Lecture- 03
Fundamentals of Remote Sensing
Electromagnetic Radiation
The first requirement for remote sensing is to have an energy source to
illuminate the target (unless the sensed energy is being emitted by the
target). This energy is in the form of electromagnetic radiation. Two
characteristics of electromagnetic radiation are particularly important
for understanding remote sensing. These are the wavelength and
frequency.
The Electromagnetic Spectrum and Bands
What is the information being collected? It is measurements of
electromagnetic radiation. The electromagnetic spectrum is a continuum
of energy from short wave high frequency cosmic waves to longer
wavelength low frequency radio waves. Our eyes are sensitive to the
visible part of the electromagnetic spectrum. Within the visible spectrum
our eyes can see the different colours which are variations in the
wavelengths.
Blue, green, and red are the primary colors or wavelengths of the visible
spectrum. They are defined as such because no single primary color can be
created from the other two, but all other colors can be formed by combining
blue, green, and red in various proportions. Although we see sunlight as a
uniform or homogeneous color, it is actually composed of various
wavelengths of radiation in primarily the ultraviolet, visible and infrared
portions of the spectrum. The visible portion of this radiation can be shown
in its
Violet:
Blue:
Green:
Yellow:
Orange:
Red:
0.4 - 0.446 μm
0.446 - 0.500 μm
0.500 - 0.578 μm
0.578 - 0.592 μm
0.592 - 0.620 μm
0.620 - 0.7 μm
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Active Sensor
Active sensors, provide their own energy source for illumination.
The sensor emits radiation which is directed toward the target to be
investigated. The radiation reflected from that target is detected and
measured by the sensor. Advantages for active sensors include the
ability to obtain measurements anytime, regardless of the time of
day or season.
Passive Sensor
Remote sensing systems which measure energy that is naturally
available are called passive sensors. Passive sensors can only be used
to detect energy when the naturally occurring energy is available. For
all reflected energy, this can only take place during the time when the
sun is illuminating the Earth.
Example of Active Sensor & Passive sensors
Interactions with the Atmosphere
Before radiation used for remote sensing reaches the Earth's surface it has to
travel through some distance of the Earth's atmosphere. Particles and gases in
the atmosphere can affect the incoming light and radiation. These effects are
caused by the mechanisms of Scattering and Absorption.
Scattering
Scattering occurs when particles or large gas molecules present in the
atmosphere interact with and cause the electromagnetic radiation to be
redirected from its original path. How much scattering takes place depends
on several factors including the wavelength of the radiation, the abundance
of particles or gases, and the distance the radiation travels through the
atmosphere.
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There are three (3) types of scattering which take place.
 Rayleigh Scattering
 Mie Scattering
 Nonselective Scattering
Rayleigh Scattering occurs when particles are very small compared to the
wavelength of the radiation. These could be particles such as small specks of
dust or nitrogen and oxygen molecules. Rayleigh scattering causes shorter
wavelengths of energy to be scattered much more than longer wavelengths.
The fact that the sky appears "blue" during the day is because of this
phenomenon.
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Mie Scattering
Mie scattering occurs when the particles are just about the same size as the
wavelength of the radiation. Dust, pollen, smoke and water vapour are
common causes of Mie scattering which tends to affect longer wavelengths
than those affected by Rayleigh scattering. Mie scattering occurs mostly in
the lower portions of the atmosphere where larger particles are more
abundant, and dominates when cloud conditions are overcast.
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Nonselective Scattering
Nonselective Scattering occurs when the particles are much larger than the
wavelength of the radiation. Water droplets and large dust particles can
cause this type of scattering. This type of scattering causes fog and clouds
to appear white to our eyes because blue, green, and red light are all
scattered in approximately equal quantities (blue+green+red light = white
light).
Nonselective Scattering by a cloud
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Absorption
Absorption
is
the
other
main
mechanism
at
work
when
electromagnetic radiation interacts with the atmosphere. In contrast to
scattering, this phenomenon causes molecules in the atmosphere to
absorb energy at various wavelengths. Ozone, carbon dioxide, and
water vapour are the three main atmospheric constituents which
absorb radiation.
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Example
Spectral Signature
Every natural and artificial object reflects and emits EMR over a range
of wavelengths in its own chemical composition and physical state. The
distinctive reflectance and emission properties of objects are called
spectral signature.
Within some limited wavelength region, a particular object will usually
exhibit a diagnostic spectral response patterns that differs from other
objects.
Spectral Signature
It is hoped that each material on the earth would have a distinctive
spectral response patterns that would allow it to be reliably detected
by visual and digital means. Finding distinctive spectral response
patterns is the key to most procedures for remote sensing image
interpretation.
Pixel and DN values
A photograph could also be represented and displayed in a digital
format by subdividing the image into small equal-sized and shaped
areas, called picture elements or pixels, and representing the
brightness of each area with a numeric value or digital number.
Resolutions
There are four types of resolutions in Remote Sensing
 Spatial resolution – ‘Area’ aspects
 Spectral resolution -‘ Band’ aspect
 Radiometric resolution- ‘Radiance’ aspect
 Temporal resolution- ‘Frequency’ aspect
Spatial Resolution
The earth surface area covered by a pixel of an image is known as
spatial resolution. Large area covered by a pixel means low spatial
resolution and vice versa.

Spatial resolution



Pixel: smallest unit of an
image
Pixel size
Spatial coverage


Field of view (FOV), or
Area of coverage, such as
MODIS: 2300km
Spatial Resolution
30 meter, Spatial Resolution
1 meter, Spatial Resolution
Spatial
Resolution
Spectral Resolution
Is the ability to resolve spectral features and bands into their separate
components. More number of bands in a specified bandwidth means
higher spectral resolution and vice versa
 Spectral resolution describes the
ability of a sensor to define fine
wavelength intervals
 The finer the spectral resolution,
the narrower the wavelength
range for a particular channel or
band
Radiometric Resolution
The radiometric resolution of an imaging system describes its ability
to discriminate very slight differences in energy The finer the
radiometric resolution of a sensor, the more sensitive it is to detecting
small differences in reflected or emitted energy. The maximum
number of brightness levels available depends on the number of bits
used in representing the energy recorded. Thus, if a sensor used 8 bits
to record the data, there would be 28=256 digital values available,
ranging from 0 to 255.
2-Bit Image
8-Bit Image
Radiometric Resolution
2-bit range
0
4
6-bit range
0
63
8-bit range
0
255
10-bit range
0
1023
Temporal resolution
• Temporal resolution is the revisit period, and is the length of time
for a satellite to complete one entire orbit cycle, i.e. start and back to
the exact same area at the same viewing angle. For example,
 Landsat needs 16 days,
 MODIS needs one day,
 NEXRAD needs 6 minutes for rain mode and 10 minutes for clear
sky mode.
July 2
July 18
August 3
16 days
Time
11 days
July 1
July 12
July 23
August 3
Thanks