Radar remote sensing, P K MANI

Radar Remote sensing

Dr. P. K. Mani

Bidhan Chandra Krishi Viswavidyalaya
E-mail: pabitramani@gmail.com
Website: www.bckv.edu.in

What is RADAR?
• Radio Detection and Ranging
• Radar is a ranging instrument
• (range) distances inferred from time elapsed
between transmission of a signal and
reception of the returned signal

3

Types of radar:
• Nonimaging radar
– Traffic police use hand held 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, such as
NEXRDA
– Satellite-based radar altimeters
(low spatial resolution but high vertical resolution)

• Imaging radar
– Usually high spatial resolution,
– Consists of a transmitter, a receiver, one or more

The most common form of imaging active
microwave sensors is
RADAR
Non-imaging microwave sensors include
Altimeters and scatterometers.
imaging radars (side-looking) used to acquire
images (~10m - 1km)
altimeters (nadir-looking) to derive surface height
variations
scatterometers to derive reflectivity as a function of
incident angle, illumination direction,
polarisation, etc

NonImaging
Radar
To provide a polar-coordinate maplike display of targets, NRL
originated the radar PLANPOSITION INDICATOR (PPI)-the
well-known radar scope with the
round face and the sweeping handbetween 1939 and 1940. The PPI is
now universally used by military and
commercial interests around the
world for the display of radar
information for such functions as air
and surface detection, navigation,
air traffic control, air intercept, and
object identification

Two imaging radar systems
In World War II, ground based radar was used to detect incoming
planes and ships (non-imaging radar).
Imaging RADAR was not developed until the 1950s (after World
War II).
Since then, side-looking airborne radar (SLAR) has been used to
get detailed images of enemy sites along the edge of the flight
field.
SLAR is usually a real aperture radar. The longer the antenna (but
there is limitation), the better the spatial resolution
• Real aperture radar (RAR)
– Aperture means antenna
– A fixed length (for example: 1 - 15m)
• Synthetic aperture radar (SAR)
– 1m (11m) antenna can be synthesized electronically into a
600m (15 km) synthetic length.
– Most (air-, space-borne) radar systems now use SAR.

Advantages
•
•
•
•
•
•

All time / all weather capability
Information on surface roughness at the “human” scale
Centimeters rather than microns
Penetration of soil : function of the dielectric constant
Rule of thumb is that for dry soils, penetration depth (cm) = 10
For hyper-arid environments, radar can penetrate 3-5 m

Disadvantages
• Very costly
• Imagery is complex and typically hard to interpret
• Little to no information on composition of the surface materials

Imaging Radar - Advantages
• Active system (works day or night).
– There is also passive microwave imaging
(radiometer) mode. This senses surface radioemission, which can be converted to radiant
temperatures.
• Not affected by cloud cover or haze if λ > 2 cm. It
operates independent of weather conditions. Water
clouds have a significant effect on radar with wavelength
λ < 2 cm.
• Unaffected by rain λ > 4 cm.
• Can penetrate well-sorted dry sand in hyper-arid regions
to a depth of about 2 m.

Active and

Passive
Systems

Radar Imaging
Active radar systems
transmit short bursts or
'pulses' of
electromagnetic energy
in the direction of
interest and record the
origin and strength of
the backscatter received
from objects within the
system's field of view.
Passive radar systems
sense low level
microwave radiation
given off by all objects
in the natural envt.

Component of RADAR
• A Radar system has three primary functions:
- It transmits microwave (radio) signals
towards a
scene
- It receives the portion of the transmitted
energy backscattered from the scene
- It observes the strength (detection) and the
time
delay (ranging) of the return signals.
• Radar is an active remote sensing system &
can operate day/night
14

How Radar Works
Microwave energy pulses (A) are
emitted at regular intervals and
focused by the antenna into a radar
beam (B) directed downwards and
to the side. The radar beam
illuminates the surface obliquely at
a right angle to the motion of the
platform. Objects on the ground
reflect the microwave energy
depending on factors such as
roughness and attitude. The
antenna receives this reflected (or
backscattered) energy (C).

Principle
of
ranging
and
imaging
in Sidelooking
Airborne
Radar
(SLAR)
Tree is less reflective of radar waves than the house, a weaker response is recorded in
16
the graph

Tree is less reflective of radar waves than the house, a
weaker response is recorded in the graph

By electronically measuring the return time of signal ec
hoes, the range or distance, between the transmitter and
reflecting objects, may be determined.
Since the energy propagates in air at approximately the
velocity of light c, the slant range, SR, to any given object is
given by,
SR= ct/2
the factor 2 enters into the equation because the time is
measured for the pulse to travel both the distance to and
from the target

How Radar Works
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. As the sensor platform moves
forward, recording and processing of the backscattered
signals builds up a two-dimensional image of the surface.

Radar Geometry
• In airborne and spaceborne radar imaging systems, the
platform travels forward in the flight direction (A) with the
nadir (B) directly beneath the platform. The microwave beam
is transmitted obliquely at right angles to the direction of flight
illuminating a swath (C) which is offset from nadir. Range (D)
refers to the across-track dimension perpendicular
to the flight direction, while
azimuth (E) refers to the
along-track dimension
parallel to the flight
direction.

