GIS AND REMOTE SENSING

UNIT:2
REMOTE SENSING PLATFORMS

Sensors are devices used for making observations. These consist of mechanisms, usually
sophisticated lenses with filter coatings to focus the area observed on a plane in which the detectors
are placed. These detectors are sensitive to a particular region in which the sensor
is designed to
operate and produce outputs which are representative of the observed area. The major
characteristics of an imaging remote sensing instrument operating in the visible and infrared
spectral bands are described in terms of its spatial, spectral and radiometric resolution.
These three
types ofresolutions vary from sensor to sensor. Each se

energy reflected from the earth's surface feafures. M.E.(rr.nsporralon Elpg,), phD
Associate Prolessor

In order for a sensor to collect and record energy

must reside on a stable platform removed from the target or surface being
observed. platforms for
remote sensors may be situated on the ground, on an aircraft or balloon (or
some other platform
within the Earth's atmosphere), or on a spacecraft or satellite outside of the Earth,s
atmosphere.

Ground-based sensors are often used to record detailed information

about the surface which is
compared with information collected from aircraft or satellite sensors.
In some cases, this can be
used to better chatacterize the target which is being imaged by these
other sensors, making it
possible to better understand the information in the imagery.

Figure I Ground-based sensors

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Sensors may be placed on a ladder, scaffolding, tall building, cherry-picker, crane, etc. Aerial
platforms are primarily stable wing aircraft, although helicopters are occasionally used. Aircraft
are often used to collect very detailed images and facilitate the collection of data
over virtually any
portion of the Earth's surface at any time. Dr. MOHD. MTNHAJUDDIN AQUI L
'. M.E.(fransportaflon Eneg,), PhU
Associate Professor

DF g)

Figure 2 Space shuttle

In space, remote sensing is sometimes conducted from the space shuttle or, more commonly, from
satellites. Satellites are objects which revolve around another object - in this case, the Ear1h.
For
example, the moon is a natural satellite, whereas man-made satellites include those platforms
launched for remote sensing, communication, and telemetry (location and navigation)
purposes.
Because of their orbits, satellites permit repetitive coverage of the Earth's surface
on a continuing
basis. Cost is often a significant factor in choosing among the various platform
options.

Figure 3 Coverage of the Earth's surlhce Figure 4 Coverage of the Earth's surface

Prepared by: Dr. Molid. Minhajuddin Aquil

Associate Professor, CED, DCET
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Figure 5 Remote Sensing platforms

The sensors recording the energy that they receive are placed in a near- polar sun synchronous
orbit at an altitude of 700 - 900 km. These satellites are known as remote sensing satellites (e.g.
Indian Remote Sensing Series). As against these satellites, the weather monitoring and
telecommunication satellites are placed in a Geostationary position (the satellite is always
positioned over its orbit that synchronises with the direction of the rotation of the earth) and
revolves around the earth (coinciding with the direction of the movement of the
earth over its axis)
at an altitude of nearly 36,000 km (e.g. INSAT series of satellites).

Table I comparison between Sun-synchronous and Geostationary satellites

Geostationary Satellites

@ 36,000 km
81u N to 8l of the Globe
Orbital period @ 14 orbits per day
Fine (182 metre to I metre) Coarse (1 km x 1 km)
Earth Resources Applications Telecommunication and
Weather monitoring

neputncnr of Civil
Deocan Coll ofEuct
Prepared by: Dr. Mohd. Minhajuddin Aquil
Associate Professor, CED, DCET
 Figure 6 Orbit of Sun Synchronous Figure 7 Geostationary Satellites

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Figure 8
(a) Illustrating the principles ofa Sun-synchronous orbit.

(b) Example of Sun-synchronous orbit.

