Principles of Remote Sensing

PRINCIPLES OF REMOTE SENSING
Sunil Kumar,
Director, NWA, Pune
What is Remote Sensing?

"Remote sensing is the science (and to some extent, art) of acquiring information about the
Earth's surface without actually being in contact with it. This is done by sensing and recording
reflected or emitted energy and processing, analyzing, and applying that information."
PRINCIPLES OF REMOTE SENSING

Detection and discrimination of objects or surface features means detecting and recording of
radiant energy reflected or emitted by objects or surface material (Fig. 1). Different objects
return different amount of energy in different bands of the electromagnetic spectrum, incident
upon it. This depends on the property of material (structural, chemical, and physical), surface
roughness, angle of incidence, intensity, and wavelength of radiant energy. The Remote
Sensing is basically a multi-disciplinary science which includes a combination of various
disciplines such as optics, spectroscopy, photography, computer, electronics and
telecommunication, satellite launching etc. All these technologies are integrated to act as one
complete system in itself, known as Remote Sensing System. There are a number of stages in
a Remote Sensing process, and each of them is important for successful operation. The
process involves an interaction between incident radiation and the targets of interest. This is
exemplified by the use of imaging systems where the following seven elements are involved.
Note, however that remote sensing also involves the sensing of emitted energy and the use of
non-imaging sensors.
1. Energy Source or Illumination (A) - the first requirement for remote sensing is to have
an energy source which illuminates or provides electromagnetic energy to the target of interest.
2. Radiation and the Atmosphere (B) - as the energy travels from its source to the target,
it will come in contact with and interact with the atmosphere it passes through. This interaction
may take place a second time as the energy travels from the target to the sensor.
3. Interaction with the Target (C) - once the energy makes its way to the target through
the atmosphere, it interacts with the target depending on the properties of both the target and
the radiation.
104 TRAINING FOR OFFICIALS OF “FLOOD WARNING SECTION” ROYAL GOVT. OF BHUTAN (20-24 April 2015)
4. Recording of Energy by the Sensor (D) - after the energy has been scattered by, or
emitted from the target, we require a sensor (remote - not in contact with the target) to
collect and record the electromagnetic radiation.
5. Transmission, Reception, and Processing (E) - the energy recorded by the sensor has
to be transmitted, often in electronic form, to a receiving and processing station where the
data are processed into an image (hardcopy and/or digital).
6. Interpretation and Analysis (F) - the processed image is interpreted, visually and/or
digitally or electronically, to extract information about the target which was illuminated.
7. Application (G) - the final element of the remote sensing process is achieved when we
apply the information we have been able to extract from the imagery about the target in order
to better understand it, reveal some new information, or assist in solving a particular problem.
These seven elements comprise the remote sensing process from beginning to end. We will be
covering all of these in sequential order throughout the five chapters of this tutorial, building
upon the information learned as we go.
Electromagnetic Radiation
Experiments with electricity and magnetism in the 1800's developed a body of knowledge
which led James Clerk Maxwell to predict in 1886 on purely theoretical grounds that it might
be possible for electric and magnetic fields to combine, forming self-sustaining waves which
could travel great distances. These waves would have many of the behavior characteristics of
waves on water (reflection, refraction, defraction, etc.) and would travel at the speed of light.
These properties gave rise to the possibility that light was an electromagnetic wave, but at
that time, there was no proof that electromagnetic waves really existed. In 1888, Heinrich
Hertz built an apparatus to send and receive Maxwell's waves. In this case the waves were
around 5 meters long. The apparatus worked and, in addition, proved that the waves could be
polarized which turns out to be an important property from a remote sensing point of view.
After this, it was learned that light, x-rays, infrared, ultraviolet, radio, microwaves, and
gamma rays were all electromagnetic waves. The only property dividing them was their
wavelength ranges. The names for these divisions arise from the interaction properties each
wavelength range exhibits. (For instance, we see light, radio waves are useful for
communication, x-rays pass through objects, etc.)
EMR is a dynamic form of energy that propagates as wave motion at a velocity of c =

