PSPOLSYNC

Description

PSPOLSYNC creates a synthesized backscatter image from a fully polarimetric SAR (POLSAR) data set. The transmitted and received polarizations used for synthesis are determined to maximize the contrast between two targets that are provided as two bitmaps or vector layers.

Parameters

Name	Type	Caption	Length	Value range
FILI*	String	Input polarimetric SAR image	1 - 192
TARGETS*	String	Input targets bitmap or geocoded vector file	1 - 192
FILO	String	Output synthesized detected SAR image	0 - 192
MONITOR	String	Monitor mode	0 - 3	ON, OFF Default: ON

* Required parameter

Parameter descriptions

FILI

The name of the input polarimetric SAR data set, which must be either nonsymmetrized or symmetrized fully polarimetric (quad-polarization) complex data . The input data set must be in scattering, covariance, coherence, or Kennaugh matrix format.

The input data set must be a data set that has already been imported into the PCIDSK (.pix) format by SARINGEST, or it can be the key-file name of any GDB-supported POLSAR data set in its distribution format. For more information, including a complete list of supported POLSAR sensors and data products, follow the link to SARINGEST under Related functions at the end of this topic.

TARGETS

The name of the file that contains vector targets, bitmap targets, or both. The file must be in PCIDSK (.pix) format. If necessary, you can specify the same file as your input polarimetric SAR data set.

The two targets with the lowest segment number will be used. All other targets in the file are ignored.

Bitmap targets must have the same dimensions as the input data. Vector targets must overlap and have the same georeferencing as the input data.

If necessary, you can create targets in CATALYST Professional Focus or import targets from another image. You can generate geocoded vector targets in Focus or CATALYST Professional SAR Polarimetry Target Analysis, or you can import the targets from another source.

You can use PSPOLSYNC to find the maximum contrast between a point target and its background. The first target is usually small, consisting of only one or very few pixels. The second target is usually the background (clutter) in proximity to the first target. You can also use PSPOLSYNC to synthesize a backscatter image to show the maximum contrast between two extended targets.

FILO

The name of the output synthesized backscatter SAR image to create. The output file has the same dimensions as the input SAR image, and one channel. The output channel contains the synthesized radar backscatter image, which is stored as floating-point pixel values. The pixel values are in the linear scale. The physical quantity represented by pixel values is the same as that of the input SAR data set. The polarizations used for synthesis are determined automatically by PSPOLSYNC to maximize the contrast between the two specified targets. If the synthesized backscatter cannot be computed at a pixel, the pixel value is set to zero. The output values are in the linear scale.

The optimal transmit and receive orientation and ellipticity values are written to the metadata of the output file.

The specified file must not already exist.

MONITOR

The program progress can be monitored by printing the percentage of processing completed. A system parameter, MONITOR, controls this activity.

Available options are:

• ON: Turns the monitor ON (default)
• OFF: Turns the monitor OFF (recommended if running in batch or background mode)

Details

PSPOLSYNC creates a synthesized backscatter image from a fully polarimetric SAR data set. The polarizations used for synthesis are selected automatically by PSPOLSYNC to maximize the contrast (ratio) between two targets. The polarizations are determined by searching the full ranges of the orientation and ellipticity angles for the transmitted and received polarizations.

The identified transmitted and received polarizations are then used to synthesize the radar backscatter at every pixel in the input data set. The synthesis algorithm is the same as in PSPOLSYN.

The input file must represent a nonsymmetrized or symmetrized fully polarimetric (quad-polarization, complex) data set in the scattering (s4c or S3c), covariance (c4r6c or C3r3c), coherency (t4r6c or T3r3c), or Kennaugh (k16r or K9r) matrix format.

Example

Create a synthesized backscatter image that maximizes the contrast between a small target and the surrounding clutter. The input image is a RADARSAT-2 quad-polarized SLC image that is already converted to the PCIDSK (.pix) format. The two targets are represented by two bitmaps that already exist in the input file.

