Transcript basics of remote sensing

BASICS OF REMOTE SENSING
Developed By
Dr. Mohamed A. Mohamed
With assistance from
Ms. Sungmi Park
Pixoneer Geomatics Inc.
Phone: (703) 852 2162
E-mail: [email protected]
Summer 2003
LECTURE 1
Introduction to Remote Sensing
FROM IMAGE TO INFORMATION
Energy
Source
Data
Acquisition
Maps
Receiver
Image
Atmosphere
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Film
Reports
Photograph
Tape
Ground
Reference
Earth Surface Features
CD-ROM
Data
Products
and
Storage
Computer
Image
Interpretation
and
Analysis
Geographic
Information
Systems
Products
and
Information
Extraction
Users
and
Decision
Makers
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DEFINITION OF REMOTE SENSING
The science and art of obtaining information
about features or phenomena from data acquired
by a device that records reflected, emitted, or
diffracted electromagnetic energy, and is not in
direct contact with the features or phenomena
under investigation.
Partially adapted from Lillesand and Keifer, 2000
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HISTORY OF REMOTE SENSING
• Born in 1839, photography was first used in topographic
surveying in 1840’s
• First aerial photograph was taken from a balloon in 1858
• Three-Color photographic process was developed in 1861
• Invented in 1903, airplane was first used as a camera
platform in 1909.
• Aerial photography was extensively used for reconnaissance
during World War I.
• Photo interpretation and photogrammetric mapping
techniques and instruments were greatly developed during
World War II
• The lunar missions in 1960’s marked the era of space
imaging
• First imaging satellites were launched in early 1970’s
Adapted from Lillesand and Keifer, 2000
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RADIATION PRINCIPLES
•Basic Wave Theory
• Electromagnetic Spectrum
• Particle Theory
• Sources of Electromagnetic Energy
• Stephan Boltzmann Law
• Blackbody Radiator
• Wien’s Displacement Law
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ELECTROMAGNETIC WAVE

V = Frequency
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BASIC WAVE THEORY
• Electromagnetic energy travels at the speed
of light in a harmonic sinusoidal fashion
• Wave frequency (v) is the number of peaks
passing a point in space per unit time
• The wavelength () is the distance between
two successive Peeks
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WAVELENGTH AND FREQUENCY
• Wavelength:
₋ Distance between two
successive peaks
• Frequency:
₋ Number of peaks (crests)
that pass a given point in
space per unit time
• Amplitude:
₋ Height of peak
Wavelengthh
Wavelengt
Amplitude
4 cycles
8 cycles
Frequency
1 Second
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WAVELENGTH MEASUREMENT UNITS
• Angstrom (Å) = 10-10 m or one 10 billionth of a meter
• Nanometer (nm) = 10-9 m or one billionth of a meter
• Micrometer (µm) = 10-6 m or one millionth of a meter
• Millimeter (mm) = 10-3 m or one thousandth of a meter
• Centimeter (cm) = 10-2 m or one hundredth of a meter
• Decimeter (dm) = 10-1 m or one tenth of a meter
• Meters (m) = 100 m or one meter
• Kilometer (dm) = 103 m or one thousand meter
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FREQUENCY MEASUREMENT UNITS
• Hertz (Hz) =
one cycle per second
• Kilohertz (KHz) =
1000 cycles per second
• Megahertz (MHz) =
106 Hz or million Hz
• Gigahertz (GHz) =
109 Hz or billion Hz
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BASIC WAVE EQUATION
C = v
Where:
C = Speed of light
 = Wavelength
v = Wave frequency
v is inversely related to 
The longer the wavelength the lower the frequency
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ELECTROMAGNETIC SPECTRUM
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PARTICLE THEORY
Electromagnetic radiation is composed of many
discrete units called photons or quanta
Q = hv
Where:
Q = Energy of a photon
h = Planck’s constant
v = Wave frequency
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ENERGY/WAVELENGTH
RELATIONSHIP
C = v ------ 1
Q = hv ------ 2
From Equation 1 & 2
Q = hC / 
The photon energy is inversely related to 
The longer the wavelength the lower its energy content
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SOURCES OF ELECTROMAGNETIC
ENERGY
• The Sun
• All matter at temperature above absolute
zero (zero degree K or -273 degree C)
Examples are terrestrial objects
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STEPHAN BOLTZMANN LAW
M = s T4
Where:
M = Total radiant from the surface
s = Boltzmann constant
T = Absolute temperature
Total energy increases very rapidly
with increase in temperature
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BLACKBODY RADIATOR
• A hypothetical ideal radiator that totally absorbs
and re-emits all energy incident upon it
• All earth surface features are not ideal radiators
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WIEN’S DISPLACEMENT LAW
m = A/T
Where:
m = Wavelength of maximum spectral radiant
A = Constant
T = Absolute temperature
Wavelength and temperature are inversely related
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GRAPHICAL REPRESENTATION OF
WIEN’S DISPLACEMENT LAW
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LECTURE 2
Energy Interaction with the Atmosphere
and Earth Surface Features (Objects)
ENERGY INTERACTION WITH THE
ATMOSPHERE & EARTH FEATURES
Absorbed
