ATMOSPHERIC WINDOW:-
In the range of visible light were the sun
emits highest intensities, atmospheric transmittance is the highest (see
figure). In the range of higher wavelengths transmittance is reduced to narrow
bands. This includes the optical windows in the thermal infrared, where the
Earth's surface emits radiation. In the range of microwaves the atmosphere is
nearly transmissive, but the sun and earth's radiation are low; therefore, this
range is used by active radar systems. Wavelengths smaller than the ultraviolet
are nearly totally absorbed by the atmosphere and are therefore less relevant
for remote sensing. Remote sensing concentrates on the transmissive ranges, the
so called atmospheric
windows.
In particular, the
molecules of water, carbon dioxide, oxygen, and ozone in our atmosphere block
solar radiation. The wavelength ranges in which theatmosphere is
transparent are called atmospheric windows. Remote sensing projects
must be conducted in wavelengths that occur withinatmospheric windows.
Source of Energy
As noted earlier in lesson 1, the first requirement for remote
sensing is to have an energy source to illuminate the target. Just as our eyes
need objects to be illuminated by light so that we can see them, sensors also
need a source of energy to illuminate the earth’s surface. The sun is the
natural source of energy. This energy is in the form of electromagnetic
radiation (EMR). The following subsections explain the classification of remote
sensing.
Depending on the predominant source of electromagnetic energy in
the remote sensing system, the remote sensing can be passive or active.
Passive Remote Sensing depends on a natural source to
provide energy. The sun is the most commonly used source of energy for passive
remote sensing. The satellite sensor in this case records primarily the
radiation that is reflected from the target. Remote sensing in the visible part
of the electromagnetic spectrum is an example of passive (reflected) remote
sensing.
A portion of the sun’s radiation that is not reflected back to the
sensor is absorbed by the target, raising the temperature of target material.
The absorbed radiation is later emitted by the material at a different
wavelength. Passive remote sensing can also be carried out in the absence of
the sun. In this latter case, the source of energy is the target material
itself and the sensor records primarily emitted radiation. Remote sensing in
the thermal infrared portion of the electromagnetic spectrum is an example of
passive (emitted) remote sensing.
Active Remote Sensing uses an artificial source for energy. For example the satellite itself
can send a pulse of energy which can interact with the target. In active remote
sensing, humans can control the nature (wavelength, power, duration)
of the source energy. Remote sensing in the microwave region of the
electromagnetic spectrum (radar remote sensing) is an example of active remote
sensing. Active remote sensing can be carried out during day and night and in
all weather conditions.
Remote Sensing Classification
Based on wavelength regions the remote sensing can be classified
as:
1. Visible and
reflective infrared remote sensing.
2. Thermal infrared
or emitted remote sensing.
3. Microwave remote
sensing.
The energy source used in the visible and reflective infrared remote sensing is the sun.
The sun radiates EM energy with a peak wavelength of about 0.5 μm. Remote
sensing data obtained in the visible and reflective infrared regions mainly
depends on the reflectance of objects on the ground surface. Therefore,
information about objects can be obtained from the spectral reflectance.
However laser radar is exceptional because it does not use the solar energy but
the laser energy of the sensor.
The source of the radiant energy in the thermal infrared remote
sensing is the object itself, because any object with a normal temperature of
about 27oC will emit EM radiation with a peak at about 9.7 μ m.
In microwave region, there are two types of microwave remote sensing, passive
microwave remote sensing and active microwave remote sensing. In passive
microwave remote sensing, the microwave radiation emitted from an object is
detected, while the back scattering co-efficient is detected in active
microwave sensing.
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