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Summary REMOTE SENSING APPLICATIONS IN AGROMETEOROLOGY
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- University Of The Freestate (UFS)
introduction to remote sensing, instruments used and how they work
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THEME 8: REMOTE SENSING APPLICATIONS INAGROMETEOROLOGY
8.1 DEFINITION AND PLATFORMS
Remote sensing is the science of obtaining information about an object through
the analysis of data acquired by a device that is not in contact with the object
(Lillesand & Keifer, 1994). Remote sensing provides spatial coverage by
measurement of reflected, emitted and backscattered radiation, across a wide
range of wavebands, from the earth’s surface and surrounding atmosphere.
Remotely sensed data can be obtained from a variety of platforms. These
platforms can be fixed or moving, terrestrial or operating from different altitudes,
and it can be either manned or automatic. In general, the tools used in remote
sensing can be grouped into three main categories, namely satellite, radar and
near-surface instruments. Examples of remote sensing platforms include
satellites, airplanes, remotely piloted vehicles, hand-held radiometers or even
trucks. Considering the operating time, the platform can be classified as
temporary, semi-permanent or virtually permanent. These aspects are important
in order to understand the quality and quantity of the data available to the
agrometeorological community.
In relation to the technology used in remote sensing, the electromagnetic
spectrum (EMS) can be divided into two large wavelength regions known as
‘optical’ and ‘microwave’ (Figure 8.1). Optical remote sensing detects energy
reflected and emitted by the Earth and its atmosphere, typically at wavelengths
between 0.4 and 14 µm. Microwave remote sensing targets much longer
wavelengths, between 1 mm and 1 m.
166
,Figure 8.1 Optical and microwave regions of the electromagnetic spectrum (Turner et
al., 2003).
Various devices can be mounted on remote sensing platforms such as sensors,
film cameras, digital cameras and video recorders. Instruments capable of
measuring electromagnetic radiation are called sensors. Such sensors operate
across a range of wavebands (Figure 8.1), from the ultraviolet (UV) (< 0.4 µm),
visible (VIS) (0.4 to 0.7 µm), near-infrared (NIR) (0.7 to ~1.1 µm), shortwave
infrared (SWIR) (~1.1 to ~3.7 µm), mid-infrared (MIR) (~3.7 to ~7 µm), thermal
infrared (TIR) (~7 to ~14 µm) and microwave (MW) (0.75 to 100 cm) regions of
the electromagnetic spectrum (EMS). These bands are located in the so-called
“atmospheric windows” so that a signal can be detected (otherwise total
absorption (or scattering) of the radiation by the atmosphere will occur). Sensors
can be divided into two main groups:
a) Passive sensors, which do not have their own source of radiation. They
only observe radiation emitted from distant objects. Examples include
satellites, cameras and lightning detection systems.
b) Active sensors, which have a built-in source of radiation. They use their
own radiation source to “illuminate” distant objects, where after the
reflected radiation is observed. Examples include radar, lidar and sodar
systems.
Remote sensing can be analogue (photography) or digital (multispectral
scanning, thermography, radar). The elements of a digital image are called
resolution cells (during the acquisition of data) or pixels (after the creation of the
image). The use of remote sensing data requires some knowledge about the
167
,technical capabilities of the various sensor systems. The technical capabilities of
the sensor systems can be categorised according to three kinds of resolution:
a) Spatial resolution, which concerns the size of the resolution cell on the
ground in the direction of the flight and across. The size of the pixel
determines the smallest detectable terrain feature;
b) Spectral resolution, which concerns the number, location in the
electromagnetic spectrum, and bandwidth of the specific wavelength
bands or spectral bands. This resolution differs among sensors and
largely determines their potential use;
c) Temporal resolution, which concerns the time lapse between two
successive images of the same area. This is determined primarily by the
platform used and secondarily by atmospheric conditions.
The distance between the instrument and the target directly affects the resolution
and the precision of the information. The resolution of observation can vary from
a few square metres (with a sensor mounted on a vehicle) to continental scale
(using a meteorological satellite). Due to the large volumes of data generated, an
increase in temporal resolution is usually at the expense of spatial and spectral
resolution. Earth resource satellites have high spatial resolutions while
meteorological satellites have high temporal resolutions.
There are a number of stages in the remote sensing process (Figure 8.2), and
each of them is important for successful operation:
A. Emission of electromagnetic radiation (sun/self-emission)
B. Transmission of energy from the source to the surface of the earth, as well
as absorption and scattering
C. Interaction of electromagnetic radiation with the earth’s surface: reflection
and emission
D. Transmission of energy from the surface to the remote sensor
E. Sensor data output
F. Data transmission, processing and analysis
168
, Figure 8.2 The remote sensing process (GISCEU, 2013).
8.2 SATELLITE REMOTE SENSING
Satellite data provide better coverage in time and in area extent than any
alternative. Most polar satellite instruments observe the entire planet once or
twice in a 24-hour period. Each geostationary satellite’s instruments cover about
25% of the planet almost continuously and together their combined coverage
amount to about 75% of the planet. Satellites cover the world’s oceans (about
70% of the planet), its deserts, forests, polar regions and other sparsely inhabited
places.
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