Radiation
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REFRACTION
• The bending of light when it passes from one
medium to another due to differing densities
• The index of refraction (n) is a measure of the
optical density of a substance
n = c / cn
Where:
c = speed of light in vacuum
cn = speed of light in a substance
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SCATTERING
• Unpredicted diffusion of radiation by
particles in the atmosphere
• Three types of scatter:
- Rayleigh scatter
- Mie scatter
- Non-selective scatter
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RAYLEIGH SCATTER
• Atmospheric molecules and tiny particles are
much smaller in diameter than wavelength of
the interacting radiation
- Example is a blue sky
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MIE SCATTER
• Atmospheric molecule and particle diameters
are equal to the wavelength of the interacting
radiation
• Water vapor and dust are major causes
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NON-SELECTIVE SCATTER
• Atmospheric molecule and particle diameters
are much larger than the wavelength of the
interacting radiation
• Water droplets scatter all visible and near-to-mid
infrared wavelengths equally
- Examples are fog and white clouds
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ABSORPTION
• Effective loss of energy to atmospheric constituents
• Absorption band is a range of wavelengths in the
electromagnetic spectrum within which radiant
energy is absorbed by a substance
• Most efficient absorbers are:
- Water vapor
- Ozone
- Carbon dioxide
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ATMOSPHERIC WINDOWS
Courtesy of NASA Goddard Space Flight Center
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ENERGY INTERACTION WITH
EARTH SURFACE FEATURES
Energy incident on an element are reflected,
absorbed, and/or transmitted
EI () = ER () + EA () + ET ()
Where:
EI () = Incident energy
ER () = Reflected energy
EA () = Absorbed energy
ET () = Transmitted energy
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ENERGY INTERACTION WITH
EARTH SURFACE FEATURES
EI () = Incident energy
ER () = Reflected energy
EA () = Absorbed energy
ET () = Transmitted energy
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ENERGY REFLECTION BY
EARTH SURFACE FEATURES
ER () = EI () - [EA () + ET ()]
The geometric manner in which objects reflect
energy is a function of surface roughness
• Specular reflector
• Diffuse (Lambertian) reflector
• In-between (near specular, spread, near diffuse)
- Examples are earth surface features
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SURFACE REFLECTANCE
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IDEAL SPECULAR REFLECTOR
Angle of
Reflection
Angle of
Incidence
r
i
i=r
Flat Surface that Manifest Mirror-like Reflection
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LAMBERTIAN SURFACE
Uniform reflectance in all directions
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DIFFUSE REFLECTION
It contains information on the “color”
of the reflecting surface
In remote sensing, we are most often interested
in measuring the diffuse reflectance properties
of terrain (earth surface) features
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SPECTRAL REFLECTANCE
• The portion of incident energy that is reflected
• It is often expressed as a percentage
r = ER () / EI () * 100
Where:
r
= Spectral reflectance
ER () = Reflected energy
EI () = Incident energy
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SPECTRAL REFLECTANCE CURVE
Is a graph of spectral reflectance of an object
as a function of wavelength
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SPECTRAL REFLECTANCE CURVE
60
Blue
% Reflectance
50
Green
Red
MSS 4
MSS 5 MSS 6
Near IR
40
MSS 7
Dead grass
30
20
Dry bare soil
10
Green grass
0.5
0.6
0.7
Wavelength
0.8
1.1
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ALBEDO
50
• % reflection off surfaces at
particular wavelengths
• Artificial turf has a low albedo in
the Near Infrared (IR) region
40
Percent Reflectance
• Healthy natural grass has a
high albedo in the Near
Infrared (IR) region
Natural
Grass
30
20
Artificial
Turf
10
0
BLUE GREEN
0.4
0.5
RED
INFRARED
0.6
0.7
Microns
0.8
0.9
Visible Light
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SPECTRAL SIGNATURE CURVE
Is a spectral response measured to assess the type
and/or condition of the feature
Main characteristic
Tends to imply an absolute or unique pattern
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SPECTRAL RESPONSE PATTERN
Is the spectral reflectance or emittance
of a terrain feature
Main characteristics
• Quantitative but not absolute
• Distinctive but not unique
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SPATIAL EFFECT
Factors that cause the same type of feature at a
given point of time to have different spectral
characteristics at different locations
Example
• Same crop in different fields
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TEMPORAL EFFECT
Factors that change the spectral characteristics
of a feature over time
Example
• Vegetation in different seasons
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IDEAL REMOTE SENSING SYSTEM
• Uniform energy source
• Non-interfering atmosphere
• Series of unique energy/matter interactions
at the earth’s surface
• A super sensor
• A real-time data handling system
• Multiple data users.