In the 1960s, a revolution in remote sensing technology began with the deployment
of space
satellites' From their high vantage-point, satellites have a greatly extended
view of the Earth,s
surface' The first meteorological satellite, TIROS-1, was launched by the
United States using an
Atlas rocket on April l, 1960. This early weather satellite used vidicon cameras to scan
wide areas
of the Earth's surface- Early satellite remote sensors did not use conventional
film to produce their
images' Instead, the sensors digitally capture the images using a device
similar to a television
camera. Once captured, this data is then transmitted electronically to receiving
stations found on
the Earth's surface.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Figure TIROS-f satellite (NASA)
9
p.r. MOHD. MINHAJUDDTN AeUIt
M. E.(fransportafl on Engtr. ), pht
GOES
Today, the GOES (Geostationrry"mtm: ) system of satellites
provides most of the remotely +Gftn America. To cover the
complete continent and adjacent oceans two satellites are employed in a geostationary orbit. The

western half of North America and the eastem Pacific Ocean is monitored by GOES-10, which is

directly above the equator and 135. West longitude.

Figure 10 Color image from GOES-8 of hurricanes Madeline and Lcster off the coast of Mexico,
october 17 , 1998- (source: NASA - Looking at Earth From space).

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 The eastern half of North America and the western Atlantic are cover
by GOES-g. The GOES-g
satellite is located overhead of the equator and.75" West longitude. Advanced
sensors aboard the
GOES satellite produce a continuous data stream so images can be viewed at
any instance, The
imaging sensorproduces visible and infrared images of the Earth's terrestrial
surface and oceans.
Infrared images can depict weather conditions even during the night. Another
sensor aboard the
satellite can determine vertical temperature profiles, vertical moisture profiles,
total perceptible
water, and atmospheric stability. Dr. MOHD. MINHAJUDDIN AQUIL
M.E.(Inmporrabn EngE. ), PhD

Dep ering
collegc of Engg' n
A rist of the sensors that have been used in Indian n.-oBffi StHf*
Satellite Microwave Radiometer (SAMIR)
SAMIR was the payload for BHASKAR I and II satellites launched in 1979 and 19g1. They
successfully provided data on the sea surface temperature, ocean winds, moisture
content over the
land and sea. It was a dicke type radiometer with a temperature resolution
better than I degree
kelvin. Two Band T.V. Payload
The Bhaskara satellites I and II had a two band TV payload for land applications. It gave images
of earth from a height of 525 Km. The data were used in meteorology, hydrology, and forestry.
Smart Sensor Rohini Rs-D2, (the successor to the failed Rs-Dl) was launched
on Apr. 19g3. It
carried a Smart sensor, which was a 2-Band, solid-state device. It had
the first CCD camera
developed in house.

LISS-I,II and III

LISS-I (Linear Imaging self Scanner) was a payload for the IRS-1A satellite. This camera operated
in four spectral bands. It operated in
push-broom scanning mode using a CCD array. It was again
a

used in IRS-lB. It used 7-bit quantization, and had a swath of 148

Kms. Images of LISS-I were
extensively used in forestry, crop acreage, yield estimation, drought monitoring,
flood monitoring
etc' LISS-II was similar to LISS-I, but with higher spatial resolution
and smaller swath. it was on
payload in three satellites: IRS-lA, IRS-1B, IRS-P2.
LISS-ru is onboard two satellites IRS-IC and IRS-ID. This is a multi-spectral
camera which
operates in four bands' It provides color images. Its images were used
widely in the area of
agriculture, mapping, crop acreage etc. The Panchromatic Camera This was
carried by IRS-lc and

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 IRS-ID satellites. Pan camera enables the acquisition of images at the resolution
of 5.gm, which
was the highest resolution offered by a civilian satellite until recently,
when American satellite
Ikonos with a resolution of lm surpassed it. The Pan camera uses CCD's to capfure images.
Wide Field Sensor
IRS-lC, IRS-1D, IRS-P3, which are all second-generation Indian remote sensing
satellites, carried
the WIFS sensor' The WIFS camera uses an 8 element refractive optics like
in LISS-III. Two such
cameras are mounted with overlapping pixels of imaging. WIFS data was
used in assessment of
rabi cropped area, crop inventory observation ofcrop phenology etc.
Ocean Color Monitor
IRS-P4, also called Oceansat, carried the ocean color monitor, launched on
board pSLV-C l. This
payload is meant for oceanographic applications. The OCM is a solid-state
camera operating in
the push-broom scanning mode, using linear array CCD'S as detectors for generating
ocean
biological parameters.
Very High-Resolution Radiometer
All the INSAT-l and the INSAT-2, INSAT-3 series communications
satellites carry the VHRR to
provide various remote sensing applications. Since INSAT satellites
are geostationary VHRR
provides round the clock meteorological earth observations,
disaster warning signals.
lln MOHD. MINHAJUDDTN AeUIL
M.E.Cfr.Gpoilatton Engg.), phD

