1. Electromagnetic
energy is a major component of most remote sensing systems for environmental,
namely as a medium for the transmission of information from the target to the
sensor. Electromagnetic energy travels in waves with some characters that can
be measured, namely: wavelength / wavelength, frequency, amplitude / amplitude,
speed. Wave amplitude is high, while the wavelength is the distance between the
two peaks.
The electromagnetic (EM) spectrum is the continuous range of
electromagnetic radiation, extending from gamma rays (highest frequency &
shortest wavelength) to radio waves (lowest frequency & longest wavelength)
and including visible light. The EM spectrum can be divided into seven
different regions - gamma rays, X-rays,
ultraviolet, visible light, infrared, microwaves and radio waves (Robert Sanderson)
Figure 1. electromagnetic (EM) energy
Frequency is the number of waves that pass a point in one unit of
time. The frequency depends on the speed of the wave climbed. Because of the
speed of electromagnetic energy is constant (speed of light), wavelength and
frequency is inversely proportional. The longer a wave, the lower the
frequency, the shorter the wave and the higher the frequency. Wavelength of
electromagnetic energy figure 2:
Figure 2 :
Elektromagnetic energy
Electromagnetic
energy radiates in accordance with the basic wave theory. This theory describes
the EM energy as travelling in a harmonic sinusoidal fashion at the velocity of
light. Although many characteristics of EM energy are easily described by wave
theory, another theory known as particle theory offers insight into how electromagnetic
energy interacts with matter. It suggests that EMR is composed of many discrete
units called photons/quanta (Shefali
Aggarwal).
Electromagnetic
energy emitted, or released, by all the time in the universe in which different
levels. Higher levels of energy in an energy source, the lower the wavelength
of the energy produced, and the higher the frequency. Differences in the
characteristics of the wave energy is used to classify electromagnetic energy.
a. Temperature
: the origin of all energy (electromagnetic energy or radiant energy) begins
with the vibration of subatomic particles called photos (figure 3). All objects
at a temperature above absolute zero vibrate and therefore emit some form of
electromagnetic energy. Temperature is a measurement of this vibrational energy
emmited from an object. Humans are sensitive to sensation of heat. A “hot”
object emits relatively large amounts of energy. Conversely: “cold” object
emits relatively little energy.
Figure 3. As an electron jumps from a
higher to lower energy level, shown in top figure, a photon of energy is
released. The absoption of photon energy by an atom allows electrons to jump
from a lower to a higher energy state (Departement of the Army).
b. Scattering
:
Scattering
is the redirection of EMR by particles suspended in the atmosphere or by large
molecules of atmospheric gases. Scattering not only reduces the image contrast
but also changes the spectral signature of ground objects as seen by the
sensor. The amount of scattering depends upon the size of the particles, their
abundance, the wavelength of radiation, depth of the atmosphere through which
the energy is traveling and the concentration of the particles. The
concentration of particulate matter varies both in time and over season. Thus
the effects of scattering will be uneven spatially and will vary from time to
time or Particles or gas molecules in the atmosphere that interact with light
large size and cause changes in the direction of the light.
Theoretically
scattering can be divided into three categories depending upon the wavelength
of radiation being scattered and the size of the particles causing the
scattering. The three different types of scattering from particles of different
sizes are summarized below:
Tabel 1. Type scattering (Shefali
Aggarwal)
c. Atmospheric
Windows :
Atmospheric Windows
an areas that are not affected by the absorption of atmosphere is up, so
it's handy for Remote Sensing. The general atmospheric transmittance across the
whole spectrum of wavelengths is shown in Figure 6. The
atmosphere electively transmits energy of certain wavelengths. The spectral
bands for which the atmosphere is relatively transparent are known as atmospheric
windows. Atmospheric windows are present in the visible part (.4 μm - .76 μm)
and the infrared regions of the EM
spectrum. In the visible part transmission is mainly effected by ozone
absorption and by molecular scattering. The atmosphere is transparent again
beyond about λ= 1mm, the region used for microwave remote sensing.
2. Characteristics
Visible, Infrared, Microwave against The Earth Objects in the Wavelength :
a. Visible
The
position of the electromagnetic spectrum visible light is in the middle. This
type of energy can be detected by the human eye, film and electronic detectors.
