SAR Image Acquisition and
Characteristics
(Lecture I- Monday 21 December 2015)
Training Course on Radar Remote Sensing and Image Processing
21-24 December 2015, Karachi, Pakistan
Organizers: IST & ISNET
Parviz Tarikhi, [email protected]
http://parviztarikhi.wordpress.com
Alborz Space Center, ISA, Iran
OutlineMicrowaves and Radar,
Non-Imaging radar and Imaging Radar,
SLAR,
RAR and SAR,
Radar Image Geometry,
Wavelength,
Incidence Angle,
Polarization,
SAR Spatial Resolution,
Speckle,
Radar Looks,
Radar Shadow,
Layover
Foreshortening.
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SLAR and SAR
Geometric Properties
Radiometric Properties
sigma nought
Imaging modes and similar basic topics
(Mani 2104)
RADAR:
• Radio Detection And Ranging
• It is a ranging instrument
• Range means distances inferred from time
elapsed between transmission of a signal and
reception of the returned signal
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Main types of microwave data acquisition:
• Non-imaging
– Traffic police radars 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– Usually high spatial resolution,
– Consists of a transmitter, a receiver, one or more antennas, GPS, computers
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RADAR is the most common form of active
microwave imaging sensor
Non-imaging microwave sensors include
Altimeters and scatterometers
imaging radars (side-looking) used to acquire
images (~10m - 1km)
altimeters (nadir-looking) to estimate surface height
variations
scatterometers to derive reflectivity as a function of
incidence angle, illumination direction,
polarization, etc
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Two imaging radar systems
• Real aperture radar (RAR)
– Aperture means antenna
– A fixed length (for example: 1 - 15m)
– SLAR is usually a real aperture radar. The longer the antenna (but there is limitation), the better the spatial resolution
• 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
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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
(Man
i 2
10
4)
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Active and Passive Radar Imaging Systems
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 envent.
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Component of RADAR
• A Radar system performs 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 and can
operate day/night
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How Radar imaging system works
Microwave pulses (A) are emitted at regular intervals and focused by
the antenna into a radar beam (B) directed downwards. 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).
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Principle of
ranging and
imaging in
Side-looking
Airborne
Radar
(SLAR)
Tree is less reflective of radar waves than the house, a
weaker response is recorded in the graph13
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By electronically measuring the return time of signal
echoes, 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
Presence of the factor 2 in the equation i,mplird
thabecause the time is measured for the pulse to travel
both the distance to and from the target
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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.
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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.
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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
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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".
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Range resolution (across track): RAR
τ
Since A-B is < PL/2 then cannot resolve A & B
Dependence of range
resolution on pulse length
Pulse
of length
PL (duration
of the pulse
transmission)
has been
transmitted
towards
buildings A and B
The slant range distance (the direct sensor to target distance) between the
buildings is less than PL/2
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(Man
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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.
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Relationship between slant-range resolution and
ground-range resolution
Relationship between slant-range resolution and ground-range
resolution 111
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
τ is the pulse duration.
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Range (or across-track) Resolution
cos2
ctRr
• t.c is 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
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(Man
i 2104
)
Azimuth (or along-track) Resolution
L
SRa
L
SRa
L = antenna length
S = slant range = height H/sin
λ = wavelength
L sinγ
H
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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 spacecraft or aircraft, the azimuth resolution
deteriorates. Objects at points A and B would be resolved (imaged separately) at GR1 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
The beamwidth of the antenna is then
λ : the wavelength
AL : the antenna length
Azimuth resolution: SAR
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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 scatterer is
Pt Gt, where the value of Gt in that direction is used. 2718-Jan-16
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Pr = Pt Gt Gr λ2σ
(4π)3 R4
Reeves, (1979)
G = Gt = Gr
Pr = Pt G2 λ2σ
(4π)3 R4
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-
section
The Radar Equation
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Amount of backscatter per unit areahttp://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar_Course_III/parameters_affecting.htm29
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sin8h
Intermediate wrong
•Peake and Oliver
(1971) – surface
height variation h
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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
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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
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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.
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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
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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)
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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.
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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.
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Radar Image Geometry - Shadow
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5
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A slope away from the radar illumination
with an angle that’s steeper than the
sensor depression angle provokes radar
shadows.
European Space Agency
Incidence Angle
Radar Image Geometry - Shadow
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Shadow is more of a problem at far range
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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.49
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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 slant-
range direction such that the slope A-B appears as compressed in the
image (A'B') and slope C-D is severely compressed (C'D') 5018-Jan-16
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Penetration ability
into subsurface
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57
Nicobar
Islands
December 2004
tsunami flooding
in red
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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 resolution58
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SIR-C image of Nile Paleochannel, Sudan
• 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 SIR-C/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
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Nov. 2002 Oil spill in Spain• 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
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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.
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Acquisition mode is directly linked with
the resolution of the resulting image and
the size of the scene area covered.
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5
Wavelength
As detailed in table 2, radar remote sensing uses the microwave portion of
the electromagnetic spectrum, from a frequency of 0.3 GHz to 300 GHz.
Most radar satellites operate at wavelengths between 0.5 cm and 75 cm.
Shorter wavelengths—e.g., X-band imagery at 3 cm—are reflected from the
top of the canopy, while longer wavelengths—e.g., L-band imagery at 24
cm—normally go down to the ground and are reflected from there. Using
this characteristic of different wavelengths makes it possible to discern
information about the canopy structure of a forested area from a
multiwavelength image and thus estimate above-ground biomass.
Furthermore, the choice of wavelength needs to be matched to the size of
the surface feature that should be distinguishable. Small features are best
recognized with X-band imagery—i.e., short wavelengths—while large
features, such as geology, are better marked in L-band imagery.
(JA
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Wavelength
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Radar bands, frequencies and wavelengths
(Chri
stia
n W
olf
f-ra
dar
tuto
rial
)
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Band Frequency Wavelength Key CharacteristicsX Band 12.5-8 GHz 2.4-3.75 cm Widely used for military reconnaissance, mapping and surveillance (TerraSAR-X, TanDEM-X, COSMO-
SkyMed)
C Band 8-4 GHz 3.75-7.5 cm Penetration capability of vegetation or solids is limited and restricted to the top layers. Useful for sea-ice
surveillance (RADARSAT, ERS-1).
S Band 4-2 GHz 7.5-15 cm Used for medium-range meteorological applications—e.g., rainfall measurement, airport surveillance
L Band 2-1 GHz 15-30 cm Penetrates vegetation to support observation applications over vegetated surfaces and for monitoring ice
sheet and glacier dynamics (ALOS PALSAR)
P Band 1-0.3 GHz 30-100 cm To date only used for research and experimental applications. Significant penetration capabilities regarding
vegetation canopy (key element for estimating vegetation biomass), sea ice, soil, glaciers.
Canopy penetration varies with
different wavelengths.
Wavelength
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Thank you!
&
any question