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REAL REMOTE SENSING SYSTEM
• Variable and non-uniform energy sources
• Interfering atmosphere
• Energy/matter interactions at the earth’s surface
are not unique
• Sensors have limited spectral sensitivity
• Data handling system have limited capabilities
• Concerns and issues about multiple data usage
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LECTURE 5
• Electromagnetic Energy Detection with
Optical and Thermal Imaging Systems
• Concepts of resolution
SCANNER SYSTEMS
Build up two-dimensional images of the terrain
for a swath beneath the plane using either
across-track (whiskbroom) scanning
or
along-track (pushbroom) scanning
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WHISKBROOM SCANNING
• Uses a set of detectors, each of which is
designed to have its peak sensitivity at a specific
wavelength
• Uses a rotating mirror to scan the terrain along
scan lines perpendicular to flight line
₋ The scanner repeatedly measure energy on both
sides of the platform
• Successive scan lines (contiguous) compose a
two-dimensional image
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WHISKBROOM SCANNING
Scanning Mirror
Detectors
Flight
Direction
Scanning
Direction
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RESOLUTION CELL SIZE
• At Nadir:
D = H’ b
• At a Scan Angle of q:
D = (H’ secq) b
Where:
D = Diameter of resolution cell
H’ = Flying height above the terrain
b = IFOV
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RESOLUTION CELL SIZE VARIATION
θ
H’
ß
ß
H’ß
Adapted from Lillesand and Keifer, 2000
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SWATH WIDTH
W = 2H’ tanq
Where:
W = Swath width
H’ = Flying height above the terrain
q = Half the total field of view
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INSTANTANEOUS FIELD OF VIEW
• Commonly referred to as IFOV
• At any instant, the scanner sense the energy
within the IFOV
• IFOV is expressed as the angle b within which
incident energy is focussed on the detector
• The ground area covered by the IFOV is often
expressed as a circle and called resolution cell
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PUSHBROOM SCANNING
• Uses linear arrays of detectors to scan in
The direction perpendicular to flight line
• Linear arrays normally consist of
charged-coupled devices (CCDs)
• A single array may contain > 10000 CCD
• Each detector is dedicated to sensing the
radiation in a single resolution cell
• All scan lines are viewed by all arrays
simultaneously
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PUSHBROOM SCANNING
Detectors
Flight
Direction
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ADVANTAGES OF PUSHBROOM
SCANNING
• Each detector has a longer dwell time, over which
to measure energy from the res. Cell
₋ Better spatial and radiometric resolution
• Greater geometric integrity because of the fixed
relationship among detectors
• CCDs are smaller in size, lighter in weight, and
require less power for their operation
• Having no moving parts, a linear array system has
higher reliability & longer life
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DISADVANTAGES OF PUSHBROOM
SCANNING
• Need to calibrate more detectors
• Current CCDs have a relatively limited
range of spectral sensitivity
• Commercially available CCDs are not
sensitive to wavelength longer than
the Near Infrared
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MULTISPECTRAL SCANNERS
• Sensitive to the region from 0.3 - 14 mm
• Three or more bands
• Bands are relatively broader in range
• Different bands may have different
spatial resolution
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ATLAS SENSOR
Courtesy of NASA Stennis Space Center
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ATLAS COLOR-IR IMAGE OF ATLANTA
Courtesy of NASA Stennis Space Center
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HYPERSPECTRAL SCANNERS
• Sensitive to the region from 0.3 - 2.5 mm
• Many to several hundred bands
• Bands are very narrow in range
• Normally all bands have the same
spatial resolution
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AVIRIS SENSOR
Courtesy of NASA Jet Propulsion Laboratory
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FOREST FIRE IN BRAZIL (AVIRIS)
Courtesy of NASA Jet Propulsion Laboratory
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THERMAL SCANNERS
• Sensitive to the region from 3 - 14 mm
• Few to several bands
• Band ranges are variable
• Normally all bands have the same
spatial resolution
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THERMAL INFRARED SCANNER
Tape
Recorder
Electrical
Signal
Amplifier
Dewar
(liquid
nitrogen)
70mm
Film
Recorder
Scanning
Optics
Lens
Glow Tube
Motor
Radiation
Source
Oscillating
Mirror
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VOLCANOLOGY STUDIES
TIMS image draped over 1:4000 DEM
Courtesy of NASA Ames Research Center
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SPATIAL RESOLUTION
A measure of the smallest angular or linear separation
between two objects that can be resolved by the sensor
i.e.
