Geology Reference
In-Depth Information
polarizations simultaneously: HH, HV, VV, and VH in
magnitude and phase. Research on evaluation of SAR
polarimetric data for sea ice applications started in the late
1980s and early 1990s after the NASA/JPL airborne SAR
(AIRSAR) system became operational in late 1987. The
system operated in fully polarimetric mode in P‐, L‐, and
C‐bands simultaneously (see Table 7.3 for frequency and
wavelength of each band).
Drinkwater et al
. [1992] summa-
rized the potential applications of AIRSAR polarimetric
data in sea ice. The momentum of this research declined in
the late 1990s due to lack of data after decommissioning of
the AIRSAR. Canada Centre for Remote Sensing (CCRS)
upgraded the airborne SAR facility onboard its Convair
580 plane to fully polarimetric system in 1991. Data were
used to generate prelaunch simulation of the future space‐
borne polarimetric SAR on Radarsat‐2 and to develop
SAR data processors and surface parameter retrieval algo-
rithms. Research on applications of SAR polarimetric data
revived later when data became available in 2006 from the
Phased Array L-band Synthetic Aperture Radar (PALSAR)
onboard JAXA's Advanced Land Observing Satellite
(ALOS) and in the following year onboard the C‐band
Radarsat‐2. It should be mentioned, however, that polar-
imteric data were available from the Japanese satellite in the
fine beam only and from the Canadian satellite in the fine
and standard beams. These data have not been available so
far from the scanSAR mode, which is most suitable for sea
ice applications. (ScanSAR operated only in single polari-
zation, HH or VV mode). A few space agencies in Europe,
Canada, and Japan are planning to launch polarimetric
SAR systems with linear and circular polarization capabili-
ties. The difference between these two polarization configu-
rations is explained in section 7.6.2.3.
During its 6 year mission, ALOS PALSAR provided
useful data for sea ice applications. A few studies were
carried out to compare ice mapping using the L‐band
PALSAR against the C‐band Radarsat from single-
channel co-polarization data [e.g.,
Arkett et al
., 2008]
(see section 9.1). As the L‐band has deeper penetration
into the surface, a few studies focused on exploring the
possibility of estimating ice thickness from PALSAR
data.
Toyota et al
. [2011] found good correlation between
backscatter coefficient from PALSAR in HH polariza-
tion and in situ ice thickness measurements (correlation
coefficient 0.86) as well as surface roughness (correlation
coefficient 0.70). The ice thickness in their study varied
between 44 to 165 cm. More information on the method
and the results is given in section 10.4.4.
In addition to the most commonly used C‐band and
the less frequently used L‐band SAR, an X‐band SAR
(TerraSAR‐X) was developed through a joint venture
between the German Aerospace Centre (DLR) and the
European Aeronautic Defence and Space Company
(EADS). The sensor was launched on 15 June, 2007, but
it was not used often in sea ice application. One notable
study by
Eriksson et al
. [2010] compared polarimetric
SAR data from the L‐band ALOS, the C‐band ASAR,
the C‐band Radarsat‐2, and the X‐band TerraSAR‐X
satellites. The study evaluated their usefulness for sea ice
monitoring in the Baltic Sea in 2009. The C‐band co‐
polarization data, which have been used for operational
sea ice mapping since the early 1990s, was selected as a
reference. A main conclusion from the study is that the
information content in the X‐ and C‐band data is largely
equivalent, whereas L‐band data provide complementary
information. L‐band SAR is better for identifying ridges
and seems to be less sensitive to wet snow cover on the
ice. The cross‐polarized data improve the discrimina-
tion between sea ice and open water. Though not relevant
to the subject of this topic, it is worth mentioning
that a spacecraft called TanDEM‐X, very similar to
TerraSAR‐X, was launched on 21 June 2010. The two
satellites fly at a distance of a few hundred meters from
each other. This unique constellation allows generation
of digital elevation models of Earth's land surface with
an unprecedented vertical accuracy of 2-10 m. These
data will be available for distribution in 2014.
The most recent satellite SAR mission was launched in
April 2014 on ESA's Sentinel-1A. This is a C-band sys-
tem to provide continuity after Envisat mission. Data are
expected to be used in monitoring sea ice zones and arctic
environment. Sentinel-1B is planned to be launched in
2018 to pair with Sentinel-1A. The pair will image the
entire planet every six to 12 days. The sensors will have
four modes of operation: wave, strip map, interferometric
wide swath, and extra‐wide swath. While the wave mode
will have selectable single polarization (HH or VV), the
other three modes will have selectable dual polarization,
VV and VH or HH and HV.
Another category of space‐borne radar sensors that has
been used for polar sea ice mapping is the scatterometer.
This instrument has been designed to measure the sur-
face wind speed and direction over the ocean [
Pan et al.,
2003]. However, it has later proven useful in generating
important hemispheric sea ice information on a daily basis
at a coarse spatial resolution (25-50 km) [
Ezraty and
Cavanié
, 1999;
Anderson and Long
, 2005]. With this coarse
resolution, and subsequently wide swath, scatterometers
provide more frequent coverage and measurements over a
broad range of incidence angles. The first scatterometer
flew onboard NASA's Seasat in 1978, which was a Ku‐
band (13.8 GHz) fan beam. It was followed by the scat-
terometer systems onboard ERS‐1 and ERS‐2, which
were called active microwave instruments (AMI). These
scatterometers operated in the C‐band (5.3 GHz). In 1996
NASA launched another Ku‐band fan‐beam scatterom-
eter known as NSCAT, but it failed after 9 months. As
a quick replacement, NASA launched the SeaWinds
