Sensors for remote sensing

APPLICATION OF REMOTE SENSING AND
GEOGRAPHICAL INFORMATION SYSTEM IN
CIVIL ENGINEERING
Date:
INSTRUCTOR
DR. MOHSIN SIDDIQUE
ASSIST. PROFESSOR
DEPARTMENT OF CIVIL ENGINEERING

Optical Remote Sensing
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Optical remote sensing makes use of visible, near infrared and short-wave
infrared sensors to form images of the earth's surface by detecting the solar
radiation reflected from targets on the ground
Photography
(Photogrammetry)
Thermal Scanner
Multispectral

Wave Length Band for Principal Sensor
4

We shall concentrate the discussion on
optical-mechanical-electronic radiometers
and scanners, leaving the subjects of
camera-film systems and active radar for
Seminar assignment !!
Major elements of Electro-optical Scanner
Optical System: lenses, mirrors, apertures,
modulators and dispersion devices
Detectors: provides an electrical signal
proportional to the irradiance on its active
surface, generally some type of
semiconductors
Signal processors: perform specified
functions on the electrical signal to provide
the desired output data
Sensor: Optical-Mechanical-Electrical Sensors
A typical electro-optical sensor design.
5

The two broadest classes of sensors are
Passive (energy leading to radiation received comes from an external
source, e.g., the Sun; the MSS is an example) and
Active (energy generated from within the sensor system is beamed
outward, and the fraction returned is measured; radar is an example)
Sensor for Remote Sensing
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Another attribute in this classification is whether the sensor operates in a
non-scanning or
a scanning mode.
If the scene is sensed point by point (equivalent to small areas within the
scene) along successive lines over a finite time, this mode of measurement
makes up a scanning system.
And if the entire scene is sensed directly with the sensor then its terms as non-
scanning system
Most non-camera sensors operating from moving platforms image the scene
by scanning.
For example, A film camera held rigidly in the hand is a non-scanning device
that captures light almost instantaneously when the shutter is opened, then
closed. But when the camera and/or the target moves, as with a movie
camera, it in a sense is performing scanning as such.
7

Sensors can be
Non-imaging (measures the radiation received from all points in the sensed
target, integrates this, and reports the result as an electrical signal strength
or some other quantitative attribute, such as radiance) or
Imaging (the electrons released are used to excite or ionize a substance
like silver (Ag) in film or to drive an image producing device like a TV or
computer monitor or a cathode ray tube or oscilloscope or a battery of
electronic detectors
8

Radiometer is a general term for
any instrument that quantitatively
measures the EM radiation in some
interval of the EM spectrum.
When the radiation is light from the
narrow spectral band including the
visible, the term photometer can be
substituted.
If the sensor includes a component,
such as a prism or diffraction
grating, that can break radiation
extending over a part of the
spectrum into discrete wavelengths
and disperse (or separate) them at
different angles to an array of
detectors, it is called a
spectrometer.
9

The term spectro-radiometer is
reserved for sensors that collect the
dispersed radiation in bands rather
than discrete wavelengths.
Most air/space sensors are
spectroradiometers.
Sensors that instantaneously measure
radiation coming from the entire
scene at once are called framing
systems. The eye, a photo camera,
and a TV vidicon belong to this
group.
10

The optical setup for imaging sensors will be either an image plane or
an object plane set up depending on where lens is before the photon rays are
converged (focused), as shown in the illustration.
For the image plane
arrangement, the lens receives
parallel light rays after these
are deflected to it by the
scanner, with focusing at the
end.
For the object plane setup, the
rays are focused at the front
end (and have a virtual focal
point in back of the initial optical
train), and are intercepted by
the scanner before coming to a
full focus at a detector.
11

Two broad categories of most scanners are defined by the terms
"optical-mechanical" and
"optical-electronic",
Both are distinguished by the former containing an essential mechanical
component (e.g., a moving mirror) that participates in scanning the scene and
by the latter having the sensed radiation move directly through the optics onto
a linear or two-dimensional array of detectors
optical-mechanical
optical-
electronic
12

Another attribute of remote sensors, not shown in the classification, relates to
the modes in which those that follow some forward-moving track (referred to
as the orbit or flight path) gather their data.
Cross-track scanners
Along track scanners
In doing so, they are said to monitor the path over an area out to the sides of the
path; this is known as the swath width.
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This is sometimes referred to as the
Whiskbroom mode from the vision of
sweeping a table side to side by a
small handheld broom.
The Cross Track mode normally uses
a rotating (spinning) or oscillating
mirror (making the sensor an optical-
mechanical device) to sweep the
scene along a line traversing the
ground
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Also known as Pushbroom Scanners
Sense a swath with an linear array of
CCD’s
Because pushbroom scanners have no
mechanical parts, their mechanical
reliability can be very high
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Spatial Resolution
Spectral Resolution
Radiometric Resolution
Temporal Resolution
Sensor Resolutions
The ratio of distance on an image or map, to actual ground distance is
referred to as scale.
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radiometric

