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, PhDparviz_tarikhi@hotmail.com

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

i 2104

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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

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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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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