The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008
Figure 6: Data Processing Flow Diagram
4. ICE APPLICATIONS
Comparing to RADARSAT-1 single polarization SAR data,
RADARSAT-2 is capable of acquiring dual-polarization and
fully polarimetric SAR data. The RADARSAT-2 dual
polarization options include HH-HV or VV-VH modes. The
additional information provided by the cross-polarization
channel could be very useful, as the cross polarization channel
responds to different scattering mechanisms than the co
polarization channel (Scheuchl et al. 2004). These dual
polarization data are available in all beam configurations,
giving a wide choice in resolution, coverage, and incidence
angle. Most importantly, dual polarization is available in
ScanSAR modes, which are mostly used for ice and oil
monitoring at CIS from RADARSAT-1 data. Operational
experience by CIS analysts results in a preference of HH over
VV for co-polarization channels.
The cross-polarization HV and VH backscatter response from
water are generally low and are relatively independent of wind-
induced surface roughness conditions, whereas their backscatter
from sea ice are affected by surface roughness, volume
scattering, and multi-bounce scattering. Thus, the ice-ocean
contrast of cross-polarization can be expected to be greater than
that for either of the co-polarization channels, especially at high
wind conditions as shown in Figure 7. The cross-polarization
data can enhance the structural information of sea ice and have
demonstrated some utility for improving discrimination
between smooth and deformed ice as well. The combined use of
co-polarization and cross-polarization channel will give better
results across a wider range of incidence angles.
Co-Polarization Channel: VV Cross-Polarization Channel: VH
Figure 7: Co-Polarization vs Cross-Polarization at High Wind
Condition
5. OIL SPILL APPLICATIONS
CIS is an important partner to Canada’s NASP (National Aerial
Surveillance Program) by using earth observation technology
(RADARSAT imagery) to look for oil-like signatures
(anomalies) on the ocean’s surface that could be indicative of
an oil spill. The CIS operational ISTOP (Integrated Satellite
Tracking of Pollution) program currently uses RADARSAT-1
ScanSAR HH polarization data to identify potential oil spills, to
track ship targets (Gauthier et al. 2007), and to direct pollution
surveillance flights to the locations of potential pollution
incidents.
The SAR signal is sensitive to the roughness of the sea surface,
which is modulated by wind speed and direction; imagery
acquired at VV polarization is the most sensitive to wind speed
variability (as shown in Figure 7). The suppression of the
capillary waves by oil from either anthropogenic sources, such
as an oil spill, or from natural biological slicks, reduces the
surface roughness resulting in less radar backscatter and darker
image tones. The detection of oil slicks has been found to be
best in moderate wind conditions in the range of 3 to 10 m/s.
Although imagery in both VV and HH polarizations from
RADARSAT-2 can be used for slick detection, the VV imagery
might be preferred as, in general, it offers a better signal to
clutter ratio than other polarization choices (i.e., HH, VH, or
HV). Although VV is more sensitive than HH for slick
detection, there may not be any advantage to using the co
polarized or cross-polarized signatures as oil-free and oil-
covered surfaces tend to have similar contrast and polarization
ratios. Slick thickness and the inability to differentiate oil slicks
from "look-alikes" such as areas of low-wind, grease ice, or
biological surfactants remain problematic.
One of CIS research priority is to assess the utility of
RADARSAT-2 VV/VH dual channel ScanSAR data for the
ISTOP program. It is known that VV polarization provides
superior CNR for oil detection over HH polarization. On the
other hand, VV is less appropriate for ship detection. However,
VH polarization has shown promise in detecting ships (figure
3)[2], ISTOP will be working with others to investigate more
fully whether the VH channel of RADARSAT-2 can perform