Department of Civil Engineering (CE)
Spring 2025
CE 531
Geographic Information System(GIS)
& Remote Sensing
Dr. Tahsina Islam
Microwave Remote Sensing
❖ Microwave remote sensing covers EM
spectrum in the range from approximately
1mm to 1m
❖ Longer wavelength microwave radiation can
penetrate through cloud cover, haze, dust.
The longer wavelengths are not susceptible
to atmospheric scattering which affects
shorter optical wavelengths.
❖ This property allows detection of microwave
energy under almost all weather and
environmental conditions so that data can be
collected at any time
Microwave Remote Sensing
❖ It can be active or passive
❖ It uses Radar (Radio Detection and Ranging) technology to send and
detect EMR as well as to measure the distance between energy
source and the object, on which the energy is being scattered.
❖ A microwave remote sensor measures backscattered microwave
❖ Most of the microwave remote sensors are active sensors having
their own source of energy. For example RADARSAT
Characteristics of Microwave Remote Sensing
Advantages compared to optical remote sensing
❖ All weather capability (small sensitivity of clouds, light rain)
❖ Day and night operation (independence of sun illumination)
❖ No effects of atmospheric constituents (multi temporal analysis)
❖ Sensitivity to dielectric properties (water content , biomass, ice)
❖ Sensitivity to surface roughness ( ocean wind speed)
❖ Sensitivity to man made objects
❖ subsurface penetration
Characteristics of Microwave Remote Sensing
❖ This covers the longest
wavelengths used for remote
sensing (1mm to 1m).
❖ The remote sensing using
microwave spectrum is
termed as microwave sensing
Type of Microwave Remote Sensing
❑ Passive RS ❑ Active RS
Natural (EMR from Sun) Technology assisted radiation
RS using reflected solar radiation
RS using emitted terrestrial radiation
Passive Microwave Remote Sensing
❑ Passive microwave sensing is similar in concept to thermal remote sensing.
❑ All objects emit microwave energy of some magnitude, but the amounts are generally
very small.
❑ A passive microwave sensor detects the naturally emitted microwave energy within its
field of view. This emitted energy is related to the temperature and moisture properties
of the emitting object or surface.
❑ Because the wavelengths are so long, the energy available is quite small compared to
optical wavelengths. Thus, the fields of view must be large to detect enough energy to
record a signal.
❑ Most passive microwave sensors are
therefore characterized by low spatial
resolution.
❑ Applications of passive microwave
remote sensing include meteorology,
hydrology, and oceanography
Active Microwave Remote Sensing
❑ Active microwave sensors provide their own
source of microwave radiation to illuminate
the target
❑ The most common form of imaging active
microwave sensors is RADAR.
❑ RADAR is an acronym for Radio Detection
And Ranging
❑ RADAR transmits a microwave (radio) signal
towards the target and detects the
backscattered portion of the signal.
❑ The strength of the backscattered signal is
measured to discriminate between different
targets and the time delay between the
transmitted and reflected signals determines
the distance (or range) to the target
How RADAR Works
A radar is essentially a ranging or distance A radar system consists of a transmitter
measuring device. producing electromagnetic waves in the
radio or microwaves domain, an emitting
It consists a transmitter, an emitting antenna, antenna, a receiving antenna (separate or
a receiving antenna and an electronics the same as the previous one) to capture
system to process and record the data. any returns from objects in the path of the
emitted signal, a receiver and processor to
The transmitter generates successive pulses determine properties of the object(s).
of microwave (A) at regular intervals which
are focused by the antenna into a beam (B).
The radar beam illuminates the surface
obliquely at a right angle to the motion of the
platform.
The antenna receives a portion of the
transmitted energy reflected (or
backscattered) from various objects within
the illuminated beam (C).
By measuring the time delay between the transmission of a pulse and the reception of the
backscattered "echo" from different targets, their distance from the radar and thus their location
can be determined
Fundamental Radar Equation
the gain describes how well the antenna converts
input power into radio waves headed in a specified
direction. As a receiving antenna, the gain describes
how well the antenna converts radio waves arriving
from a specified direction into electrical power
Wavelength ranges or bands of microwave
❖ Ka, K, and Ku bands: very short wavelengths
used in early airborne radar systems but
uncommon today.
❖ X-band: used extensively on airborne systems
for military reconnaissance and terrain
mapping.
❖ C-band: common on many airborne research
systems, ERS-1 and 2 and RADARSAT).
