Why a large HH minus VV phase difference is interpreted as a double bounce scattering mechanism

This Help Article explains the reason why a large HH minus VV phase difference in polarimetric SAR is interpreted as indicating a double bounce scattering mechanism. ENVI has a tool, which performs a scattering classification. In this classification, if the phase difference for a pixel is large, and the overall backscatter is high, the pixel is interpreted to have a large contribution from the double bounce scattering mechanism.

Note:
HH = Horizontal transmitted-Horizontal received
VV = Vertical transmitted-Vertical Received 

Question:
Why does a large HH minus VV phase difference in polarimetric SAR become interpreted as indicating a double bounce scattering mechanism?

Answer:(Contributed by Dr. Anthony Freeman - JPL SAR: Understanding the Imagery)
During double-bounce scattering the vertically polarized wave usually flips its orientation, e.g. from oscillating upwards to downwards. This constitutes a phase shift of 180 degrees. The H-polarized wave does not do this. Hence the two are out of phase by 180 degrees after double-bounce has occurred.

To understand this in more detail, you have to go in to the behaviour of scattering at a surface and how for a given dielectric constant and a given incidence angle, the phase changes during scattering.

Review on 12/31/2013 MM