what does "desying" mean in polarimetric radar studies?
Since no one else has answered, I'll go ahead and make an answer out of my comments.
Downvotes and corrections welcome. :)



Psi (Ψ) is used in some of the formulas for polarimetric radar as the symobol for an angle that represents the orientation between the target and the radar beam. The 9 parameters that Huynen defines must be derived without Ψ, since they are meant to be used regardless of the orientation. Removing the factor Ψ from the measured data is called desying. 

Since Ψ is central to much of what goes on, I would suggest you read at least the relevant parts of the papers you linked to, or perhaps better yet find a copy of Huynen's book "Phenomenological theory of radar targets" in which he defines the term "desying."

The term "desying" makes more sense if you think of it as "deΨing" or "dePsiing."

My understanding of this comes from page 76 of "Target Scattering Decomposition in Terms of Roll-Invariant Target Parameters" by Ridha Touzi.