Remote sensing data may be collected using either

In
the passive remote sensing, the reflected or emitted elect i nsors
operating in different selected spectral bands where the original source
of energy is the sun. But in
active remote sensing method the earth surface is illuminated
by an artificial source of energy. The
emitted and reflected energy detected by the sensors onboard are transmitted
to the earth station.
The data then it is processed (after various corrections) and
made ready for the users.
Passive sensors record nafurally occurring electromagnetic radiation
that is reflected or emitted
from the terrain. Remote sensing in the day light under the influence
of solar energy falls under
this category. when man-made electromagnetic energy is used to illuminate
the ground and
backscatters are recorded by the sensor (e.g. in microwave radar), such
sensors are called passive
sensors.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET 32
 Remote sensing images are collected from suitable platforms located at various altitudes aerial
(balloons, helicopters and aircraft) and space-borne (rockets, manned and unmanned satellites).
Hydraulic platforms and handheld spectroradiometers are used to generate ground truth data. Each
remote sensing system is characterizedby four types of resolutions e.g. spectral, spatial, temporal

and radiometric.
Dr. IflOHD. MINHAJUDDIN AQL; L
M.E.Or.ncpo.t llon Engg.), PhD

For some remote sensing instruments, the distance -and the

platform, plays a large role in determining the detail of information obtained and the total area
imaged by the sensor. Sensors onboard platforms far away from their targets, typically view a
Iarget area, but cannot provide great detail. Compare what an astronaut onboard the space shuttle
sees of the Earth to what you can see from an airplane. The astronaut might see your whole
province or country in one glance, but couldn't distinguish individual houses. Flying over a city
or
town, you would be able to see individual buildings and cars, but you would be viewing a much
smaller area than the astronaut. There is a similar difference between satellite images and air
photos. The detail discernible in an image is dependent on the spatial resolution of the
sensor and
refers to the size of the smallest possible feature that can be detected. Spatial resolution of passive

sensors (we will Iook at the special case of active microwave sensors later) depends primarily on
their Instantaneous Field of View (IFOV).
The IFOV is the angular cone of visibility of the sensor and determines the area on the Earth's
surface which is "seen" from a given altitude at one particular moment in time. The
size of the area
viewed is determined by multiplying the IFOV by the distance from the ground to the
sensor. This
area on the ground is called the resolution cell and determines a sensor's maximum
spatial
resolution.
For a homogeneous feature to be detected, its size generally has to be equal to or larger
than the
resolution cell. If the feature is smaller than this, it may not be detectable as the average brightness
of all feahrres in that resolution cell will be recorded. However, smaller features may sometimes
be detectable if their reflectance dominates within a particular resolution cell allowing
sub-pixel
or resolution cell detection.

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET 3l
 Dr. MOHD. MTNHAJUDDIN AQUTL
M.E'(kenoPorratbn Engg' )' PhD

De gY

Figure 11 Spatial Resolution

Most remote sensing images are composed of a matrix of picture elements,

or pixels, which are
the smallest units of an image' Image pixels are normally
square and represent a ceftain area on an
image' It is important to distinguish between pixel size and spatial
resolution - they are not
interchangeable. If a sensor has a spatial resolution of 20 metres and an image
from that sensor is
displayed at full resolution, each pixel represents an areaof 20m
x20mon the ground. ln this case
the pixel size and resolution are the same. However, it is possible
to display an image with a pixel
size different than the resolution. Many posters of satellite images
of the Earth have their pixels
averaged to represent larger areas, although the original spatial resolution of the sensor that
collected the irnagery remains the same.