❖ S-band: used on board the Russian ALMAZ
satellite.
❖ L-band: used onboard American SEASAT and
Japanese JERS-1 satellites and NASA
airborne system.
❖ P-band: longest radar wavelengths, used on
NASA experimental airborne research system.
Wavelength ranges or bands of microwave
Band Descriptions
Band Name Wavelength (l) in cm Frequency (v) in GHz
Ka (0.86) 0.75 – 1.18 40.0 to 26.5
K 1.18 – 1.67 26.5 to 18.0
Ku 1.67 – 2.4 18.0 to 12.5
X (3.0 and 3.2 cm) 2.4 – 3.8 12.5 – 8.0
C (7.5, 6.0 cm) 3.8 – 7.5 8.0 – 4.0
S (8.0, 9.6, 12.6 cm) 7.5 – 15.0 4.0 – 2.0
L (23.5, 24.0, 25.0 cm) 15.0 – 30.0 2.0 – 1.0
P (68.0 cm) 30.0 – 100 1.0 – 0.3
Wave length and Radar Images
o The two radar images of the same agricultural fields, each image having been
collected using a different radar band.
o The top one was acquired by a C-band radar and the one below was acquired by an
L-band radar.
o It is clearly seen that there are significant C-Band Image
differences between the way the various
fields and crops appear in each of the two
images.
o This is due to the different ways in which
the radar energy interacts with the fields
and crops depending on the radar
wavelength.
L-Band Image
Types of Radar
Non-imaging radar
❖ Traffic police use handheld Doppler radar
system determine the speed by measuring
frequency shift between transmitted and
return microwave signal
❖ Plan position indicator (PPI) radars use a
rotating antenna to detect targets over a
circular area
❖ Ground Penetration Radar (GPR) – to detect
object/fault underneath the surface
Doppler Effect and Doppler Radar
• The frequency and wavelength of an electromagnetic field are affected by relative
motion. This is known as the Doppler effect . Only the radial (approaching or
receding) component of motion produces this phenomenon.
• A special type of RADAR, called Doppler radar , uses the Doppler effect to ascertain
wind velocity in heavy thundershowers, tornadoes, and hurricanes.
• A Doppler radar is a specialized radar that uses the Doppler effect to produce
velocity data about objects at a distance. It does this by bouncing a microwave
signal off a desired target and analyzing how the object's motion has altered the
frequency of the returned signal. This variation gives direct and highly accurate
measurements of a target's velocity relative to the radar.
Types of Radar
Imaging radar
❖ Usually high spatial resolution,
❖ Consists of a transmitter, a receiver, one or more antennas, GPS,
computers etc.
Radar Terminology
❖ nadir
❖ azimuth flight direction
❖ look direction
❖ range (near and far)
❖ depression angle (γ)
❖ incidence angle (θ)
❖ altitude above-ground-level, H
❖ polarization
Radar Terminology
Azimuth Direction
– direction of travel of aircraft or orbital track of satellite
Range angle
– direction of radar illumination, usually perpendicular to azimuth direction
Depression angle
– angle between horizontal plane and microwave pulse (near range
depression angle > far range depression angle)
Incident angle
– angle between microwave pulse and a line perpendicular to the local
surface slope
Polarization
– linearly polarized microwave energy emitted/received by the sensor
(HH, VV, HV, VH)
Incident and Depression Angle
The incident angle (θ) is the angle between the radar pulse of EMR
and line perpendicular to the Earth’s surface where it makes contact.
When the terrain is flat, the incident angle
(θ) is the complement (θ = 90 - γ) of the
depression angle(γ). If the terrain is sloped,
there is no relationship between
depression angle and incident angle. The
incident angle best describes the
relationship between the radar beam and
surface slope.
Many mathematical radar studies assume
the terrain surface is flat (horizontal)
therefore, the incident angle is assumed to
be the complement of the depression
angle.
Azimuth Direction
- direction of travel of aircraft or orbital track of satellite
Range Direction
The range or look direction for any
radar image is the direction of the
radar illumination that is at right
angles to the direction the aircraft or
spacecraft is traveling.
Generally, objects that strike in a
direction that is perpendicular to the
range or look direction are enhanced
much more than those objects in the
terrain that lie parallel to the look
direction.
The terrain illuminated nearest the
aircraft in the line of sight is called the
near-range. Consequently, linear features that appear
dark in a radar image using one look
The farthest point of terrain illuminated direction may appear bright in another
by the pulse of energy is called the radar image with a different look direction
far-range.