EASI>FILI	=	"radarsat2.pix" 
EASI>TARGETS	=	"radarsat2.pix" 
EASI>FILO	=	"max_c_1_2.pix"

EASI>run PSPOLSYNC
            

Algorithm

The contrast (alpha) between targets a and b is defined as the ratio of their radar backscatters:

alpha = (sigma0)a / (sigma0)b 
            

The radar backscatter sigma0 is shown, but the computations can also use the radar backscatter gamma0 or radar brightness beta0. The physical quantity is always the same as in the input data set.

In PSPOLSYNC, the radar backscatter of each target is expressed in terms of the Kennaugh matrix of the target, and the transmitted and received Stokes vectors. For targets a and b, the expressions are as follows:

(sigma0)a = Transpose(Fr) * ka * Ft
(sigma0)b = Transpose(Fr) * kb * Ft
            

Ft and Fr represent Stokes vectors for the transmitted and received polarizations, respectively; ka and kb are the averaged nonsymmetrized Kennaugh matrices for targets a and b. For symmetrized data, the matrices Ka and Kb are used.

A Stokes vector (F) is defined by its orientation (psi) and ellipticity (chi) angles, as follows:

F = Transpose( 1, cos(2*psi)*cos(2*chi), sin(2*psi)*cos(2*chi), sin(2*chi) )
            

The transmitted and received orientation and ellipticity angles are used for Ft and Fr, respectively.

If required, PSPOLSYNC first converts the input data to the Kennaugh matrix format. A symmetrized or nonsymmetrized matrix matches the symmetry of the input data set. PSPOLSYNC then computes the average Kennaugh matrices for the two targets by averaging every matrix element over all pixels of each target.

PSPOLSYNC then searches for the transmitted and received polarizations that maximize the contrast between the two targets as follows:

• Initialize (alpha)max = 0
• For psi, from -90 degrees to +90 degrees, in 1-degree increments
  • For chi, from -45 degrees to +45 degrees, in 1-degree increments
    • Derive the transmitted Stokes vector Ft with the current values of the psi and chi angles
    • Derive the scattered waves from the targets a and b as follows:

      Fa = Transpose(Fa1, Fa2, Fa3, Fa4) = ka * Ft
      Fb = Transpose(Fb1, Fb2, Fb3, Fb4) = kb * Ft
                                          

    • Derive the maximum contrast alpha for the current Ft as follows:

              Fa1*Fb1 - fa.fb       ( (Fa1*Fb1 - fa.fb)^2   Fa1^2 - fa.fa )
      alpha = --------------- + sqrt( ------------------- - ------------- ) 
               Fb1^2 - fb.fb        (  (Fb1^2 - fb.fb)^2    Fb1^2 - fb.fb )
                                          

      The vectors fa and fb are defined as follows:

      fa = Transpose(Fa2, Fa3, Fa4)
      fb = Transpose(Fb2, Fb3, Fb4)
                                          

      The notation fa.fb represents the inner (scalar) product of the two vectors.

    • If the computed contrast (alpha) represents a new maximum, the processing parameters are updated as follows:

      (alpha)max = alpha
      (Ft)max = Ft
                         (     fa - alpha*fb  )
      (Fr)max = Transpose( 1, --------------- )
                         (    |fa - alpha*fb| )
                                          

      When the wave that is scattered by target b is completely polarized, then Fb1^2 = fb.fb and the contrast is infinite. In this case, the search terminates and the Stokes vector for the receive polarization is set as follows:

                         (       -fb       )
      (Fr)max = Transpose( 1, ------------ )
                         (     sqrt(fb.fb) )
                                          

When the search is completed, the output image is synthesized from the Stokes vectors (Ft)max and (Fr)max, and from the input Kennaugh matrix k(i,j) at every image pixel, (i,j). The synthesized backscatter is computed as follows:

               1
sigma0(i,j) =  - * Transpose((Fr)max) * k(i,j) * (Ft)max
               2 
            

Acknowledgements

PCI Geomatics gratefully acknowledges the financial support provided by the Canadian Space Agency through the Earth Observation Application Development Program (EOADP), contract number 9F028-034946.

References

Ulaby, F.T. and C. Elachi. Radar Polarimetry for Geoscience Applications. PUBLICATION PLACE: Artech House, 1990.

Dubois, P.C. and J.J. van Zyl, "Polarization filtering of SAR data", Proc. IGARSS'89, VOL., NO. (1989): 1816-1819.