RADAR Imaging

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Introduction

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Introduction – Active microwave Radar

•• PassivePassive remote sensing systems record electromagnetic energy that remote sensing systems record electromagnetic energy that was reflected or emitted from the surface of the Earth. There arwas reflected or emitted from the surface of the Earth. There are e also active remote sensing systems that are not dependent on thealso active remote sensing systems that are not dependent on theSunSun’’s electromagnetic energy or the thermal properties of the Earth.s electromagnetic energy or the thermal properties of the Earth.

•• ActiveActive remote sensors create their own electromagnetic energy that remote sensors create their own electromagnetic energy that 1) is transmitted from the sensor toward the terrain (and is lar1) is transmitted from the sensor toward the terrain (and is largely gely unaffected by the atmosphere), 2) interacts with the terrain unaffected by the atmosphere), 2) interacts with the terrain producing a backscatter of energy, and 3) is recorded by the remproducing a backscatter of energy, and 3) is recorded by the remote ote sensorsensor’’s receiver.s receiver.

ActiveActive remote sensing systems: remote sensing systems: •• Active microwave (RADAR),Active microwave (RADAR), (e.g., 3 (e.g., 3 –– 25 cm);25 cm);•• LIDARLIDAR, laser light (e.g., 0.90 mm). Records the amount of light back, laser light (e.g., 0.90 mm). Records the amount of light back--scattered from the terrain; scattered from the terrain; •• SONARSONAR, transmission of sound waves, and then recording the amount of , transmission of sound waves, and then recording the amount of energy backenergy back--scattered.scattered.

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Introduction – Radar procedures

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Sending and receiving a pulse in microwave

The pulse is of specific duration (in μsec)

Real aperture side looking airborne radar (SLAR)Synthetic aperture radar (SAR)

transmitted pulse

backscattered pulse

antenna

TransmitterDuplexer

• sends and receives

Pulse Generator

CRT Display or Digital Recorder

Receiver

b.

a. antenna

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Radarsat I

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PALSAR modes

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TerraSAR-X modes

•Spotlight(ανάλυση έως και 1 m)•StripMap(ανάλυση έως και 3 m)•ScanSAR(ανάλυση έως και 16 m)

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Spotlight

Stripmap

ScanSAR

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Radar commonly used frequencies

•• Band DesignationsBand Designations•• (common wavelengths (common wavelengths Wavelength Frequencies Wavelength Frequencies •• shown in parentheses)shown in parentheses) in cmin cm in GHzin GHz•• ______________________________________________________________________________________________•• KK 1.18 1.18 -- 1.671.67 26.5 to 18.026.5 to 18.0•• Ka (0.86 cm)Ka (0.86 cm) 0.75 0.75 -- 1.181.18 40.0 to 26.540.0 to 26.5•• Ku Ku 1.67 1.67 -- 2.42.4 18.0 to 12.518.0 to 12.5•• X (3.0 and 3.2 cm)X (3.0 and 3.2 cm) 2.4 2.4 -- 3.83.8 12.5 12.5 -- 8.08.0•• C (7.5, 6.0 cm)C (7.5, 6.0 cm) 3.8 3.8 -- 7.57.5 8.0 8.0 -- 4.04.0•• S (8.0, 9.6, 12.6 cm)S (8.0, 9.6, 12.6 cm) 7.5 7.5 -- 15.015.0 4.0 4.0 -- 2.02.0•• L (23.5, 24.0, 25.0 cm)L (23.5, 24.0, 25.0 cm) 15.0 15.0 -- 30.0 30.0 2.0 2.0 -- 1.01.0•• P (68.0 cm)P (68.0 cm) 30.0 30.0 -- 100100 1.0 1.0 -- 0.30.3

The names (e.g., The names (e.g., K, Ka, Ku, X, C, K, Ka, Ku, X, C, S, L,S, L, and and PP) are ) are an artifact of the an artifact of the original secret original secret work on radar work on radar remote sensing.remote sensing.

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Atmospheric windows

• Here is a generalized diagram showing relative atmospheric radiation transmission of different wavelengths.

Blue zones mark minimal passage of incoming and/or outgoing radiation, whereas, white areas denote atmospheric windows.

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Advantages of RADAR in Remote sensing

•• It is an It is an allall--weatherweather remote sensing system. remote sensing system. •• Synoptic viewsSynoptic views of large areas, for mapping at 1:25,000 to 1:400,000; of large areas, for mapping at 1:25,000 to 1:400,000; •• Permits imaging at Permits imaging at shallow look anglesshallow look angles, resulting in different , resulting in different

perspectives. perspectives. •• Provides information on surface roughness, dielectric propertieProvides information on surface roughness, dielectric properties, s,

and moisture content.and moisture content.•• May May penetratepenetrate vegetation, sand, and surface layers of snow.vegetation, sand, and surface layers of snow.•• Enables Enables resolution to be independent of distance to the objectresolution to be independent of distance to the object, with , with

the size of a resolution cell being as small as 1 x 1 m.the size of a resolution cell being as small as 1 x 1 m.•• May operate simultaneously in several wavelengths (frequencies) May operate simultaneously in several wavelengths (frequencies)

and thus has and thus has multimulti--frequency potentialfrequency potential..•• Can Can measure ocean wave propertiesmeasure ocean wave properties, even from orbital altitudes., even from orbital altitudes.•• Can produce overlapping images suitable for stereoscopic viewingCan produce overlapping images suitable for stereoscopic viewing..•• Supports Supports interferometricinterferometric operation using two antennas for 3operation using two antennas for 3--D D

mapping.mapping.