3 x 108 m/sec. The parameters that characterize a wave motion are wavelength (λ),
frequency (f) and velocity (c).
As was noted in the previous section, 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.
All electromagnetic radiation has fundamental properties and behaves in predictable ways
according to the basics of wave theory. Electromagnetic radiation consists of an electrical
field(E) which varies in magnitude in a direction perpendicular to the direction in which the
radiation is traveling, and a magnetic field (M) oriented at right angles to the electrical field.
Both these fields travel at the speed of light (c).
Two characteristics of electromagnetic radiation are particularly important for understanding

remote sensing. These are the wavelength and frequency.
The wavelength is the length of one wave cycle, which can be measured as the distance
between successive wave crests. Wavelength is usually represented by the Greek letter
lambda (&lambda). Wavelength is measured in metres (m) or some factor of metres such as
nanometres (nm, 10-9 metres), micrometres (µm, 10-6 metres) (µm, 10-6 metres) or
centimetres (cm, 10-2 metres). Frequency refers to the number of cycles of a wave passing a
fixed point per unit of time. Frequency is normally measured in hertz (Hz), equivalent to one
cycle per second, and various multiples of hertz.
Wavelength and frequency are related by the following formula:
Therefore, the two are inversely related to each other. The shorter the wavelength, the higher
the frequency. The longer the wavelength, the lower the frequency. Understanding the
characteristics of electromagnetic radiation in terms of their wavelength and frequency is
crucial to understanding the information to be extracted from remote sensing data. Next we
will be examining the way in which we categorize electromagnetic radiation for just that
purpose.
Characteristics Of The Electromagnetic Spectrum

Photon Description: It is useful to think of radiation in terms of photons when considering
concepts like detector efficiency, the number of photons required to produce a recognizable
signal. Many modern radiation detectors actually count (at ultra high speed) photons as they
arrive and send these counts back to earth in digital form. These counts are useful when
determining quantities such as signal-to-noise ratios. They are used to answer the question "Is
a useful signal even theoretically possible from that object using this system under these
circumstances?"
The Electromagnetic Spectrum
The electromagnetic spectrum ranges from the shorter wavelengths (including gamma and
x-rays) to the longer wavelengths (including microwaves and broadcast radio waves). There
are several regions of the electromagnetic spectrum which are useful for remote sensing.
For most purposes, the ultraviolet or UV portion of the spectrum has the shortest
wavelengths which are practical for remote sensing. This radiation is just beyond the violet
portion of the visible wavelengths, hence its name. Some Earth surface materials, primarily
rocks and minerals, fluoresce or emit visible light when illuminated by UV radiation.
The light which our eyes - our "remote sensors" - can detect is part of the visible spectrum.
It is important to recognize how small the visible portion is relative to the rest of the spectrum.
There is a lot of radiation around us which is "invisible" to our eyes, but can be detected by
other remote sensing instruments and used to our advantage. The visible wavelengths cover a
range from approximately 0.4 to 0.7 µm. The longest visible wavelength is red and the
shortest is violet. Common wavelengths of what we perceive as particular colours from the
visible portion of the spectrum are listed below. It is important to note that this is the only
portion of the spectrum we can associate with the concept of colours.
Violet: 0.4 - 0.446 µm
Blue: 0.446 - 0.500 µm
Green: 0.500 - 0.578 µm
Yellow: 0.578 - 0.592 µm
Orange: 0.592 - 0.620 µm
Red: 0.620 - 0.7 µm
Blue, green, and red are the primary colours

or wavelengths of the visible spectrum. They are defined as such because no single primary
colour can be created from the other two, but all other colours can be formed by combining
blue, green, and red in various proportions. Although we see sunlight as a uniform or
homogeneous colour, 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 component colours when sunlight is passed through a prism,
which bends the light in differing amounts according to wavelength.
The next portion of the spectrum of interest is the infrared (IR) region 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.
The portion of the spectrum of more recent interest to remote sensing is the microwave
region from about 1 mm to 1 m. This covers the longest wavelengths used for remote sensing.
The shorter wavelengths have properties similar to the thermal infrared region while the longer
wavelengths approach the wavelengths used for radio broadcasts. Because of the special
nature of this region and its importance to remote sensing in Canada, an entire chapter
(Chapter 3) of the tutorial is dedicated to microwave sensing.
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 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. There are three (3) types of scattering which take place.
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. Rayleigh
scattering is the dominant scattering mechanism in the upper atmosphere. The fact that the
sky appears "blue" during the day is because of this phenomenon. As sunlight passes through
the atmosphere, the shorter wavelengths (i.e. blue) of the visible spectrum are scattered more
than the other (longer) visible wavelengths. At sunrise and sunset the light has to travel
farther through the atmosphere than at midday and the scattering of the shorter wavelengths
is more complete; this leaves a greater proportion of the longer wavelengths to penetrate the
atmosphere.
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.
The final scattering mechanism of importance is called