It is the limit on how small an object can be and still be
“seen” by a sensor as being separate from its
surroundings
The smaller the object the higher the resolution
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SPATIAL RESOLUTION
4 meter x 4 meter
16 meter x 16 meter
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Map Usage
SPATIAL RESOLUTION & APPLICATIONS
IKONOS
( 1 ~ 4 m)
IRS/Kompsat
( 5 ~ 10 m)
SPOT/Landsat
10 ~ 30 m
Standard Error in
Position (RMSE meters)± < 0.2 ± 0.4
1:500 1:1,000
SCALE
±1
1:2,500
±2
1:5,000
±5
1:10,000
±6
1:25,000
± 12
1:50,000
Typical Users
LAND MANAGEMENT
CITY
Developed Area
Regional Ar ea
Rural
Urban
COUNTY
Undeveloped
STATE
INFRASTRUCTURE
UTILITIES
TRANSPORTATION
RESOURCE MANAGEMENT
AGRICULTURE
MINERALS & PETROL
Cross Country T ransmission
Urban - Distr ibution
Urban Area & Construction
Regional Sys.
Row Cr ops, Or chard, Small Fields
Facilities
Lease H oldings Dev. Ar ea
State - M ulti State R egion
Field Cr ops, L arge Fields
Gr azing
Lease H ol d Undeveloped
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SPECTRAL RESOLUTION
The number and dimensions of specific wavelength
intervals in the electromagnetic spectrum to which a
remote sensing instrument is sensitive.
i.e.
The ability to discriminate fine spectral differences
The higher the number of bands
the higher the resolution
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SPECTRAL RESOLUTION
Red
Green
Blue
Grey
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RADIOMETRIC RESOLUTION
The sensitivity of a remote sensing detector to differences
in signal strength as it records the radiance flux reflected
or emitted from the terrain.
i.e.
The ability to discriminate very slight energy differences
The higher the number of bits,
the wider the range of values,
and the higher the resolution
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RADIOMETRIC RESOLUTION
8 bit (0 ~ 255 )
11 bit (0 ~ 2047 )
In 2 bit case, target A and target B has brightness value of 1(can not be
recognized as different objects in the image). However, in 4 bit case , target A
has value of 3 and target b has value of 2.