Near Range is the portion of the image swath closest to the nadir track
 Far Range is the portion of the swath farthest from the nadir track.
Depression or Grazing Angle is the angle between the horizontal and a
radar ray path.
Slant Range Distance is the radial line of sight distance between the
radar and each target on the surface.
Ground Range Distance is the true horizontal distance along the ground
corresponding to each point measured in slant range.
 Incidence Angle is the angle
between the radar beam and
ground surface
 Look Angle is the angle
at which the radar "looks“
at the surface, or the
angle between vertical
and a ray path

Backscatter
• The portion of the outgoing
radar signal that the target
redirects directly back
towards the radar antenna.
• When a radar system
transmits a pulse of energy to
the ground (A), it scatters off
the ground in all directions
(C). A portion of the
scattered energy is directed
back toward the radar
receiver (B), and this portion
is referred to as
"backscatter".

Range resolution (across track): RAR
Pulse of length PL (duration of the pulse transmission)
has been transmitted towards buildings A and B

τ

Note that the slant range distance (the direct sensor to target distance)
between the buildings is less than PL/2
i.e. A-B is < PL/2 cannot resolve A & B

Dependence of range resolution on pulse length

22

For a SLAR system to image separately two ground features
that are close to each other in the range direction, it is necessary
for all parts of the two objects reflected signals to be
received separately by the antenna. Any time overlap
between the signals from two objects will cause their images to
be blurred together.
Because of this propagation of
wavefront, pulse has had time to
travel to B and have its echo
returns to A while the end of
the pulse at A continues to be
reflected. Consequently, the two
signals are overlapped and will
be imaged as one large object
extending from building A to
building B. If the slant range
distance betweenA and B were
anything greater than Pl/2, the
two signals would be received
separately, resulting in two
separate image responses.

Although the slant-range resolution of an SLR system does
not change with distance from the aircraft, the corresponding
ground-range resolution does. As shown in Figure 8.6, the
ground resolution in the range direction varies inversely
with the cosine of the depression angle. This means that the
ground-range resolution becomes smaller with increases in
the slant-range
distance.
Accounting for the depression angle effect, the ground
resolution in the range direction Rr is found from
where τ is the pulse duration.

Range (or across-track) Resolution

t ⋅c
Rr =
2 cos γ
• t.c called pulse
length. It seems the
short pulse length
will lead fine range
resolution.
• However, the
shorter the pulse
length, the less the
total amount of
energy that
illuminates the
target.

t.c/2

t.c/2

Azimuth (or along-track) Resolution
S ⋅λ
Ra =
L

Ra =

Sλ
L

=

Hλ
L sinγ

L = antenna length
S = slant range = height H/sinγ
λ = wavelength

As shown in Figure 8.7, the resolution of an SLR system in the azimuth
direction, Ra, is determined by the angular beam width β of the antenna
and the ground range GR. As the antenna beam "fans out" with increasing
distance from the aircraft, the azimuth resolution deteriorates. Objects at
points A and B in Figure 8.6 would be resolved (imaged separately) at GR 1
but not at GR2. That is, at distance GR1 , A and B result in separate return
signals. At GR2, distance, A and B would be in the beam simultaneously and
would not be resolved.
Azimuth resolution Ra is given by

A given SLAR system has a 1.8-mrad
antenna beamwidth. Determine the
azimuth resolution of the system at ranges
of 6 and 12 km.

The Radar Equation

Relates characteristics of the radar, the target, and the
received signal
The geometry of scattering from an isolated radar target
(scatterer) is shown.
When a power Pt is transmitted by an antenna with gain Gt ,
the power per unit solid angle in the direction of the scatterer31
is
P G , where the value of G in that direction is used.

The Radar Equation
2

Pr = Pt Gt Gr λ σ
(4π)3 R4
G = G t = Gr
Pr = Pt G2
(4π)3 R4

λ

2

σ

Reeves, (1979)
Pt= transmitted power
Pr= received power
Gt= gain of transmitted
antenna
Gr= gain of receiver
antenna
R= distance between
target and sensor
λ= wavelength of
radiation
σ = scattering cross-

Amount of backscatter per unit area
http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/parameters_affecting.htm

Intermediate

λ
h=
8 sin γ

•Peake and Oliver
(1971) – surface height
variation h

wrong

Penetration of the radar signal
• Can penetrate vegetative cover and soil
surface
• Depth of penetration is assessed by the
skin depth – the depth to which the
strength of a signal is reduced.
• Skin depth increases with increasing
wavelength and in the absence of
moisture

Penetration of the radar signal
• Optimum penetration is in arid and long
wavelength radiation
• Penetration also related to surface
roughness and incidence angle. The
steeper the incidence angle the greater
the penetration.
• There is no clear defined way to assess
penetration

Polarization
• Denotes the orientation of the field of EM
energy emitted and received by the
antenna.
• Radar systems can be configures to
transmit and receive either horizontally or
vertically.
• Unless otherwise specified, an imaging
radar transmits and receives horizontal
polarized EM waves.