nonselective scattering. This 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.
Nonselective scattering gets its name from the fact that all wavelengths are scattered about
equally. 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).
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.
Ozone serves to absorb the harmful (to most living things) ultraviolet radiation from the sun.
Without this protective layer in the atmosphere our skin would burn when exposed to sunlight.
You may have heard carbon dioxide referred to as a greenhouse gas. This is because it tends
to absorb radiation strongly in the far infrared portion of the spectrum - that area associated
with thermal heating - which serves to trap this heat inside the atmosphere. Water vapour in
the atmosphere absorbs much of the incoming longwave infrared and shortwave microwave
radiation (between 22µm and 1m). The presence of water vapour in the lower atmosphere
varies greatly from location to location and at different times of the year. For example, the air
mass above a desert would have very little water vapour to absorb energy, while the tropics
would have high concentrations of water vapour (i.e. high humidity).
Because these gases absorb electromagnetic energy in very specific regions of the spectrum,
they influence where (in the spectrum) we can "look" for remote sensing purposes. Those
areas of the spectrum which are not severely influenced by atmospheric absorption and thus,
are useful to remote sensors, are called atmospheric windows. By comparing the
characteristics of the two most common energy/radiation sources (the sun and the earth) with
the atmospheric windows available to us, we can define those wavelengths that we can use
most effectively for remote sensing. The visible portion of the spectrum, to which our eyes
are most sensitive, corresponds to both an atmospheric window and the peak energy level of
the sun. Note also that heat energy emitted by the Earth corresponds to a window around 10
µm in the thermal IR portion of the spectrum, while the large window at wavelengths beyond 1
mm is associated with the microwave region.
Now that we understand how electromagnetic energy makes its journey from its source to the
surface (and it is a difficult journey, as you can see) we will next examine what happens to
that radiation when it does arrive at the Earth's surface.
Radiation - Target Interactions
Radiation that is not absorbed or scattered in the atmosphere can reach and interact with the
Earth's surface. There are three (3) forms of interaction that can take place when energy
strikes, or is incident (I) upon the surface. These are: absorption (A); transmission (T);
and reflection (R). The total incident energy will interact with the surface in one or more of
these three ways. The proportions of each will depend on the wavelength of the energy and
the material and condition of the feature.
Absorption (A) occurs when radiation (energy) is absorbed into the target while transmission
(T) occurs when radiation passes through a target. Reflection (R) occurs when radiation
"bounces" off the target and is redirected. In remote sensing, we are most interested in
measuring the radiation reflected from targets. We refer to two types of reflection, which
represent the two extreme ends of the way in which energy is reflected from a target:
specular reflection and diffuse reflection.
When a surface is smooth we get specular or mirror-like reflection where all (or almost all) of
the energy is directed away from the surface in a single direction. Diffuse reflection occurs
when the surface is rough and the energy is reflected almost uniformly in all directions. Most
earth surface features lie somewhere between perfectly specular or perfectly diffuse reflectors.
Whether a particular target reflects specularly or diffusely, or somewhere in between, depends
on the surface roughness of the feature in comparison to the wavelength of the incoming
radiation. If the wavelengths are much smaller than the surface variations or the particle sizes
that make up the surface, diffuse reflection will dominate. For example, fine-grained sand
would appear fairly smooth to long wavelength microwaves but would appear quite rough to
the visible wavelengths.
Let's take a look at a couple of examples of targets at the Earth's surface and how energy at
the visible and infrared wavelengths interacts with them.
Leaves: A chemical compound in leaves called chlorophyll strongly absorbs radiation in the
red and blue wavelengths but reflects green wavelengths. Leaves appear "greenest" to us in
the summer, when chlorophyll content is at its maximum. In autumn, there is less chlorophyll
in the leaves, so there is less absorption and proportionately more reflection of the red
wavelengths, making the leaves appear red or yellow (yellow is a combination of red and
green wavelengths). The internal structure of healthy leaves act as excellent diffuse reflectors
of near-infrared wavelengths. If our eyes were sensitive to near-infrared, trees would appear
extremely bright to us at these wavelengths. In fact, measuring and monitoring the near-IR
reflectance is one way that scientists can determine how healthy (or unhealthy) vegetation
may be.
Water: Longer wavelength visible and near infrared radiation is absorbed more by water than
shorter visible wavelengths. Thus water typically looks blue or blue-green due to stronger
reflectance at these shorter wavelengths, and darker if viewed at red or near infrared
wavelengths. If there is suspended sediment present in the upper layers of the water body,
then this will allow better reflectivity and a brighter appearance of the water. The apparent
colour of the water will show a slight shift to longer wavelengths. Suspended sediment (S) can
be easily confused with shallow (but clear) water, since these two phenomena appear very
similar. Chlorophyll in algae absorbs more of the blue wavelengths and reflects the green,
making the water appear more green in colour when algae is present. The topography of the
water surface (rough, smooth, floating materials, etc.) can also lead to complications for
water-related interpretation due to potential problems of specular reflection and other
influences on colour and brightness.
We can see from these examples that, depending on the complex make-up of the target that is
being looked at, and the wavelengths of radiation involved, we can observe very different
responses to the mechanisms of absorption, transmission, and reflection. By measuring the
energy that is reflected (or emitted) by targets on the Earth's surface over a variety of
different wavelengths, we can build up a spectral response for that object. By comparing the
response patterns of different features we may be able to distinguish between them, where we
might not be able to, if we only compared them at one wavelength. For example, water and
vegetation may reflect somewhat similarly in the visible wavelengths but are almost always
separable in the infrared. Spectral response can be quite variable, even for the same target
type, and can also vary with time (e.g. "green-ness" of leaves) and location. Knowing where to
"look" spectrally and understanding the factors which influence the spectral response of the
features of interest are critical to correctly interpreting the interaction of electromagnetic
radiation with the surface.
Passive vs. Active Sensing
So far, throughout this chapter, we have made various references to the sun as a source of
energy or radiation. The sun provides a very convenient source of energy for remote sensing.
The sun's energy is either reflected, as it is for visible wavelengths, or absorbed and then re-
emitted, as it is for thermal infrared wavelengths. 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. There is no
reflected energy available from the sun at night. Energy that is naturally emitted (such as
thermal infrared) can be detected day or night, as long as the amount of energy is large
enough to be recorded.
Active sensors, on the other hand, 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. Active sensors can be used for examining wavelengths that are not sufficiently
provided by the sun, such as microwaves, or to better control the way a target is illuminated.
However, active systems require the generation of a fairly large amount of energy to
adequately illuminate targets. Some examples of active sensors are a laser fluorosensor and a
synthetic aperture radar (SAR).
Characteristics of Images
We need to define and understand a few fundamental terms and concepts associated with
remote sensing images.
Electromagnetic energy may be detected either