Images where only large features are visible are said to have coarse
or low resolution. In fine or
high-resolution images, small objects can be detected. Military sensors
for example, are designed

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 to view as much detail as possible, and therefore have very fine resolution.
Commercial satellites
provide imagery with resolutions varying from a few metres to several
kilometers. Generally
speaking, the finer the resolution, the less total ground area can be seen. The
ratio of distance on
an image or map, to actual ground distance is referred to as scale. If you had a map with a scale of
1:100,000, an object of lcm length on the map would actually be an object 100,000cm (lkm) long
on the ground' Maps or images with small "map-to-ground ratios" are referred to as
small scale
(e.g. 1:100,000), and those with larger ratios (e.g. l:5,000) are called large
scale.
Dr. MOHD. MINHAJUDDIN AQUIL
M.E.Fr.tEportttlon EngO.). PhD
Associate
Different classes of features and details in an
ring their
responses over distinct wavelength ranges.
tion, can
usually be separated using very broad wavelength ranges - the visible and near infrared.
Other
more specific classes, such as different rock types, may not be easily distinguishable
using either
of these broad wavelength ranges and would require comparison at much finer wavelength
ranges
to separate them. Thus, we would require a sensor with higher spectral resolution. 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. Black and
white film records wavelengths extending over much, or all of the visible portion
of the
electromagnetic spectrum. Its spectral resolution is fairly coarsg, as the various
wavelengths of the
visible spectrum are not individually distinguished and the overall reflectance
in the entire visible
portion is recorded. Colour film is also sensitive to the reflected
energy over the visible portion of
the spectrum, but has higher spectral resolution, as it is individually sensitive
to the reflected
energy at the blue, green, and red wavelengths of the spectrum.

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Figure 12 Spectral Resolution

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 Thus, it can represent features ofvarious colours based on their reflectance
in each ofthese distinct
wavelength ranges. Many remote sensing systems record energy over
several separate wavelength
ranges at various spectral resolutions. These are referred to as multi-spectral
sensors and will be
described in some detail in following sections. Advanced multi-spectral sensors called
hyperspectral sensors, detect hundreds ofvery narow spectral bands throughout
the visible, near-
infrared, and mid-infrared portions of the electromagnetic spectrum. Their very
high spectral
resolution facilitates fine discrimination between different targets based on their
spectral response
in each ofthe narrow bands, DT. MOHD. MINHAJUDDIN AQUIL
M.E.(Trrn po.tflon Engg.), pho

Dep
Deccan g)
While the arrangement of pixels describes iP*," ,uaiometric
characteristics describe the acfual information content in an image. Every time an image is
acquired on film or by a sensor, its sensitivity to the magnitude of the electromagnetic energy
determines the 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.
Imagery data are represented by positive digital numbers which vary from
0 to (one less than) a
selected power of 2. This range coresponds to the number of bits used
for coding numbers in
binary format. Each bit records an exponent of power 2 (e.g. I bit:z l:2). 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 2g:256 digital values
available, ranging from 0 to 255.

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Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 However, if only 4 bits were used, then only 24:l6values ranging from 0 to l5 would
be available.
Thus, the radiometric resolution would be much less, Image data are generally displayed in
a range
of grey tones, with black representing a digital number of 0 and white representing the maximum
value (for example, 255 in 8-bit data). By comparing a 2-bit image with an g-bit image, we can
see that there is a large difference in the level of detail discernible depending on their radiometric
resolutions. Dr. MOHD. MINHAJUDDIN AQUIL
M.E.ffnrrPorletbn EqE')' PTlo

