Geology Reference
In-Depth Information
wave interactions with matter: reflection, transmission,
absorption, scattering, and emission. The discussions
highlight a few points relevant to understanding the data
from optical and TIR sensors. These processes are fre-
quency dependent, but the resulting signal frequency does
not change as a result of any process (except emission).
The section addresses also the concepts of brightness tem-
perature, and emissivity. It is concluded with information
on penetration depth of the signal, which determines the
depth from which the recorded observations carries infor-
mation. For more detailed and comprehensive discussions
on these fundamental concepts the reader may check topics
on physics of remote sensing such as
Ulaby et al.
[1981],
Elachi and van Zyl
[2006], and
Reese
[2013].
In section 6.1.1 rudimentary aspects of polarization of
EM waves were introduced to highlight the use of polar-
ized light, particularly plane polarized light, for examin-
ing the grain structure and texture of polycrystalline ice.
In this section more information on polarization of EM
waves is presented and discussed in the context of remote
sensing applications.
Most sources of light are classified as unpolarized or at
least partially polarized. This makes the reflected/emitted
observations from optical/TIR sensors also unpolarized
and therefore recording the polarization of the observations
becomes irrelevant. In passive microwave remote sensing
the emitted radiation (section 7.3.3) can have a varying
degree of polarization. In radar remote sensing the polari-
zation becomes most crucial because the transmitted signal
is polarized. Depending on the ground target or surface, the
signal is scattered with similar or different polarization.
Alternatively, it can be partially polarized. Characterization
of the polarization of the received radar backscatter carries
information about the imaged surface.
Figure 7.12 is an illustration of a linearly polarized wave
(
I
) incident at an interface and the reflected (
R
) and trans-
mitted (
T
) waves. The propagation direction of each wave
is indicated by the arrow (
I
,
R
and
T
). While the electric
field of the incidence wave can be in any plane, it is custom-
arily resolved into two orthogonal components, one being
the plane of incidence. This plane is defined as the plane
containing the normal to the interface and the propagation
vector of the EM wave. If the electric field vector lies in the
7.3.1. Polarization of EM Waves
Polarization is a fundamental characteristic of an EM
wave since it can be depicted as a two‐dimensional trav-
erse wave. It refers to the alignment of the electric field
vector in an EM wave. If the vector is aligned to a certain
plane or varies according to a predictable alignment while
the EM wave is propagating, the wave is called polarized.
In the first case the wave is referred to as linearly polar-
ized, and in the second it may become elliptically or circu-
larly polarized as explained later. If, on the other hand,
the wave has a random time‐varying electric field vector,
it is called unpolarized. Between these two ends the signal
can be partially polarized.
I
h
Figure 7.12
Illustration showing an Incident EM wave (
I
) with its electric waveform resolved into two orthogonal
components denoted
h
(horizontal) and
v
(vertical). The
h
plane is perpendicular to the plane of incidence but
not necessarily coincident with the interface plane. The reflected (
R
) and transmitted (
T
) waves are also shown
with the two wave components of the former.