Slant Range Vs Ground Range
❖ Radar imagery has a different geometry
than that produced by most
conventional remote sensor systems.
❖ Uncorrected radar imagery is called
slant-range geometry, i.e., it is based on
the actual distance from the radar to
each of the respective features in the
scene.
❖ It is possible to convert the slant-range
display into the true ground-range
display on the x-axis so that features in
the scene are in their proper planimetric
(x,y) position relative to one another in
the final radar image.
Geometric Distortions of Radar Images
Slant-range scale distortion
o The radar is fundamentally a distance measuring device (i.e. measuring range).
o Slant-range scale distortion occurs because the radar is measuring the distance to
features in slant-range rather than the true horizontal distance along the ground.
o This results in a varying image scale, moving from near to far range.
o For Example, targets A1 and B1 are the same size on the ground, but their apparent
dimensions in slant range are A2 and B2, which are different.
o This causes targets in the near range to appear compressed relative to the far
range.
o Using trigonometry, ground-range distance can be calculated from the slant-range
distance and platform altitude to convert to the proper ground-range
Slant-Range Scale Distortion
Corrected image
Geometric Distortions of Radar Images
Foreshortening
When the radar beam reaches the base of a tall feature before it reaches the
feature’s top, then foreshortening occurs. Again, as the radar measures
distance in slant-range, the slope length (A to B) will appear compressed and
the length of the slope will be represented incorrectly (A' to B'). The severity
of foreshortening will vary due to the angle of the hillside or mountain slope
in relation to the incidence angle of the radar beam.
Maximum foreshortening occurs when the radar beam is perpendicular to
the slope such that the slope, the base, and the top are imaged
simultaneously (C to D). The length of the slope will be reduced to an
effective length of zero in slant range (C'D').
Geometric Distortions of Radar Images
Layover
Layover occurs when the radar beam reaches the top of a tall feature (B) before it
reaches the base (A). The return signal from the top of the feature will be received
before the signal from the bottom. As a result, the top of the feature is displaced
towards the radar from its true position on the ground, and "lays over" the base of the
feature (B' to A'). Layover effects on a radar image look very similar to effects due to
foreshortening. As with foreshortening, layover is most severe for small incidence
angles, at the near range of a swath, and in mountainous terrain.
Geometric Distortions of Radar Images
Shadowing
Both foreshortening and layover result in radar shadow. Radar shadow occurs when
the radar beam is not able to illuminate the ground surface. Shadows occur towards
the far range, behind vertical features or slopes with steep sides. Since the radar beam
does not illuminate the surface, shadowed regions will appear dark on an image as no
energy is available to be backscattered. As incidence angle increases from near to far
range, the shadow will be more as the radar beam looks more and more obliquely at
the surface. This image illustrates radar shadow effects on the right side of the
hillsides which are being illuminated from the left.
Red surfaces are completely in
shadow. Black areas in image are
shadowed and contain no
information.
Geometric Distortions of Radar Images
Comparison of Radar Distortions:
Distortion Type Cause Effect
Radar wave returns from
Foreshortening the base of a steep object Slope appears compressed
before its top
Radar wave returns from
Object appears tilted
Layover the top of a steep object
forward
before its base
Radar cannot reach the far Dark, featureless areas in
Shadowing
side of a tall object the image
Polarization
In case of microwave energy, the polarization of the radiation is important.
Polarization refers to the orientation of the electric field.
Most radar sensors are designed to transmit microwave radiation either horizontally
polarized (H) or vertically polarized (V).
Similarly, the antenna receives either the horizontally or vertically polarized
backscattered energy, and some radars can receive both.
These two polarization states are designated by the letters H for horizontal, and V, for
vertical.
Polarization
Thus, there can be four combinations of both transmit
and receive polarizations as follows:
HH - for horizontal transmit and horizontal receive,
VV - for vertical transmit and vertical receive,
HV - for horizontal transmit and vertical receive,
VH - for vertical transmit and horizontal receive.
▪ HH and VV configurations produce
like-polarized radar imagery.
▪ HV and VH configurations produce
cross-polarized imagery.
Radar antennas send and receive polarized energy. This means that the
pulse of energy is filtered so that its electrical wave vibrations are only in a
single plane that is perpendicular to the direction of travel.
The pulse of electromagnetic energy sent out by the antenna may be
vertically or horizontally polarized.
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