calculation of coefficients of compensation for each image pixel individually. Thus time for realization of operation is increased.
In a mode "Fast" the coefficients of compensation calculated for the central column and the central line of a picture are used.
In a mode "No flattening" flat Earth phase correction operation will not be executed.

Simultaneously with interferogram calculation, coherence calculation is carried out. Coherence is a parameter describing a synchronization of values of the main and slave images. More close to 1 value of coherence in pixel speaks about a good signal/noise ratio for interferogram image in that pixel.
Coherence values:
0.0 - 0.3 - poor. Typically for interferogram areas where there are phase discontinuity.
0.3 - 0.5 - low. Typically for interferogram areas with changed between observations surface characteristics.
0.5 - 0.7 - good.
0.7 - 1.0 - excellent.

 

 

Interferogram
Coherence

Filtration

Filtration of interferogram is performed in order increase the signal-noise ratio. Speckle noise, which is inherent for the coherent wave systems, presents on both the radar images and the interferogram. Speckle noise is not only decreases the visual quality of interferogram but additionally considerably complicates and worsens quality of work of phase unwrapping algorithm, induces the errors in absolute phases matrix.
It’s necessary to note that filtration decreases the spatial resolution on interferogram. Therefore the filter parameters should be selected taking into account this fact. It’s desirable to avoid too intensive interferogram filtration.

The following filters are accessible to use:
Average filter. Most simple of accessible filters. From the mathematical point of view this filter is equivalent to construction of a plane on some set of points by the least squares method. Works quickly but filtration quality not always high. The filtration the more strongly, than is more size of a filtration window. The filtration with a window of the size 1x1 means absence of a filtration.
Spectral adaptive filter. At work uses two dimensional Fourie transformation. Varying Min and Max values of parameter of filtration intensity it is possible to achieve necessary quality of a filtration. The more value of parameter of intensity, the more strongly will be filtration.

 

Filtered interferogram

Phase unwrapping

Phase unwrapping procedure have to solve a problem of phase ambiguity. Before phase unwrapping interferogram contains phase values in limits 0-360 degrees but the knowledge of absolute phase values is necessary. The problem is reduced to addition of the necessary number of phase cycles to each phase measurement. The decision of a problem is complicated by presence on interferogram phase noise and so called residues, that makes the decision of a problem difficult theoretically and expensive computational. There is a set of methods of the phase unwrapping problem decision.

The following methods are accessible to use:
Unweighted phase unwrapping. The most simple algorithm on the base of the least squares method.
Picard iteration method. Algorithm on the base of the least squares method with use of Picard iterations method for decision of system of linear algebraic equations.
Conjugate gradients method. Algorithm on the base of the least squares method with use of conjugate gradients method for iterative decision of system of linear algebraic equations.
Growing pixels method. This algorithm realizes the local approach in phase unwrapping. Phase unwrapping will be carried out from the unwrapped pixel the nearest not unwrapped. Quality of algorithm work is checked by threshold tests.

There is also an opportunity of the passing of a phase unwrapping stage. It may be useful in case of area with insignificant differences of heights, when on interferogram change of a phase does not cross border of 0-360 degrees.

 

Absolute phase

Absolute phase values to height recalculation

Values of an absolute phase can be recalculated both in relative or in absolute values of height.
For recalculation of an absolute phase in relative values of height it is only necessary to specify a point concerning which recalculation will be made. Result of recalculation is the image for which the difference of heights between two neighbour pixels is kept.
For recalculation of absolute phase values in height values it is necessary to specify several ground control points, heights in which are authentically known. It is desirable, that ground control points were evenly distributed on the image. Result of recalculation is the image of the heights above reference ellipsoid.

Geocoding

At this stage geocoding of matrix of the heights received at a stage of recalculation of absolute phase values in height is carried out. The relief thus is translated in geographical system of coordinates: latitude-longitude-height above reference ellipsoid. The opportunity of a choice of the geometrical sizes of an output file grid on latitude and longitude is given. In result the file representing digital elevation model turns out.

 

 

Digital elevation model
3D perspective

Differential SAR interferometric data processing

Interferometric Processor software allows to process SAR data, acquired in interferometric mode (a 3-path and a 4-path interferometry). The differential SAR interferometric data set consists of 3 or 4 images (regarding PHOTOMOD Radar), each of them makes interferometric pair to any other image data of this set. Processing images of such set in pairs makes possible to get the differential interferogram, the picture of a phase changes (interferograms) for a some period of time. Considering a viewing geometry of all data of the set, you can re-calculate phase values of differential interferogram into field of relief displacement, occurred between observations of the processed pairs. Such changes can be caused both by vertical and horizontal motions of an underlying surface. As values of the differential interferogram directly depend on SAR wavelength, displacements on the surface can be defined in millimeter scale.
The master
image
The first
slave image
The second
slave image
The third
slave image

The processing of differential SAR interferometric data in the processor module is made consistently using the same tabs, as a processing of interferometric pairs with following features.

 Image coregistration
Two or three slave images are transformed into image geometry chosen as a master. Control points measuring and the affine transformation for each slave image are the same as in case of interferometric processing. Interferogram is a result of complex multiplication of master image and an image that is complex conjugated to slave.
The first
interferogram
The second
interferogram

 Filtration
At the stage of the filtration both interferograms are filtered independently by the same algorithms, as at the relief formation by the interferometric method.

 Phase unwrapping
The phase unwrapping is made individually for each interferogram, the differential interferogram is formed on the same tab.
The first
interferogram
The second
interferogram

 Recalculation of phase values to height
In case of differential SAR interferometric processing phase values on this tab are recalculated into a surface displacement along a sight line of a radar-surface in a direction to the radar or from it.
Matrix of the surface
displacement
Map of the displacement

 Geocoding
Matrix of surface displacement is recalculated during the stage of geocoding to geographic coordinate system (latitude-longitude-height) above reference ellipsoid.
Geocoded matrix
of the surface displacement
Interferometric
DEM

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Last modified: 18.09.2018© Racurs, 2004-2018