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Polarization Signatures Background

Polarization Signatures Background

Polarization Signatures Background


The first imaging SAR systems collected data with only one polarization state. For example, the SEASAT satellite, launched in 1978, measured the backscatter return for a horizontally polarized transmitted signal, and a horizontally polarized return signal. Several more recent systems have also utilized single polarization SAR, including ERS-1, JERS-1, SIR-A, and SIR-B. Only in the past decade have SAR sensors been capable of measuring more than one polarization state while preserving phase information. These systems, called POLSAR, transmit and receive both vertically and horizontally polarized microwave signals. Currently, the most commonly available POLSAR datasets are those generated from JPL’s AIRSAR or SIR-C instruments.

Ellipticity and Orientation Angles

As an electromagnetic wave propagates through space, its electric field vector rotates in a plane perpendicular to the direction of propagation. In rotating, the electric field traces out an ellipse. The shape of this polarization ellipse is completely described by two parameters: the ellipticity angle (E) and the ellipse orientation angle (O) The V and H axes are in a plane perpendicular to the direction at which the radar signal is transmitted.

If the electromagnetic wave has a linear polarization, then the polarization ellipse will be a straight line which corresponds to an ellipticity angle of 0 or 180. For linear polarizations, an orientation angle of 0 or 180 degrees represents horizontal polarization, while an orientation angle of 90 degrees represents vertical polarization. POLSAR sensors usually transmit and receive both vertical and horizontal linearly polarized radiation. An excellent source for more information on ellipticity and orientation angles is Raney (1988).

For a SAR system that coherently transmits and receives both horizontal and vertical polarizations, you can use the elements of the resulting scattering matrix to calculate images representing any desired polarization state (that is, any valid combination of ellipticity and orientation angles). Therefore, for POLSAR data, you are not limited to synthesizing images of backscatter at VV, HH, or HV polarizations. You can synthesize an image that shows what the backscatter would be with transmitted and received signals of any polarization, including non-linear polarizations.

Polarization Signatures

Although coherent POLSAR data allows the calculation of radar backscatter at any ellipticity and orientation angle, it is generally not practical to analyze POLSAR data by synthesizing a large number of images with different polarizations. Polarization signatures, which are 3D representations of the complete scattering characteristics of a pixel or ROI, are commonly used for this purpose. Polarization signature plots show the variation in scattering intensity, normalized scattering cross-section, or dB as a function of ellipticity and orientation angles. In a polarization signature plot, linear vertical polarization is shown in the center of the plot, while linear horizontal polarization is shown at the center of the X axis, at both the maximum and minimum of the Y axis. See the following figure for an example.

Polarization signatures provide a wealth of information about various properties of a surface. Radiation with different polarizations scatters off a given surface in different ways. The polarization signatures can help to determine which polarization images may optimize the signal from a particular feature. They can also be used to determine which polarization images may best distinguish two features of interest.

One polarization signature can show either co-polarized or cross-polarized polarizations. Co-polarized polarizations have the same transmitted and received polarizations. HH and VV are the two linear co-polarized options. Cross-polarized polarizations have orthogonal transmitted and received polarizations. HV or VH are the linear cross-polarized options.

References

Evans, D.L., T.G. Farr, J.J. van Zyl, and H.A. Zebker, 1988, Radar polarimetry: analysis tools and applications. IEEE Transactions on Geosciences and Remote Sensing, Vol. 26, No. 6, pp. 774-789.

Raney, R.K., 1998, Radar fundamentals: technical perspective, in Principles and Application of Imaging Radar (F.M. Henderson and A.J. Lewis, eds.), Manual of Remote Sensing, Third Edition, Volume 2, John Wiley and Sons, Inc.

van Zyl, J.J., H.A. Zebker, and C. Elachi, 1987, Imaging radar polarization signatures: theory and observation. Radio Science, Vol. 22, No. 4, pp. 529-543.

van Zyl, J. J., 1989, Unsupervised classification of scattering behavior using radar polarimetry data, IEEE Transactions on Geosciences and Remote Sensing, vol. 27, No. 1, pp. 36-45.

Zebker, H.A., J.J. van Zyl, and D.N. Held, 1987, Imaging radar polarimetry from wave synthesis. Journal of Geophysical Research, Vol. 92, No. 31, pp. 683-701.



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