Dic
In addition to spatial, spechal, and r"Oior"t
also important to consider in a remote sensing system. The concept of revisit period, which refers

to the length of time it takes for a satellite to complete one entire orbit cycle. The revisit period of
a satellite sensor is usually several days. Therefore, the absolute temporal resolution of a remote
sensing system to image the exact same area at the same viewing angle a second time is equal
to
this period. However, because of some degree of overlap in the imaging swaths of a-djacent orbits

for most satellites and the increase in this overlap with increasing latitude, some areas of the Earth
tend to be re-imaged more frequently. Also, some satellite systems are able to point their
sensors
to image the same area between different satellite passes separated by periods from one to five
days' Thus, the actual temporal resolution of a sensor depends on a variety of factors, including

the satellite/sensor capabilities, the swath overlap, and latitude. The ability to collect
imagery of
the same area of the Earth's surface at different periods of time is one of the most important
elementi for applying remote sensing data. Spectral characteristics of features may change
over
time and these changes can be detected by collecting and comparing multi-temporal
imagery.

Figure 14 Temporal Resolution

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 For example, during the growing season, most species of vegetation are in a continual
state of
change and our ability to monitor those subtle changes using remote sensing is dependent
on when
and how frequently we collect imagery. By imaging on a continuing basis at different
times we are
able to monitor the changes that take place on the Earth's surface, whether they are naturally
occurring (such as changes in natural vegetation cover or flooding) or induced by humans (such
as urban development or deforestation). The time factor in imaging is important when: persistent
clouds offer limited clear views of the Earth's surface (often in the tropics) shortJived phenomena
(floods, oil slicks, etc.) need to be imaged multitemporal comparisons are required (e.g. the spread
of a forest disease from one year to the next) the changing appearance of a feature over time can
be used to distinguish it from near similar features (wheat / maize).

F There are two main categories of passive sensor:

1. A mechanical scanning radiometer (Whisk Broom)
This is an electro-optical imaging system on which an oscillating or rotating mirror directs
the
incoming radiation onto a detector as a series of scan-lines perpendicular to the line of flight.
The
collected energy on the detector is converted into an electrical signal. This signal is then recorded
in a suitably coded digital format, together with additional data for radiometric and geometric
calibration and correction, directly on magnetic tape on board the sensor platform.
2. APush Broom Radiometer
This uses a wide angle optical system in which all the scenes across the AFOV
are imaged on a
detector anay at one time, i.e. there is no mechanical movement. As the sensormoves
along the
flight line, successive lines are imaged by the sensor and sampled by a multiflexer for transmission.
The push broom system is generally better than the mechanical scanner since
there is less noise in
the signal, there are no moving parts and it has a high geometrical aceuracy,

Dr. MOHD. MINHAJUDDIN AQUIL

M.E.(t n Portatbn Engg')' PnO
Associate ProlessoT

D g)'

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 fICIURE
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Figure 15 Characteristics of a Push Broom Radiometer

Dr. MOHD. MINHAJUDDIN AeUIt

M.E.Orancpotufion Engg.), phl

De Et

Figure 16 Push Broom Radiometer

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

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET
 DT, MOHD. MINHAJUDDIN AQUIL
fa.E{f.frF.il0on E rCg,}, pnD

De
that target is detected and measured by the sens

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 fluro-sensor and synthetic aperfure radar (SAR).

We will review briefly airborne and satellite active systems, which are commonly called Radar, and
which are generally classified either imaging or non-imaging: Imaging Radars. These display the
radar backscatter characteristics of the earth's surface in the form of a strip map or a picture
of a
selected area' A type used in aircraft is the SLAR whose sensor scans an area not directly below
the
aircraft, but at an angle to the vertical, i.e. it looks sideways to record the relative intensity of the
reflections so as to produce an image of a narrow strip of terrain. Sequential strips are recorded as
the aircraft moves forward allowing a complete image to be built up. The SLAR is unsuitable for
satellites since, to achieve a useful spatial resolution, it would require a very large antenna. A variant

used in satellites is the SAR whose short antenna gives the eflect of being several hundred
times
longer by recording and processing modified data.

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Figure 17 The Synthetic Aperture Radar System

Prepared by: Dr. Mohd. Minhajuddin Aquil

Associate Professor, CED, DCET