Geology Reference
In-Depth Information
7.6. microWave sensinG
radar polarimetry because of the wide applications of
radar in operational ice monitoring and the expected
emerging use of radar polarimtery when data become
available from future sensors. Commonly used decompo-
sition methods of radar polarimteric data are pre-
sented with derived parameters that reveal different
radar scattering mechanisms and have potential in sea ice
classification.
Passive and active microwave remote sensing are the
most commonly used tools to monitor sea ice in the polar
regions because of their ability to image the surface in the
absence of sunlight. They are also unaffected, to a large
extent, by atmospheric conditions especially for relatively
large wavelengths (>10 mm). Passive microwave (PM) sen-
sors measure emitted radiation while active sensors meas-
ure the backscatter of a transmitted signal after reflection
(scattering) off a surface. The frequency and wavelength
of the currently operational microwave sensors (active
and passive) are shown in Table 7.3. Passive sensors, with
their wide swath and frequent temporal coverage are suit-
able for synoptic‐scale polar sea ice monitoring. Active
sensors (radar) are more suitable for tactical scale moni-
toring at finer spatial resolution (hundreds of meters),
which is required to facilitate ship navigation. This section
provides a brief summary on fundamentals of microwave
remote sensing to support discussions on applications in
Chapter 9 and 10. Focus is placed on imaging radar and
7.6.1. Passive Microwave
Unlike TIR radiation, which is dominantly affected by
the physical temperature of the object, PM radiation is
dominantly affected by the emissivity of the radiating
layer. The emissivity in the microwave region is influ-
enced by physical composition and properties of the
medium such as salinity, surface roughness, moisture
contents, and atomic composition and crystalline struc-
ture. These parameters feed into the dielectric constant.
Some of these parameters (e.g., salinity and geometrical
characteristics of brine pockets) are primarily triggered
by variations in temperature. Therefore, in addition of
being an explicit parameter in determining the emitted
radiation, ice temperature influences most of the param-
eters that affect the emissivity and consequently the emit-
ted radiation. Radiation is emitted from a layer defined
by the penetration depth (the radiating layer). Within
that layer the radiation is scattered by inclusions that
have dielectric constant different than the host material.
It can also be scattered at the surface if it has appropriate
roughness. Figure 7.23 shows emitted radiation scattered
when refracted across a rough boundary between two
dielectrically mismatched media. It shows also the coher-
ent summation of scattered signals that meet at a given
point at the surface before emitted in the direction of the
observation
θ
r
.
Table 7.3
Frequency and wavelength of operational
microwave sensors (active and passive).
Band Symbol
Frequency (GHz)
Wavelength (cm)
Active
P
0.3 - 1.0
100 - 30
L
1.0 - 2.0
30 - 15
S
2.0 - 4.0
15 - 7.5
C
4.0 - 8.0
7.5 - 3.75
X
8.0 - 12.5
3.75 - 2.4
K
u
12.5 - 18.0
2.4 - 1.67
K
18.0 - 26.5
1.67 - 1.13
K
a
26.5 - 40.0
1.13 - 0.75
Passive
K
a
18.0 - 19.0
1.66 - 1.58
Q
36.0 - 38.0
0.83 - 0.79
W
85.0 - 90.0
0.35 - 0.33
(a)
(b)
Scattered radiation
triggered by the surface
roughness
θ
r
air
ice
θ
i
Figure 7.23
Emitted radiation across a rough boundary between two media of different dielectric constant.
(a) Rough surface scatters radiation. (b) Volume scattering contributes to the single brightness temperature
observation at the viewing angle
θ
r
of the receiving antenna.
