IEEE Geoscience and Remote Sensing Magazine - March 2013 - 14

Oil Platform

FIGURE 7. Oil pollution over the Gulf of Mexico acquired on April 30,

2010 (just 10 days after the explosion of "Deepwater Horizon," leading to the largest offshore oil spill in U.S. history). Oil spills appear
as dark areas in radar images because of the attenuation of the
capillary waves, leading to a smoother ocean surface and decrease
of the backscattered signal. Capillary waves are wind-caused ripplewaves with a wavelength less than a few centimeters. Image size: ca.
100 km # 100 km, acquired by TerraSAR-X in ScanSAR mode.
time results in an increased synthetic aperture length and
consequently in a better resolution. However, the Spotlight mode does not image a continuous swath but rather
individual patches along the radar flight path. Other

imaging modes exist, such as TOPS [33] or the wave
mode, each one improving certain performance parameters, but at the expense of others. It turns out that there
exist fundamental limits on single-channel SAR such that
improving azimuth resolution results in a degradation of
the swath width and vice versa. A system operated such
that it violates these limits will result in image ambiguities, i.e., scene features which appear at multiple or wrong
positions in the radar image. These limitations can be
overcome through multi-channel digital beamforming
techniques, which will be introduced in Section VII.
In the beginning, SAR images were mainly interesting
from the science and engineering point of view. The
last years have marked an important transition in
the application of SAR. This has changed and nowadays
the public awareness of the usefulness of radar remote
sensing beyond science is much higher. As an example,
radar satellites are predestined to perform disaster monitoring, mitigation and damage assessment due to their allweather day-and-night imaging capability. Fig. 7 shows a
SAR image of oil pollution over the Gulf of Mexico taken
10 days after the explosion of the oil drilling unit "Deepwater Horizon."
Another example are multi-temporal acquisitions of
the German island Sylt. Fig. 8 shows the changes during
a 5-day time span. Due to the large extent of the Wadden
Sea (ca. 10,000 square kilometers) long-term time series of
radar images offer important information to study the morphology, sediments and habitats in this area, which is being
particularly affected by sea level rise.
III. Polarimetry
SAR polarimetry is a widely used technique for the
derivation of qualitative and quantitative physical information for land, snow and ice, ocean and urban applications
based on the measurement and exploration of the polarimetric properties of man-made and natural scatterers [1],
[15], [34], [35]. Measuring the full scattering matrix allows
to build up a powerful observation space sensitive to shape,
orientation and dielectric properties of the scatterers and
allows the development of physical models for the identification and/or separation of scattering mechanisms occurring inside the same resolution cell [1].

FIGURE 8. Multi-temporal acquisition of Sylt, the most northern

island of Germany, acquired by TerraSAR-X on the 22nd, 24th
and 27th of October, 2007. Each image has been associated to a
different color channel (green, blue and red, respectively). While
the changes during the 5-day time span over land areas are relatively small, the changes in the Wadden Sea caused by the ocean
tide are clearly seen.
14

A. Polarimetric Description of the Scatterer
The basic concept of SAR polarimetry is given by the 2 # 2
complex scattering matrix that describes the transformation of the two-dimensional transmitted (e.g., incidence)
plane wave vector Ev t into the received (e.g., scattered) wave
vector Ev r (two-dimensional in the far field of the scatterer)
performed by the scatterer [15], [35], [36]

exp (- ikr)
Ev r =
[S] Ev t *
r

exp (- ikr) S HH S HV E tH *
E rH
E
; rE=
;
<
F
$
.
t
r
S VH S VV E V
EV
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2013