photographically or electronically. The photographic process uses chemical reactions on the
surface of light-sensitive film to detect and record energy variations. It is important to
distinguish between the terms images and photographs in remote sensing. An image refers
to any pictorial representation, regardless of what wavelengths or remote sensing device has
been used to detect and record the electromagnetic energy. A photograph refers specifically
to images that have been detected as well as recorded on photographic film. The black and
white photo to the left, of part of the city of Ottawa, Canada was taken in the visible part of
the spectrum. Photos are normally recorded over the wavelength range from 0.3 µm to 0.9 µm
- the visible and reflected infrared. Based on these definitions, we can say that all photographs
are images, but not all images are photographs. Therefore, unless we are talking specifically
about an image recorded photographically, we use the term image.
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. Indeed,
that is exactly what has been done to the photo to the left. In fact, using the definitions we
have just discussed, this is actually a digital image of the original photograph! The
photograph was scanned and subdivided into pixels with each pixel assigned a digital number
representing its relative brightness. The computer displays each digital value as different
brightness levels. Sensors that record electromagnetic energy, electronically record the energy
as an array of numbers in digital format right from the start. These two different ways of
representing and displaying remote sensing data, either pictorially or digitally, are
interchangeable as they convey the same information (although some detail may be lost when
converting back and forth).
In previous sections we described the visible portion of the spectrum and the concept of
colours. We see colour because our eyes detect the entire visible range of wavelengths and our
brains process the information into separate colours. Can you imagine what the world would
look like if we could only see very narrow ranges of wavelengths or colours? That is how many
sensors work. The information from a narrow wavelength range is gathered and stored in a
channel, also sometimes referred to as a band. We can combine and display channels of
information digitally using the three primary colours (blue, green, and red). The data from
each channel is represented as one of the primary colours and, depending on the relative
brightness (i.e. the digital value) of each pixel in each channel, the primary colours combine in
different proportions to represent different colours.
When we use this method to display a single channel or range of wavelengths, we are actually
displaying that channel through all three primary colours. Because the brightness level of each
pixel is the same for each primary colour, they combine to form a black and white image,
showing various shades of gray from black to white. When we display more than one channel
each as a different primary colour, then the brightness levels may be different for each
channel/primary colour combination and they will combine to form a colour image.

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PRINCIPLES OF REMOTE SENSING

What is Remote Sensing?

PRINCIPLES OF REMOTE SENSING

EMR is a dynamic form of energy that propagates as wave motion at a velocity of c =

Two characteristics of electromagnetic radiation are particularly important for understanding

Wavelength and frequency are related by the following formula:

Characteristics Of The Electromagnetic Spectrum

Blue, green, and red are the primary colours

Interactions with the Atmosphere

The final scattering mechanism of importance is called

Radiation - Target Interactions

Passive vs. Active Sensing

Electromagnetic energy may be detected either

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