Wavelengths ranging from 0.4 to 0.7 lm. Different wavelengths in the range
detected by the human eye and brain translate into color. Below is an example
of a composite of Landsat 7. Examples can be seen in figure 4:
Figure 4. Citra
landsat komposit
Landsat provides a relatively economical
way of obtaining sequential data at a scale and in a format that is appropriate
for monitoring global vegetation, using computer-aided analysis (R. M. Hoffer).
However, scanner systems operating in the optical wavelengths cannot obtain
data in areaswhere there is persistent heavy cloud cover, whereas radar can.
The capability of radar to provide data related to plant physiognomy otTers a potential
for differentiating among vegetative cover types and sizesthat cannotbe
distinguished through the use of spectral data alone. The advantages of photographic
data are that they provide a degree of detail that cannot be obtained by the
other types of sensors. Thus, each type of sensor provides the capability of
obtaining data that cannot be obtained in any other way.
The type and degree of detail of the
information needed must be carefully defined, after which the various sensor
systems can be matched to the information required. Different analysis
techniques must be utilized, depending on the sensor system involved, the scale
of imagery obtained, and the degree of detail required. Both manual and
computer-aided analysis techniques have distinct advantages and limitations
that must be recognized in order to achieve
maximum
efficiency.
When one considers the various types of
sensor systems and analysis techniques available, it is apparent that remote
sensing technology offers a powerful and relatively economical tool for
assessing the extent, characteristics and condition of the vegetation resources
of the world.
b. Infrared
Infrared is
electromagnetic radiation of a wavelength longer than visible light, but
shorter than microwave radiation. Infrared radiation (IR) can be emitted from
an object or reflected from a surface. Infrared emission detected as heat
energy and is called thermal infrared. The reflected energy is almost the same
as the beam energy is called the reflected visible and near IR or IR because of
its position on the electromagnetic spectrum near visible light.
Which
covers the wavelength range from approximately 0.7 µm to 100 µm - more than 100
times as wide as the visible portion. The infrared region can be divided into
two categories based on their radiation properties - the reflected IR, and the
emitted or thermal IR. Radiation in the reflected IR region is used for remote
sensing purposes in ways very similar to radiation in the visible portion. The
reflected IR covers wavelengths from approximately 0.7 µm to 3.0 µm. The
thermal IR region is quite different than the visible and reflected IR
portions, as this energy is essentially the radiation that is emitted from the
Earth's surface in the form of heat. The thermal IR covers wavelengths from
approximately 3.0 µm to 100 µm.
figure 5. infrared radiation
For remote sensing applications for
environmental use Landsat imagery, IR Reflected in band 4 (near IR), band 5.7
(Mid IR) and thermal IR band 6, a key characteristic for image interpretation.
For example, the following figure shows the global sea surface temperature
(with thermal IR) and the distribution of vegetation (with near IR). Infrared
figure 6:
Figure
6. Infrared (IR)
c. Microwave
Microwave remote sensing, using microwave radiation
using wavelengths from about one centimeter to a few tens on centimeters
enables observation in all weather conditions without any restriction by cloud
or rain. This is an advantage that is not possible with the visible and/or
infrares remote sensing. In adition, microwave remote sensing provides uniqe
information on for example, sea wind and wave direction, which are derived from
frequency characteristics, Dopler effect, polarization, back scattering etc.
that cannot be observed by visible and infrared sensors. Howeve, the need for
sophisticated data analysis in the disadvantage in using microwave remote
sensing.
Wavelength
of microwave radiation ranges from 0.3-300 cm. Its use is mainly in the field
of communication and information delivery through open spaces, cooking, and
remote sensing systems active. In the remote sensing system is active, the
microwave pulse is fired through a targeted and measured to study the
reflection characteristics of the target. As an application example is the
Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI), which
measures the microwave radiation emitted from the Earth's atmosphere to measure
evaporation, water content in the cloud and rain intensity.
References :
Departement
of the Army. 2003. US Army Corps of Enginers Washington, DC 20314-1000. EM
1110-2907
Journal
Fundamentals of Remote Sensing. A Canada Centre for Remote Sensing Remote sensing
Tutorial. Natural Resources Canada
Robert Sanderson. Introduction
to Remote Sensing. New Mexico State University
R. M. Hoffer. 1984.
Chapter 5Remote Sensing to Measure the
Distribution and Structure of vegetation. Department of Forestry and Natural Resources,
Purdue University, West Lafayette, Indiana, USA
Shefali Aggarwal. Principles of
Remote Sensing. Photogrammetry
and Remote Sensing Division Indian
Institute of Remote Sensing, Dehra Dun. Journal.
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