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TEMPORAL RESOLUTION
Frequency of data acquisition over the area
Terms implying temporal resolution:
• Revisit capability
• Global cycle
• Global coverage
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CHANGE DETECTION
The ability to measure temporal effects
or
The ability to quantify change over a period of time
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METHODS OF SCANNERS
CALIBRATION
• On-Board
• In laboratories
• Fly over natural and/or man-made targets
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CALIBRATION OF SCANNERS
• Signal-to-noise ratio
• Spatial resolution
• Spectral bands (ranges & co-registration)
• Radiometric values
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IMAGERY
Recorded by electronic sensors that generate
electrical signals that corresponds to
energy variations in the scene
Merits:
- Broader spectral range sensitivity
- Improved calibration potential
- Electronic transmittal of data
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LECTURE 7
Microwave Remote Sensing
PASSIVE MICROWAVE SENSORS
• Radiometers or scanners that operate similar to their optical
counterparts
• Use antenna to detect naturally emitted microwave energy
(atmosphere, surface features, and sub-surface transmittance)
• Characterized by low spatial resolution (large field of view)
due to the relatively small magnitude of emitted energy
• The emitted microwave energy is related to the temperature
and moisture properties of the object
• Data from such sensors is typical used by:
₋ Meteorologists (atmospheric profile, ozone and water content)
₋ Hydrologists (soil moisture content)
₋ Oceanographers (mapping sea ice, currents, winds, and pollutants)
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ACTIVE MICROWAVE SENSORS
• Non-Imaging sensors
₋ Profiling devices such as altimeters and scatterometers
that take measurements in one linear dimension
• Imaging sensors:
₋ Radar instruments that record two-dimensional images
of surfaces beneath the platforms
₋ In general, image acquisition is not affected by
weather (Clouds, Haze, Dust, etc.)
₋ Images can be acquired at day and/or night time
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NON-IMAGING SENSORS
• Radar Altimeters:
₋ Look straight down at nadir and transmits short microwave
pulses to determine distances of targets through measurement
of round trip time delays
₋ Often used for determination of aerial platforms’ altitudes, as
well as topographic mapping and sea surface height
estimation.
• Scatterometers:
₋ Used to precisely measure the amount of energy
backscattered from a target, which depends on surface
roughness and the incident angle at which the energy
contacts the target
₋ Typically used in oceanographic applications for estimation
of wind speeds, and identification of materials and
characterization of surfaces in land applications.
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PRINCIPLES OF IMAGING RADAR
• Radar is a range or distance measuring device which consists of a transmitter,
a receiver, an antenna, and a data processing and recording system
• The transmitter generates short successive pulses of microwave energy at
regular intervals which are focused by the antenna into a beam
• The beam obliquely illuminates a surface at a right angle to the direction of the
platform
• Targets (objects) reflects the signal back to the receiver (echo or backscatter)
• The location of an object (based on its distance from the radar) is determined
by measuring the time delay between transmission of a pulse and reception of
the echo backscattered by that object.
• The forward motion of the platform and the continued processing and recording
of the backscattered echo builds up a two-dimensional image of the surface
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MICROWAVE BANDS
Letter Code
Frequency Range (MHz)
Wavelength Range (cm)
P
UHF
L
S
C
X
Ku
K
Ka
220 – 390
300 – 1000
1000 – 2000
2000 – 4000
4000 – 8000
8000 – 12500
12500 – 18000
18000 – 26500
26500 – 40000
136 – 77
100 – 30
30 – 15
15 – 7.5
7.5 – 3.75
3.75 – 2.4
2.4 – 1.67
1.67 – 1.18
1.18 – 0.75
Wavelength (λ) in cm = 30000 / frequency in MHz
Adapted from Henderson and Lewis, 1998
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EXAMPLES OF RADAR IMAGERY
L-Band
X-Band
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POLARIZATION
Transmission of energy in either a horizontal (H)
or vertical (V) plane
Horizontal (H)
Vertical (V)
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PARALLEL POLARIZED SYSTEM
Sends and receives signal in same polarization
(HH) or (VV)
Horizontal (HH)
Vertical (VV)
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CROSS-POLARIZED SYSTEM
• Sends in one polarity and receives in another
polarity (VH) or (HV)
• Requires a second antenna
• With a second antenna two images can be recorded
simultaneously (HH-HV) or (VV-VH)
• These pairs of images (dual polarization images)
can be analyzed for differences
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CROSS-POLARIZED SYSTEM
Vertical Horizontal (VH)
Horizontal Vertical (HV)
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POLARITY EFFECT ON IMAGERY
HH Polarization
HV Polarization
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MULTI-CHANNEL RADAR
• Records four images simultaneously
• Two polarities and two different wavelengths
₋
₋
₋
₋
Band X HH
Band X HV
Band L HH
Band L HV
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SIDE-LOOKING AIRBORNE RADAR
