Phase of Target Scattering for Wetland Characterization Using Polarimetric C-Band SAR

Wetlands continue to be under threat, and there is a major need for mapping and monitoring wetlands for better management and protection of these sensitive areas. Only a few studies have been published on wetland characterization using polarimetric synthetic aperture radars (SARs). The most successful results have been obtained using the phase difference between HH and VV polarizations, phi
_HH
- phi
_VV
, which has shown promise for separating flooded wetland classes. Recently, we have introduced a new decomposition, the Touzi decomposition, which describes target scattering type in terms of a complex entity, the symmetric scattering type. Huynen's target helicity is used to assess the symmetric nature of target scattering. In this paper, the new complex-scattering-type parameters, the magnitude alpha
s
and phase Phi
_alpha

s
, are investigated for wetland characterization. The use of the dominant-scattering-type phase Phi
_alpha

s
makes it possible to discriminate shrub bogs from poor (sedge or shrub) fens. These two classes cannot be separated using phi
_HH
- phi
_VV
, or the radiometric scattering information provided by alpha
s
, the Cloude alpha, the entropy
H
, and the multipolarization HH-HV-VV channels. phi
_alpha

s
, which cannot detect deep (45 cm below the peat surface) water flow in a bog, is more sensitive to the shallower (10-20-cm) fen beneath water, and this makes possible the separation of poor fens from shrub bogs. Phi
_alpha

s
also permits the discrimination of conifer-dominated treed bog from upland deciduous forest under leafy conditions. Target helicity information is exploited to introduce a new parameter, the target asymmetry. The latter is shown very promising for detection of forest changes between leafy and no-leaf conditions. The analysis of low-entropy marsh scattering showed that both the scattering-type magnitude and phase alpha
s
and Phi
_alpha

s
, respectively, as well as the maximum polarization intensity of the dominant scattering
m
, are needed for a better understanding of marsh complex scattering mechanisms. The unique information provided by the new roll-invariant decomposition parameters are demonstrated using repeat-pass Convair-580 polarimetric C-band SAR data collected in June and October 1995 over the RAMSAR Mer Bleue wetland site near Ottawa (Canada).