Sensors for remote sensing

Introduction to Remote sensing

1. APPLICATIONOF REMOTE SENSINGAND
GEOGRAPHICAL INFORMATIONSYSTEMIN
CIVILENGINEERING
Date:
INSTRUCTOR
DR. MOHSIN SIDDIQUE
ASSIST. PROFESSOR
DEPARTMENT OF CIVIL ENGINEERING

2. Optical Remote Sensing
2
A
Optical remote sensing 
makes use of visible, near infrared and short-wave 
infrared sensors to form images of the earth's surf ace by detecting the solar 
radiation reflected from targets on the ground
A
Photography 
A
(Photogrammetry)
A
Thermal Scanner
A
Multispectral

3. Sensor for Remote Sensing
3

4. Wave Length Band for Principal Sensor
4

5. A
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 !!
A
Major elements of Electro-optical Scanner
A
Optical System:
lenses, mirrors, apertures,
modulators and dispersion devices
A
Detectors:
provides an electrical signal
proportional to the irradiance on its active
surface, generally some type of
semiconductors
A
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

6. A
The two broadestclasses of sensors are
A
Passive
(energy leading to radiation received comes from an externa l
source, e.g., the Sun; the MSS is an example)and
A
Active
(energy generated from within the sensor system is beamed
outward, and the fraction returned is measured;radar is an e xample)
Sensor for Remote Sensing
6

7. A
Another attribute in this classification is whether the sen sor operates in a
A
non-scanning
or
A
a
scanning mode
.
A
If the scene is sensed point by point (equivalent to small are as within the
scene) along successive lines over a finite time, this mode o f measurement
makes up a
scanning system
.
A
And if the entire scene is sensed directly with the sensor the n its terms as
non-
scanning system
A
Most non-camera sensors operating from moving platforms im age the scene
by scanning. A
For example, A film camera held rigidly in the hand is a
non-scanning
device
that captures light almost instantaneously when the shutte r 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.
Sensor for Remote Sensing
7

8. A
Sensors can be
A
Non-imaging
(measures the radiation received from all points in the sens ed
target, integrates this, and reports the result as an electr ical signal strength
or some other quantitative attribute, such as radiance) or
A
Imaging
(the electrons released are used to excite or ionize a substa nce
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 ba ttery of
electronic detectors
Sensor for Remote Sensing
8

9. A
Radiometer
is a general term for
any instrument that quantitatively
measures the EM radiation in some
interval of the EM spectrum.
A
When the radiation is light from the
narrow spectral band including the
visible, the term
photometer
can be
substituted.
A
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
.
Sensor for Remote Sensing
9

10. A
The term
spectro-radiometer
is
reserved for sensors that collect the
dispersed radiation in
bands
rather
than discrete wavelengths.
A
Most air/space sensors are
spectroradiometers. A
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.
Sensor for Remote Sensing
10

11. A
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.
Sensor for Remote Sensing
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

12. A
Two broad categories of most scanners are defined by the term s
A
"
optical-mechanical
" and
A
"
optical-electronic
",
A
Both are distinguished by the former containing an essentia l mechanical
component (e.g., a moving mirror) that participates in scan ning the scene and
by the latter having the sensed radiation move directly thro ugh the optics onto
a linear or two-dimensional array of detectors
Sensor for Remote Sensing
optical-mechanical
optical-
electronic
12

13. A
Another attribute of remote sensors, not shown in t he classification, relates to 
the modes in which those that follow some 
forward-moving track 
(referred to 
as the 
orbit
or 
flightpath
) gather their data. 
A
Cross-track scanners
A
Along track scanners
Sensor for Remote Sensing 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
.
13

14. A
This is sometimes referred to as the Whiskbroom mode
from the vision of
sweeping a table side to side by a
small handheld broom.
A
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
Sensor for Remote Sensing
14

15. A
Also known as
Pushbroom Scanners
A
Sense a swath with an linear array of
CCD’s A
Because
pushbroom scanners
have no
mechanical parts, their mechanical
reliability can be very high
Sensor for Remote Sensing
15

16. A
Spatial Resolution
A
Spectral Resolution
A
Radiometric Resolution
A
Temporal Resolution
Sensor Resolutions
The ratio of distance on an image or map, to actual gr ound distance is
referred to as
scale
.
16
radiometric

17. A
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
17

18. A
This is a measure of the
area
or
size
of the
smallest dimensions
on the earth’s
surface over which an independentmeasurement can be made by the sensor
A
It is expressed by the size of the pixel on the ground in
m
A
A measure of size of pixel is given by the IFOV, which is depend ent on the
altitude and the viewing angle of the sensor A
A narrow viewing angle or a lower altitude produces a small IF OV
A
If a sensor has a spatial resolution of 20 metres and an image f rom that
sensor is displayed at full resolution, each pixel represen ts an area of
20m x 20m on the ground
A
For a pushbroom system the number of detectors influences th e spatial
resolution A
A system with 1,000 detectors that images a 50 km wide swath ha s a pixel
size of 50mSpatial Resolution
18

19. Spatial Resolution
19

20. A
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 cove red 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.
20

21. Spatial Resolution
21

22. 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

23. A
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
A
Spectral resolution is defined as the
spectral bandwidth of the filter and the
sensitiveness of the detector
A
Spectral resolution describes the ability
of a sensor to define fine wavelength
intervals.
A
The finer the spectral resolution, the
narrower the wavelength range for a
particular channel or band
Spectral Resolution
Coarse
Fine
23

24. A
Many remote sensing systems record energy over several separate
wavelength ranges at various spectral resolutions. These a re referred to as
multi-spectral, superspectral, and hyperspectral sensor s
Spectral Resolution
Panchromatic
Multispectral
24

25. Multispectral and HyperspectralResolution
http://auracle.ca/news/
25

26. A
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 reflect ance) to pure
white.
A
It is measured in bits
Radiometric Resolution
Examples P1 bit (2
1
) – 2 levels
P7 bits (2
7
) – 128 levels IRS 1A& 1B
P8 bits (2
8
) – 256 levels Ladnsat TM
P11 bits (2
11
) – 2048 levels NOAA –
AVHRR
PIn 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 b it system.
26

27. Radiometric Resolution
0
1
27
1 bit (2
1
)
8 bits (2
8
) 8 bits (2
8
)

28. A
A remote sensing system with a radiometric resolution of 6 bi ts assigns a
digital number (DN) of 28 to one surface and 47 to another. Wha t would be
the equivalent DNs for the same surfaces if the measurements were taken with
a 3 bit system?
A
The DNs recorded by the 3 bit system range from 0 to 7 and this ra nge is
equivalent to 0-64 for the 6 bit system A
0 1 2 3 4 5 6 7 (3 bit)
A
0 9 18 27 36 45 54 63 (6 bit)
A
Therefore a DN of 28 and 47 on the 6 bit system will be recorded a s 3 and
5 on a 3 bit system.Radiometric Resolution
28

29. A
Temporal resolution
of a remote sensing system is a measure of how often
data are obtained for the same area
A
Applicable to satellite remote sensing only
A
Temporal resolution varies from less than one hour to approx imately 30 days.
A
ImportanceofTemporalResolution
A
Change in Land Use/ Land Cover
A
Temporal Variation
A
Monitoring of a Dynamic Event
A
Cyclone
A
Flood
A
Volcano
A
Earthquake
Temporal Resolution
29

30. Comments….
Questions…. 
Suggestions….
30
I amgreatly 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