Geology Reference
In-Depth Information
geostationary satellites positioned over the equator at
longitudes separated by 120°. Geostationary satellites are
the preferred choice for tracking weather systems; hence,
they are also referred to as meteorological satellites. A
network of operational geostationary satellites is used
to provide images of Earth's surface in the visible and
infrared bands. They include the U. S. Geostationary
Operational Environmental Satellite (GOES), the
European Space Agency's EUMETSAT operational
satellite series, the meteorological mission of the Japanese
Multi‐functional Transport Satellite (MTSAT), and
India's weather satellite series Indian National Satellite
System (INSAT). Since most of the ice in nature exists at
or near the polar regions, data from polar orbit satellites
are used more often than geostationary satellites in ice
applications.
Depending on the source of the illumination for the
received radiation, space‐borne sensors are grouped into
two categories: passive and active (Figure 7.4.). Passive
sensors measure the reflected solar radiation in the VIS/
NIR bands or the emitted radiation from the surface
or the atmosphere in the TIR or microwave bands. The
origin of the reflected or emitted radiation is the solar
radiation. Therefore, passive sensors depend on an exter-
nal source of illumination. Active sensors, on the other
hand, transmit their own illumination signal (radar or
laser pulses). They include imaging radar, scatterometers,
radar altimeters, and laser altimeters. The transmitted
signal is reflected off the surface, and the reflected/scat-
tered signal is recorded by the receiving antenna. In mon-
ostatic radar systems the same antenna is used for
transmitting and receiving the signal. Active sensors can
image Earth's surface during daytime or at night.
Measuring radiation by space‐borne instruments varies
between active and passive systems and depends on
whether the instrument is a nonscanning (profile) or scan-
ning (imaging). Active nonscanning sensors include altim-
eters that look down at the surface at nadir direction,
measuring the precise time between transmit and receive
signals and extract the height of the observed surface with
respect to a given reference. A wind scatterometer is
another example of an active nonscanning sensor that
observes the same part of the ocean surface from different
(at least three) view angles. It is mainly used to estimate
winds over ocean surface. Active scanning sensors, on the
other hand, include real imaging radar (RAR) and syn-
thetic aperture radar (SAR). Fundamentals of these sys-
tems are presented in section 7.6.2.1. Sensors classified as
passive nonscanning include space cameras (although
these are imaging systems) such as the one onboard the
Russian COSMOS satellite. Passive imaging scanning
systems are classified into space TV cameras, multispec-
tral optical‐mechanical scanners, and scanning microwave
radiometers. The latter has been heavily used in monitor-
ing sea ice in the polar regions since early 1970s.
The two most commonly used types of scanning radi-
ometers are cross‐track and conical scanners (Figure 7.5).
In cross‐track scanners the radiometer scans the swath
width (
F
) in a series of lines using a rotating mirror (
A
).
The lines are oriented perpendicular to the direction of
motion of the satellite platform (i.e., across the swath).
Obviously, the incidence angle varies across the swath. An
image is constructed from sequential scanned lines. The
scanner can record the radiation while moving in one
direction only (sidewise) or in both directions. In the sec-
ond case the successive scan lines make a zigzag. This
Passive sensor
Active sensor
Figure 7.4
Illumination source for active and passive sensors.
