Remote Sensing Principles Relevant to Sea Ice - Sea Ice: Physics and Remote Sensing - page 312

Table 7.5 Characteristics of SAR dual, compact, and fully polarization modes.
Dual Polarization Compact Polarization (CP) Fully Polarimetric (FP)

Transmission Single linear polarization

H or V

Single linear polarization
(H or V) or
single circular right (R) or left (L)

Two linear polarization (H and V)
or two circular (R and L)

Reception Noncoherently H and V Coherently H and V for any

transmitted polarization

Coherently H and V for any
transmitted polarization

Phase between channels Not known Known Known

Output polarization
combinations

(HH, HV) or (VV, VH) (HH, HV) or (VV, VH)

(RH,RV) or (LH,LV)

HH, HV, VH, VV
RH, RV, LH, LV

In general, cross‐polarization SAR data are more
suitable for mapping ice deformation features such as
ridges and rubble areas. They are also useful in identifying
marine objects and icebergs in open water. As demon-
strated in Figure 7.9, water surface does not depolarize
the scattered radar signal even if the surface is wind
roughened. This results in remarkably low values of
cross‐polarized backscatter, which are not affected by a
variation in the SAR beam incidence angle [ van der
Sanden , 2004]. Ships and icebergs, on the other hand,
have high cross‐polarized backscatter caused by multiple
scattering from the ship's superstructures and multiple
scattering from the intensive air bubble contents of ice-
bergs. It should be noted, however, that contrary to the
established knowledge that water surface does not depo-
larize the scattered radar signal, a recent study by Hwang
et al . [2010] has shown that the cross‐polarized signal HV
is as sensitive to the ocean surface roughness as the co‐
polarized HH or VV. The authors found that ocean sur-
face waves depolarize the radar signal. They used this
finding to retrieve surface wind speed over ocean from
HV data (existing operational algorithms retrieve this
parameter based on HH or VV data, especially from VV,
which is more sensitive to the wind‐driven surface
roughness). Hwang et al . [2010] show also that, unlike the
co‐polarization backscatter, the cross‐polarization back-
scatter does not saturate at high wind speed (>20 m/s).
Based on their use of composite Bragg (CB) theory, the
authors concluded that depolarization by the high winds
is driven mainly by contribution from breaking waves
(Bragg scattering is described in section 7.7.2).

in Table 7.5. The CP concept was suggested by Green
[1968]. Recall that circular polarization radar wave can be
generated by transmitting linearly polarized vertical and
horizontal signals with a phase shift of 90° (Table 7.2).
The term “hybrid polarimetric” is a synonym of compact
polarization when circular polarization is used. The ulti-
mate technology of SAR polarimetry is the FP SAR. It
contains the greatest amount of information about the
imaged area. The mathematical representation of the FP
SAR for the linear transmitted wave, along with the
derived parameters and the associated scattering mecha-
nisms are presented in the rest of this section.
A few points from Table 7.5 are worth highlighting. The
compact polarization is only a variation on the dual polar-
ization. While the latter does not retain the phase informa-
tion, the former does (similar to the FP mode). That is why
the CP is considered to be “partial polarimetry.” It is an
efficient step up from the dual polarization toward fully
polarimetric systems to realize many (but not all) of its
benefits without the attendant costs of doubling the aver-
age transmitted power [ Raney , 2007]. One advantage of
using the circular (hybrid) polarization over the linear
polarization (in the CP or FP mode) is the wider spatial
coverage. The linear polarization suffers from range ambi-
guity, which limits the polarimetric mode to a maximum
of 40° incidence angle [ Raney and Freeman , 2012]. For that
reason the linear FP mode from Radarsat‐2, for example,
is limited to the standard and fine‐beam acquisition modes
(Figure 7.8), which are not frequently used in operational
ice monitoring programs. In the future hybrid CP mode
data will be available from the ScanSAR wide acquisition.
The fully polarimetric SAR system that uses linear
polarization measures the backscatter in the four basic
linear polarization combinations: HH, VV, HV, and VH.
Due to reciprocity assumption, the cross‐polarization
components HV and VH are identical. The two co‐polar-
ization measurements are correlated, but not correlated
to the cross‐polarization measurements. Each one of the
four components is measured in terms of the amplitude
and phase of the backscatter signal, and therefore the
four components can be represented for each pixel as a
2 × 2 matrix whose elements are complex numbers. From

7.6.2.3. Radar Polarimetry
The concept of polarimetric radar was introduced in
the 1950s [ Kennaugh , 1951]. SAR polarimetry is available
in a few possible architechtures and implementation
modes. This includes dual‐polarization, compact polariza-
tion (CP), and fully polarimetric (FP) SAR systems. Data
can be generated in linear polarization from all three
architectures and in circular polarization from the com-
pact and fully polarimetric architectures. The differences
between these three SAR polarimetric systems are shown