Remote Sensing
An Introduction
Remote Sensing is the science and art of acquiring information (spectral, spatial, temporal) about
material objects, area, or phenomenon, without coming into physical contact with the objects, or
area, or phenomenon under investigation. Without direct contact, some means of transferring
information through space must be utilised. In remote sensing, information transfer is
accomplished by use of electromagnetic radiation (EMR). EMR is a form of energy that reveals its
presence by the observable effects it produces when it strikes the matter. EMR is considered to
span the spectrum of wavelengths from 10-10 mm to cosmic rays up to 1010 mm, the broadcast
wavelengths, which extend from 0.30-15 mm.
Types
1. In respect to the type of Energy Resources:
Passive Remote Sensing: Makes use of sensors that detect the reflected or emitted
electro-magnetic radiation from natural sources.
Active remote Sensing: Makes use of sensors that detect reflected responses from
objects that are irradiated from artificially-generated energy sources, such as radar.
2. In respect to Wavelength Regions:
Remote Sensing is classified into three types in respect to the wavelength regions
o Visible and Reflective Infrared Remote Sensing.
o Thermal Infrared Remote Sensing.
o Microwave Remote Sensing.
Some Interesting Links :
· Remote Sensing
An Overview of Remote Sensing
· Remote Sensing of the Global Environment
An Article by David J. Schneider, Michigan Technological University
· The Concept of Remote Sensing
Historical & Technical Perspectives of Remote Sensing
· RS Introduction and History
An Article from Earth Observatory, NASA
· The philosophical underpinnings of remote sensing
The Philosophy behind Remote Sensing can perhaps....- An Article by GDSPDS
· GOES 3.9um Channel Tutorial
An excellent tutorial on Thermal Remote Sensing
Bands Used in Remote Sensing
Emission of EMR (Electo-Magnetic Radiation) from gases is due to atoms and molecules in the
gas. Atoms consist of a positively charged nucleus surrounded by orbiting electrons, which have
discrete energy states. Transition of electrons from one energy state to the other leads to
emission of radiation at discrete wavelengths. The resulting spectrum is called line spectrum.
Molecules possess rotational and vibrational energy states. Transition between which leads to
emission of radiation in a band spectrum. The wavelengths, which are emitted by
atoms/molecules, are also the ones, which are absorbed by them. Emission from solids and
liquids occurs when they are heated and results in a continuous spectrum. This is called thermal
emission and it is an important source of EMR from the viewpoint of remote sensing.
The Electro-Magnetic Radiation (EMR), which is reflected or emitted from an object, is the
usual source of Remote Sensing data. However, any medium, such as gravity or magnetic fields,
can be used in remote sensing.
Remote Sensing Technology makes use of the wide range Electro-Magnetic Spectrum (EMS)
from a very short wave "Gamma Ray" to a very long 'Radio Wave'.
Wavelength regions of electro-magnetic radiation have different names ranging from Gamma ray,
X-ray, Ultraviolet (UV), Visible light, Infrared (IR) to Radio Wave, in order from the shorter
wavelengths.
The optical wavelength region, an important region for remote sensing applications, is further
subdivided as follows:
Name Wavelength (mm)
Optical wavelength 0.30-15.0
Reflective
1. Portion Visible
2. Near IR
3. Middle IR
0.38-3.00
0.38-0.72
0.72-1.30
1.30-3.00
Far IR (Thermal, Emissive) 7.00-15.0
Microwave region (1mm to 1m) is another portion of EM spectrum that is frequently used to
gather valuable remote sensing information.
Spectral Characteristics vis-à-vis different systems.
The sunlight transmission through the atmosphere is effected by absorption and scattering of
atmospheric molecules and aerosols. This reduction of the sunlight's intensity s called extinction.
The interrelationship between energy sources and atmospheric absorption characteristics is
shown in Figure 3
· Figure 3(a) shows the spectral distribution of the energy emitted by the sun (black body
at 58000 K and by earth features black body at 3000 K). These two curve represent the
most common sources of energy used in remote sensing.
· Figure 3(b) shows the spectral regions in which the atmosphere blocks the energy are
shaded. Remote-sensing data acquisition is limited to the unblocked spectral regions
called atmospheric windows.