Polarization
• Some systems produce combinations
– HH-image or the like-polarized mode
– HV-image or the cross-polarized mode

• Comparing the two images, the interpreter
can identify features that tend to
depolarize the signal.
• Example: bright HV image vs dark HH
image

Polarization
• Causes of depolarization is related to
physical and electrical properties (rough
surface with respect to wavelength)
• Volume scattering from an
inhomogeneous medium (occurs when the
radar penetrates the ground)

Radar Shadow
• Shadows in radar images can enhance the geomorphology
and texture of the terrain. Shadows can also obscure the
most important features in a radar image,
such as the information behind tall buildings or land use in
deep valleys. If certain conditions are met, any feature
protruding above the local datum can cause the incident
pulse of microwave energy to reflect all of its energy on the
foreslope of the object and produce a black shadow for the
backslope
• Unlike airphotos, where light may be scattered into the
shadow area and then recorded on film, there is no
information within the radar shadow area. It is black.

Shadow is more of a problem at far range

Radar Image Geometry - Layover

Layover occurs when the radar beam
reaches the top of a tall feature before it
reaches the base. The top of the feature
is displaced towards the radar sensor
and is displaced from its true ground
position - it 'lays over' the base. The
visual effect on the image is similar to
that of foreshortening.

Foreshortening
• Even if there is no layover, radar returns from facing steep
slopes will make the terrain look steeper than it is. This is
known as ‘foreshortening’. Features which show layover in
the near range will show foreshortening in the far range.

Foreshortening occurs because radar measure distance in the slantrange direction such that the slope A-B appears as compressed in the
image (A'B') and slope C-D is severely compressed (C'D')

Radar Signal Polarization
Polarization of the radar signal is the orientation of the the
electromagnetic field and is a factor in the way in which the
radar signal interacts with ground objects and the resulting
energy reflected back. Most radar imaging sensors are
designed to transmit microwave radiation either horizontally
polarized (H) or vertically polarized (V), and receive either
the horizontally or vertically polarized backscattered energy.

Penetration ability
into subsurface

Nicobar
Islands

December 2004
tsunami flooding
in red

60

SIR-C Image of Vesuvius
and Naples, Italy
•

Mt. Vesuvius, one of the best known
volcanoes in the world primarily for the
eruption that buried the Roman city of
Pompeii in AD 79, is shown in the
center of this radar image. The central
cone of Vesuvius is the dark purple
feature in the center of the volcano.
This cone is surrounded on the northern
and eastern sides by the old crater rim,
called Mt. Somma. Recent lava flows
are the pale yellow areas on the
southern and western sides of the cone.
It shows an area 100 kilometers by 55
kilometers (62 miles by 34 miles.)
Shuttle

Imagery Radar-C, April and
Sept. 1994, 10 days each.
X-, C-, L- bands multipolarization
(HH, VV, HV, VH),
10-30 m resolution

•

•

The top image is a
photograph taken with color
infrared film from Space
Shuttle Columbia in
November 1995. The radar
image at the bottom is a SIRC/X-SAR image. The thick,
white band in the top right of
the radar image is an ancient
channel of the Nile that is
now buried under layers of
sand. This channel cannot be
seen in the photograph and its
existence was not known
before this radar image was
processed. The area to the left
in both images shows how
the Nile is forced to flow
through a chaotic set of
fractures that causes the river
to break up into smaller
channels, suggesting that the
Nile has only recently
established this course. Each
image is about 50 kilometers
by 19 kilometers.
Red = Chv; Green = Lhv;
Blue = Lhh

SIR-C image of Nile
Paleochannel, Sudan

•

A damaged oil
tanker off the
northwest
coast of Spain
split in half on
November 19,
2002, creating
a series of
large oil slicks.
The image
shows the oil
slick with
RADARSAT
data. Black
areas indicate
the location of
the slick on
November 18.
The land is
shown using
Landsat
falsecolor

Nov. 2002 Oil spill in Spain

Archeology of Angor,
Cambodia
•

•
•

The city houses an ancient complex of
more than 60 temples dating to the 9th to
15th centuries. Today the Angkor complex
is hidden beneath a dense rainforest
canopy, making it difficult for researchers
on the ground. The principal complex,
Angkor Wat, is the bright square just left
of the center of the image. It is surrounded
by a reservoir that appears in this image as
a thick black line. The larger bright square
above Angkor Wat is another temple
complex called Angkor Thom.
Archeologists studying this image believe
the blue-purple area slightly north of
Angkor Thom may be previously
undiscovered structures. In the lower right
is a bright rectangle surrounded by a dark
reservoir, which houses the temple
complex Chau Srei Vibol.
Image is 55 x 85km.
Red=L hh, Green =L hv, and Blue =C hv.

Radar remote sensing, P K MANI

Radar remote sensing, P K MANI

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