Geology Reference
In-Depth Information
plane of incidence, the EM wave is said to be vertically
polarized (subscript
v
in the figure) and referred to as a
transverse magnetic (TM) wave. If it lies in the perpendicu-
lar plane, the wave is said to be horizontally polarized (sub-
script
h
in the figure) and referred to as transverse electric
(TE) wave. The figure depicts a case where the linear polari-
zation of the incident where reflected and transmitted
waves are neither vertical nor horizontal.
In passive microwave systems the radiometer is usually
designed to receive the emitted signal in each one of the
two orthogonal polarizations: horizontal or vertical.
Some surfaces emit radiation as partially polarized in one
direction or the other. In radar systems the antenna trans-
mits pulses in a selected polarization and receives the
backscatter signal (section 7.6.2) in the same or in the
perpendicular polarization. If the intention is to transmit
a nonlinear polarized radar signal, then two transmitted
plane‐polarized waves in the horizontal and vertical
planes must be generated simultaneously. The type of
polarization is determined by the vector addition of the
two perpendicular waves. If the two orthogonal waves are
in‐phase, the resultant will be a linearly polarized signal.
If out‐of‐phase the resultant will be an elliptically or, in a
special case, circularly‐polarized signal. The shape is
determined based on the phase shift between the two
orthogonal transmitted signals. These scenarios are illus-
trated in Figure 7.13. With a zero phase shift (Figure 7.13a)
the resultant waveform will oscillate within a fixed plane
at an angle of 45°. The tip of the wave traces a line at 45°
in the plane perpendicular to the wave propagation direc-
tion because the two transmitted signals have the same
amplitude. This is a linearly polarized wave. If the phase
shift between the transmitted signals is a quarter‐cycle
(i.e.,
π
/2), as shown in Figure 7.13b, the plane of oscilla-
tion will rotate as the wave propagates (shown by the
arrows in the figure). In this case, the tip of the wave
traces a circle in the plane perpendicular to the wave
propagation. This is known as a circularly polarized sig-
nal. If the phase shift is between zero and
π
/2, the tip of
the wave traces an ellipse instead, hence known as an
elliptically polarized signal. The progression of the time‐
varying direction of the electric field vector for linear,
elliptical, and circular polarizations is shown in a 3D per-
spective in Figure 7.14. As can be seen, the linear and
(a)
(b)
Figure 7.13
Illustration of two antenna‐transmitted EM waves whose electric components are orthogonal. The
resultant wave vector is shown at selected points. The phase shift between the two transmitted waves is (a) zero and
(b)
π
/2. In a plane perpendicular to the wave propagation direction the tip of the resultant vector traces a line and
a circle in (a) and (b), respectively.
y
(a)
(b)
(c)
y
y
Figure 7.14
Propagation of the electric field in an EM wave. In linear polarization, the waveform remains in (a)
the same plane, for elliptic and circular polarizations it rotates such that the tip of the wave draws an (b) ellipse
or (c) a circle in the plane perpendicular to the wave propagation, respectively.