· Figure 3(c) shows that the spectral sensitivity range of the eye (the 'visible' range)
coincides with an 'atmospheric window' and the peak level of energy from the sun.
· Figure3 (d) shows the example of atmospheric transmission characteristics and notes
some of the important 'atmospheric windows'. An 'atmospheric window' is a portion of
Electro-magnetic spectrum in which the radiation passing through the atmosphere is not
significantly altered by reflection, or absorption, or scattered by atmospheric constituents.
Some useful atmospheric windows are given in the table.
The important point to note from the figures is the interaction and the interdependence between
the primary sources of Electro-magnetic energy, the atmospheric windows through which source
energy may be transmitted to and from the earth's surface features, and the spectral sensitivity of
the sensors available to detect and record the energy. One cannot select the sensors to be used
in any given remote-sensing task arbitrarily; one must instead consider
1. the available spectral sensitivity of the sensors,
2. the presence or absence of atmospheric windows in the spectral range(s) in which one
wishes to sense, and
3. the source, magnitude, and spectral composition of the energy availabe in these ranges.
Ultimately, however, the choice of spectral range of the sensor must be based on the manner in
which the energy interacts with the features under investigation.
Energy Interactions, Spectral Reflectance and Colour Readability in Satellite Imagery
All matter is composed of atoms and molecules with particular compositions. Therefore, matter
will emit or absorb electro-magnetic radiation on a particular wavelength with respect to the inner
state. All matter reflects, absorbs, penetrates and emits Electro-magnetic radiation in a unique
way. Electro-magnetic radiation through the atmosphere to and from matters on the earth's
surface are reflected, scattered, diffracted, refracted, absorbed, transmitted and dispersed. For
example, the reason why a leaf looks green is that the chlorophyll absorbs blue and red spectra
and reflects the green. The unique characteristics of matter are called spectral characteristics.
Energy Interactions
When electro-magnetic energy is incident on any given earth surface feature, three fundamental
energy interactions with the feature are possible. See Figure 4
Spectral Reflectance & Colour Readability
Two points about the above given relationship (expressed in the form of equation) should be
noted.
1. The proportions of energy reflected, absorbed, and transmitted will vary for different earth
features, depending upon their material type and conditions. These differences permit us
to distinguish different features on an image.
2. The wavelength dependency means that, even within a given feature type, the proportion
of reflected, absorbed, and transmitted energy will vary at different wavelengths.
Thus, two features may be distinguishable in one spectral range and be very different on another
wavelength brand. Within the visible portion of the spectrum, these spectral variations result in
the visual effect called COLOUR. For example we call blue objects 'blue' when they reflect highly
in the 'green' spectral region, and so on. Thus the eye uses spectral variations in the magnitude
of reflected energy to discriminate between various objects.
A graph of the spectral reflectance of an object as a function of wavelength is called a spectral
reflectance curve. The configuration of spectral reflectance curves provides insight characteristics
of an object and has a strong influence on the choice of wavelength region(s) in which remote
sensing data are acquired for a particular application. This is illustrated in figure 5, which shows
highly generalized spectral reflectance curves of deciduous and coniferous trees. (In the
discussion, we use the terms deciduous and coniferous somewhat loosely, referring to broadleaved
trees, such as Oak and Maple, as deciduous and to needle-bearing trees, such as pine
and spruce, as coniferous.). It should be noted that the curve for each of these object types is
plotted as a 'ribbon' (or 'envelope') of values, not as a single line. This is because spectral
reflectances vary somewhat within a given material class. That is, the spectral reflectance of one
deciduous tree species and another will never be identical. Nor will the spectral reflectance of
trees of the same species ever be exactly equal.
Figure 6 shows the typical spectral reflectance curves for three basic types of earth feature:
· Green vegetation
· Soil
· Water.
The lines in this figure represent average reflectance curves compiled by measuring large sample
features. It should be noted how distinctive the curves are for each feature. In general, the
configuration of these curves is an indicator of the type and condition of the features to which they
apply. Although the reflectance of individual features will vary considerably above and below the
average, these curves demonstrate some fundamental points concerning spectral reflectance.