(SLAR) VIEWING GEOMETRY
Depression Angle
Look Angle
Radar Beam
Nadir Line or
Ground Track
Radar Pulse
90˚
90˚
Ground Range
Incidence
Angle
Image Swath
Across Track
Along Track
Radar Ground
Contact
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RADAR PARAMETERS
• Azimuth direction  Direction of flight
• Range (look) direction  Direction of illumination which is at
right angle to azimuth direction
₋ Significantly impact feature interpretation
₋ Enhancement or suppression of linear features depends on their
relative orientation to range direction
• Depression angle (γ) Angle between the range direction and
the electromagnetic pulse from the radar antenna to a point in
the ground
• Incident angle (θ)  Angle between the radar pulse and a line
perpendicular to the surface it contacts
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RANGE RESOLUTION
• The resolution in the across-track direction which
is proportional to the length of the microwave pulse
₋ The shorter the pulse length, the finer the resolution
• Range resolution (Rr) = τ.c/2cosγ
₋ τ = duration of transmission
₋ c = speed of light
₋ γ = depression angle
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SLANT RANGE RESOLUTION
Radar Pulse
Cannot distinguish
between A & B
A BC
D
Radar Pulse
All objects
distinguishable
A BC
D
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AZIMUTH RESOLUTION
• Determined by computing the width of the terrain
strip illuminated by the radar beam
₋ The angular beam width is directly proportional to
the wavelength of the transmitted pulse
₋ The beam width is inversely proportional to
antenna length
• Azimuth resolution (Ra) = S x γ / L
₋ S = slant range
₋ γ = depression angle
₋ L = antenna length
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AZIMUTH RESOLUTION
Antenna
Beam Width
A
Illuminated
Area
B
A & B are
resolvable
C
D
C & D are not
resolvable
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GEOMETRIC DISTORTION OF
RADAR IMAGERY
Inherent geometric distortions in radar imagery are
caused by the following:
• Slant-range scale distortion
• Relief displacement
₋ Foreshortening
₋ Layover
• Shadowing
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SLANT-RANGE SCALE DISTORTION
• Occurs because radar measure distance to objects
in slant ranges (not horizontal along the ground
distances)
• Objects in the near range are more distorted
(compressed) than those in the far range
• Can be easily corrected by using trigonometry to
calculate ground-range distances to objects
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RELEIF DISPLACEMENT
• A one-dimensional displacement along the range
(look) direction
• Higher Objects are displaced towards the sensor
• Radar foreshortening and layover are typical
consequences of relief displacement
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FORESHORTENING OF SLOPE
• Occurs when the radar beam reaches the base of a high object
before it reaches the top
• Slopes are compressed. Severity of compression depends on angle
of slope in relation to incident angle of radar beam
• Maximum compression occurs when the base and top are imaged
simultaneously (radar beam is perpendicular to slope)
A’ B’
A
B
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SEVERITY OF FORESHORTENING
Very severe slope
compression
Maximum slope
compression
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FORESHORTENING OF SLOPES
Courtesy of RADARSAT Corporation
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LAYOVER OF SLOPE
• Occurs when the radar beam
reaches the top of a high
object before it reaches the
base
• Very severe at the near range
with small incident angles
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LAYOVER OF SLOPE
Courtesy of RADARSAT Corporation
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RADAR SHADOWS
• Result from foreshortening
and layover
Radar Image
• Occur in the down range
direction behind vertical
features and steep slopes
• Radar beam does not
illuminate the surface
• Shadows appear dark
• Objects in shadows are
obscured
Photo Image
Shadow
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RADAR SHADOWS
Courtesy of RADARSAT Corporation
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RADAR IMAGE INTERPRETATION
• Images are a result of different factors than aerial
photos
₋ degree of reflectance in one wavelength
₋ angle of depression
₋ negative terrain: absence of data in the shadows
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TONES ON A RADAR IMAGE
• Measure of backscatter strength
• The stronger the return the brighter the area on the image
• Light tones: prominent cultural and topographic features
• Varying tones: cultivated fields and most terrain surfaces
• Dark tones: calm water bodies, smooth ice, and some
depositional landforms
• Uniform tones: relatively homogeneous feature
• Grainy or speckled tones: rough surfaces
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SUPERIOR, WISCONSIN
City
Water
Storage
Tanks
Sandy Deposits
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ALBERTA, CANADA
Selective
Clearcut
Lakes
Drumlins
Road
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DETRMINING FLIGHT DIRECTION
• What direction was the
aircraft flying if the
SLAR was mounted on
the port (left) side?
Choices…
A
C
D
B
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FEATURES IDENTIFICATION
Power
Lines
Cultivated
Fields
Eroded
Anticlines
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