Geology Reference
In-Depth Information
Radiometers are also used in SWIR and TIR to detect
thermal emission. Under cloud‐free sky, TIR sensors can
be used to determine the sea surface and ice surface tem-
perature and discriminate between ice and water based on
this parameter. They can also be used to identify multi-
year ice types because it is usually colder than the sur-
rounding ice. Observations in the far IR region (FIR) are
not used in sea ice applications. The microwave region
occupies a range of wavelengths from 1 mm to 1 m (300-
0.3 GHz frequency). The longer wavelength range (1 m to
100 km), which is not shown in the figure, is known as
radio wave. Microwave sensors in the millimeter and cen-
timeter wavelength are the primary sensors for sea ice
monitoring and parameter retrieval.
The total amount of radiative flux emitted from the
Sun or the Earth is determined from the Planck's equa-
tion (7.21). A sketch depicting the general trend defined
by this equation is presented in Figure 7.2 (bottom graph)
for temperature of 6000 K (average temperature of the
Sun) and 288 K (average temperature of the Earth). Solar
radiation peaks at wavelengths of approximately 0.5
μ
m
(blue‐green light). This band offers the highest radiation
level available for remote sensing detectors. It is also close
to the center of the wavelength range of human eyesight
(human eyes have adapted to utilize this highly radiant
spectral region). The emitted radiation from the Earth's
surface peaks near 10.0
μ
m. Solar radiation starts to
diminish in the TIR and becomes negligible in the micro-
wave region. A portion of the solar energy that strikes
the Earth's surface is reflected, and another portion is
absorbed by the surface. This absorbed energy is reemitted
at different frequencies since all objects radiate energy at
temperatures above absolute zero by virtue of their
atomic and molecular oscillations. Part of the emitted
radiation from the surface is absorbed (or scattered) by
the atmospheric gases in certain spectral bands. The
atmosphere allows radiation transmission in other bands
called atmospheric transmission windows. They are
located in the ranges 0.4-1.3
μ
m, 1.5-1.8
μ
m, 3.6-4.0
μ
m,
10.5-12.3
μ
m, and throughout the wide range of the
microwave spectrum from 10 mm to 10 cm (top of
Figure 7.2). The spectral bands used in satellite remote
sensing for surface observations fall into these transmis-
sion windows. Microwave sensors are successful in sea ice
applications partly because of the very low absorption by
the atmosphere and clouds.
The energy incident upon a point on the Earth's surface
from all directions above the surface (i.e., hemisphere) is
called irradiance. The total reflection or emission leaving
any point at the surface is known as radiance. The unit of
irradiance is power per unit area, while the unit of radi-
ance is power per unit area per solid angle. If a point
source radiates uniformly in all directions through a non-
absorptive medium, the irradiance decreases inversely
with the square of the distance from the object.
Remote sensing instruments operating in the visible or
the NIR bands measure the reflection of the incident
solar radiation (that is why the two regions are grouped
into what is commonly referred to as optical or reflective
region). The green portion (wavelength around 0.51
μ
m)
is particularly sensitive to ice regardless of its age or sur-
face conditions. However, the central channels of the VIS/
NIR channels of the Advanced Very High Resolution
(AVHRR) sensor onboard the National Oceanic and
Atmospheric windows
100
Absorbed
0
TIR
FIR
Infrared
Solar radiation
Earth emission
0.3
μ
m
1. 0
μ
m
10
μ
m
0.3
μ
m
1
mm
1
m
Wavelength
Figure 7.2
Atmospheric windows for spectral radiation in the range 0.3
μ
m to 1 m (top) and an approximate
extraterrestrial spectrum of solar radiation the Earth's emission (bottom). Outside the atmospheric windows the
radiation is absorbed by various atmospheric gases. The Earth's surface emits energy that peaks around the 10
μ
m.
Most of the radiation received by satellite sensors around this band is emitted from the surface.