Colour Discrimination based on Wavelengths of Spectral Reflectances.
(IRS-IA/IB LISS I and LISSII*)
Band wavelength
(μm)
Principal
1 0.45-0.52 Sensitive to sedimentation, deciduous/coniferous forest cover
discrimination, soil vegetation differentiation
2 0.52-0.59 Green reflectance by healthy vegetation, vegetation vigour, rocksoil
discrimination, turbidity and bathymetry in shallow waters
3 0.62-0.68 Sensitive to chlorophyll absorption: plant species discrimination,
differentiation of soil and geological boundary
4 0.77-0.86 Sensitive to green biomass and moisture in vegetation, land and
water contrast, landform/geomorphic studies.
*Spatial Resolution of Linear imaging self scanning (LISS): LISS-I (72.5 m) and LISS-II (36.25m)
Electro-Magnetic Remote Sensing of Earth's Resources -- Process & Elements
Major Components of Remote Sensing Technology:
The following are major components of Remote sensing System:
1. Energy Source
2. Passive System: sun, irradiance from earth's materials;
3. Active System: irradiance from artificially generated energy sources such as
radar.
4. Platforms:(Vehicle to carry the sensor) (truck, aircraft, space shuttle, satellite, etc.)
5. Sensors:(Device to detect electro-magnetic radiation) (camera, scanner, etc.)
6. Detectors: (Handling signal data) (photographic, digital, etc.)
7. Processing:(Handling Signal data) (photographic, digital etc.)
8. Institutionalisation: (Organisation for execution at all stages of remote-sensing
technology: international and national orrganisations, centres, universities, etc.).
Platforms
The vehicles or carriers for remote sensors are called the platforms. Typical platforms are
satellites and aircraft, but they can also include radio-controlled aeroplanes, balloons kits for low
altitude remote sensing, as well as ladder trucks or 'cherry pickers' for ground investigations. The
key factor for the selection of a platform is the altitude that determines the ground resolution and
which is also dependent on the instantaneous field of view (IFOV) of the sensor on board the
platform.
Salient feature of some important satellite platforms.
Features Landsat1,2,3 Landsat 4,5 SPOT IRS-IA IRS-IC
Natre Sun Sys Sun Sys Sun Sys Sun Sys Sun Sys
Altitude
(km)
919 705 832 904 817
Orbital
period
(minutes)
103.3 99 101 103.2 101.35
inclination
(degrees
99 98.2 98.7 99 98.69
Temporal
resolution
(days)
18 16 26 22 24
Revolutions 251 233 369 307 341
Equatorial
crossing
(AM)
09.30 09.30 10.30 10.00 10.30
Sensors RBV,MSS MSS,TM HRV LISS-I,LISSII
LISSIII,
PAN,WIFS
SENSORS
ACTIVE SENSORS
(Detect the reflected or emitted electromagnetic
radiation from natural sources.)
PASSIVE SENSORS
(Detect reflected responses from objects that
are irradiated from artificially-generated energy
sources such as radar.)
Passive
Non-Scanning
o Non-Imaging. (They are a type
of profile recorder, ex.
Microwave Radiometer.
Magnetic
sensor.Gravimeter.Fourier
Spectrometer.
o Imaging. (Example of this are
the cameras which can be:
Monochrome, Natural Colour,
Infrared etc.)
Scanning
o Imaging. Image Plane
scanning.Ex. TV CameraSolid
scanner.
Object Plane scanning.Ex.
Optical Mechanical
ScannerMicrowave radiometer.
Active
Non-Scanning
· Non-Imaging. (They are a type
of profile recorder, ex.
Microwave
Radiometer.Microwave
Altimeter.Laser Water Depth
Meter.Laser Distance Meter.
Scanning
· Imaging. (It is a radar ex.
Object Plane scanning:
1. Real Aperture Radar.
2. Synthetic Aperture
Radar.
Image Plane Scanning:
3. Passive Phased Array
Radar.
Resolution
In general resolution is defined as the ability of an entire remote-sensing system, including lens
antennae, display, exposure, processing, and other factors, to render a sharply defined image.