Instantaneous Field of View (IFOV) is
the angular cone of visibility of the
sensor (A) and determines the area on
the Earth's surface which is "seen" from
a given altitude at one particular
moment in time (B)
Spatial Resolution
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This is a measure of the area or size of the smallest dimensions on the earth’s
surface over which an independent measurement can be made by the sensor
It is expressed by the size of the pixel on the ground in m
A measure of size of pixel is given by the IFOV, which is dependent on the
altitude and the viewing angle of the sensor
A narrow viewing angle or a lower altitude produces a small IFOV
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 area of
20m x 20m on the ground
For a pushbroom system the number of detectors influences the spatial
resolution
A system with 1,000 detectors that images a 50 km wide swath has a pixel
size of 50m
Spatial Resolution
18

http://webhelp.esri.com/arcgiSDEsktop/9.3/index.cfm?TopicName=Cell_size
_of_raster_data
Spatial Resolution
In this case, resolution refers to the pixel (cell) size (the area covered on the
ground and represented by a single cell).
A higher spatial resolution implies that there are more pixels per unit area;
therefore, the graphic on the left represents a higher spatial resolution than
the graphic on the right.
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Spatial Resolution versus Scale
The spatial resolution of the data
used in the image on the left is lower
than the spatial resolution of the data
used in the image on the right. This
means the cell size of the data in the
left image is larger than that of the
data in the right image; however, the
scale at which each is displayed is
the same.
The scale of the image on the
left (1:50,000) is smaller than
the scale of the image on the
right (1:2,500); however, the
spatial resolution (cell size) of
the data is the same.
22

The spectral resolution of a sensor
characterizes the ability of the sensor
to resolve the energy received in a
spectral bandwidth to characterise
different constituents of earth surface
Spectral resolution is defined as the
spectral bandwidth of the filter and the
sensitiveness of the detector
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
Spectral Resolution
Coarse
Fine
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Many remote sensing systems record energy over several separate
wavelength ranges at various spectral resolutions. These are referred to as
multi-spectral, superspectral, and hyperspectral sensors
Spectral Resolution
Panchromatic
Multispectral
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Multispectral and Hyperspectral Resolution
http://auracle.ca/news/
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The radiometric resolution of an imaging system describes its ability to
discriminate very slight differences in energy. i.e., it is a measure of how
many grey levels are measured between pure black (no reflectance) to pure
white.
It is measured in bits
Examples
1 bit (21) – 2 levels
7 bits (27) – 128 levels IRS 1A & 1B
8 bits (28) – 256 levels Ladnsat TM
11 bits (211) – 2048 levels NOAA –
AVHRR
In a 8 bit system, black is measured
as 0 and white is measured as 255.
Imagery data are represented by positive Digital Numbers (DN) which vary from
0 to (one less than) a selected power of 2 according to bit system.
26

0
1
27
1 bit (21)
8 bits (28) 8 bits (28)

A remote sensing system with a radiometric resolution of 6 bits assigns a
digital number (DN) of 28 to one surface and 47 to another. What would be
the equivalent DNs for the same surfaces if the measurements were taken with
a 3 bit system?
The DNs recorded by the 3 bit system range from 0 to 7 and this range is
equivalent to 0-64 for the 6 bit system
0 1 2 3 4 5 6 7 (3 bit)
0 9 18 27 36 45 54 63 (6 bit)
Therefore a DN of 28 and 47 on the 6 bit system will be recorded as 3 and
5 on a 3 bit system.
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Temporal resolution of a remote sensing system is a measure of how often
data are obtained for the same area
Applicable to satellite remote sensing only
Temporal resolution varies from less than one hour to approximately 30 days.
Importance of Temporal Resolution
Change in Land Use/ Land Cover
Temporal Variation
Monitoring of a Dynamic Event
Cyclone
Flood
Volcano
Earthquake
Temporal Resolution
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Comments….
Questions….
Suggestions….
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I am greatly thankful to all the information sources
(regarding remote sensing and GIS) on internet that I
accessed and utilized for the preparation of present
lecture.
Thank you !
Feel free to contact

Sensors for remote sensing

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