Resolution of a remote-sensing is of different types.
1. Spectral Resolution: of a remote sensing instrument (sensor) is determined by the bandwidths
of the Electro-magnetic radiation of the channels used. High spectral resolution,
thus, is achieved by narrow bandwidths width, collectively, are likely to provide a more
accurate spectral signature for discrete objects than broad bandwidth.
2. Radiometric Resolution: is determined by the number of discrete levels into which signals
may be divided.
3. Spatial Resolution: in terms of the geometric properties of the imaging system, is usually
described as the instantaneous field of view (IFOV). The IFOV is defined as the
maximum angle of view in which a sensor can effectively detect electro-magnetic energy.
4. Temporal Resolution: is related ot the repetitive coverage of the ground by the remotesensing
system. The temporal resolution of Landsat 4/5 is sixteen days.
An Ideal Remote Sensing System
Having introduced some basic concepts, we now have the necessary elements to conceptualize
an ideal remote sensing system. In doing so, we can then appreciate some of the problems
encountered in the design and application of the various real remote-sensing systems examined
in subsequent chapters.
The basic components of an ideal remote-sensing system are shown in figure 8. These include
the following components.
· A uniform energy source. This source will provide energy over all wavelengths, at a
constant, known, high level of output, irrespective of time and place.
· A non-interfering atmosphere. This will be an atmosphere that will not modify the energy
from the source in any manner, whether that energy is on its way to earth's surface or
coming from it. Again, ideally this will hold irrespective of wavelength, time, place, and
sensing altitude involved.
· A series of unique energy/matter interaction at the earth's surface. These interactions will
generate reflected and/or emitted signals that are not only selective in respect to
wavelengths, but also are known, invariant, and unique to each and every earth surface
feature type and subtype of interest.
· A super sensor. This will be a sensor, highly sensitive to all wavelengths, yielding
spatially detailed data on the absolute brightness (or radiance) from a scene (a function
of wavelength), throughout the spectrum. This super sensor will be simple and reliable,
require, virtually no power or space, and be accurate and economical to operate.
· A real-time data handling system. In this system, the instant the radiance versus
wavelength response over a terrain element is generated, it will be processed into an
interpretable format and recognized as being unique to the particular terrain element from
which it comes. This processing will be performed nearly instantaneously (real time),
providing timely information. Because of the consistent nature of the energy/matter
interactions, there will be no need for reference data in the analytical procedure. The
derived data will provide insight into the physical-chemical-biological state of each feature
of interest.
· Multiple data users. These people will have comprehensive knowledge of both their
respective disciplines and of remote-sensing data acquisition and analysis techniques.
The same set of data will become various forms of information for different users,
because of their vast knowledge about the particular earth resources being used.
Unfortunately, an ideal remote-sensing system, as described above, does not exist. Real remotesensing
systems fall short of the ideal at virtually every point in the sequence outlined.
Remote Sensing Satellites
A satellite with remote sensors to observe the earth is called a remote-sensing satellite, or earth
observation satellite. Remote-Sensing Satellites are characterised by their altitude, orbit and
sensor.
TRIOS Series (1960-1965)
The Television and Infrared Observation Satelites.
NOAA It is the first generation of National Oceanic and Atmospheric Administration satellites and
was as the first operation operational remote sensing satellite system.
The third generation NOAA satellites are also successfully used for vegetation monitoring, apart
from meteorological monitoring. It is equipped with Advanced Very High Resolution Radiometer
(AVHRR) sensors, and is established at an altitude of 850 km. In polar orbit.
GMS Geo-synchronous meteorological satellite. It is established at an altitude of 36,000 km, and
its main purpose is meteorological observations
Landsat is established at an altitude of 700 Kms is a polar orbit and is used mainly for land area
observation.
Other remote sensing satellite series in operations are: SPOT, MOS, JERS, ESR, RADARSAT,
IRS etc.
Some Interesting Links :
· Indian Remote Sensing Satellites
History of Indian RS Satellites - An Article by Wim Bakker - ITC
· EROS Homepage
A key site for browsing Landsat TM and other satellite data, and browsing and ordering
declassified intelligence satellite images.