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Fundamental Radar Equation

1 1Pr = Pt x Gt ____ σ ____ Ar

4πR2 4πR2

where Pr is power received, Pt is the power transmitted toward the target, Gtis the gain of the antenna in the direction of the target, R is the range distance from the transmitter to the target, σ is the effective backscatter area of the target, and Ar is the area of the receiving antenna.

1 11 1PPrr = P= Ptt x x GGtt ____ ____ σσ ____ ____ AArr

44ππRR2 2 44ππRR22

where where PPrr is power received, is power received, PPtt is the is the power transmitted toward the target, power transmitted toward the target, GGttis the gain of the antenna in the is the gain of the antenna in the direction of the target, direction of the target, RR is the range is the range distance from the transmitter to the distance from the transmitter to the target, target, σσ is the effective backscatter is the effective backscatter area of the target, and area of the target, and AArr is the area of is the area of the receiving antenna.the receiving antenna.

The modified fundamental radar equation is: Pt x G2 x σ x λ2

Pr = ________________(4π)3 x R4

The modified fundamental radar equation is: Pt x G2 x σ x λ2

Pr = ________________(4π)3 x R4

Effects of terrain on the radar signal:the amount of radar cross-section, σ , reflected back to the receiver, per unit area aon the ground called radar backscatter coefficient (σ ˚) σ˚ = σ

a• σ˚ for a surface depends on a number of terrain parameters like geometry, surface roughness, moisture content, and the radar system parameters (wavelength, depression angle, polarization, etc.).

Effects of terrain on the radar signal:Effects of terrain on the radar signal:the amount of the amount of radar crossradar cross--sectionsection, , σσ , , reflected back to the receiver, per unit area reflected back to the receiver, per unit area aaon the ground called on the ground called radar backscatter radar backscatter coefficientcoefficient ((σσ ˚̊) ) σσ˚̊ = = σσ

aa•• σσ˚̊ for a surface depends on a number of for a surface depends on a number of terrain parameters like geometry, surface terrain parameters like geometry, surface roughness, moisture content, and the radar roughness, moisture content, and the radar system parameters (wavelength, depression system parameters (wavelength, depression angle, polarization, etc.).angle, polarization, etc.).

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Surface Roughness

• We describe the surface texture characteristics. • Surface roughness is usually measured in centimeters (i.e. the height of stones, size of leaves, or length of branches in a tree) and not thousands of meters as with mountains. • In radar imagery we are actually talking about micro-relief surface roughness characteristics rather than topographic relief.

•• We describe the surface We describe the surface texture characteristicstexture characteristics. . •• Surface roughnessSurface roughness is usually measured in centimeters (i.e. the is usually measured in centimeters (i.e. the height of stones, size of leaves, or length of branches in a treheight of stones, size of leaves, or length of branches in a tree) and e) and not thousands of meters as with mountains. not thousands of meters as with mountains. •• In radar imagery we are actually talking about In radar imagery we are actually talking about micromicro--relief surface relief surface roughnessroughness characteristics rather than topographic relief.characteristics rather than topographic relief.

• Specular reflecting surface where most of the energy bounces off the terrain away from the antenna. Dark area on the radar image. The quantitative expression of the smooth criteria is:

h < __λ__25 sin γ

• A bright return is expected if the modified Rayleigh rough criteria are used:h > __λ__

4.4 sin γwavelength (λ), the depression angle (γ), height of objects (h in cm)

•• SpecularSpecular reflecting surfacereflecting surface where most of the energy bounces off the terrain where most of the energy bounces off the terrain away from the antenna. Dark area on the radar image. The quantitaway from the antenna. Dark area on the radar image. The quantitative ative expression of theexpression of the smooth criteriasmooth criteria is: is:

h < __h < __λλ____25 sin 25 sin γγ

•• A bright return is expected if the modified A bright return is expected if the modified RayleighRayleigh rough criteriarough criteria are used:are used:h > __h > __λλ____

4.4 sin 4.4 sin γγwavelength (wavelength (λλ), the depression angle (), the depression angle (γγ), height of objects (), height of objects (hh in cm)in cm)

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Surface Roughness

Expected surface roughness Expected surface roughness backback--scatter from terrain scatter from terrain

illuminated with 3 cm illuminated with 3 cm wavelength microwave wavelength microwave

energy with a depression energy with a depression angle of 45angle of 45˚̊..

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Types of Active Types of Active Microwave Surface Microwave Surface and Volume Scattering and Volume Scattering that Take Place in a that Take Place in a Hypothetical Pine Hypothetical Pine Forest StandForest Stand

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Response to X-, C- and L-band Microwave Energy

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Polarization

•• Radar antennas send Radar antennas send and receive and receive polarizedpolarizedenergy. energy.

•• Electrical wave Electrical wave vibrations are in a vibrations are in a single plane single plane perpendicular to the perpendicular to the direction of travel. direction of travel.

•• The pulse may be The pulse may be vertically vertically ororhorizontallyhorizontallypolarizedpolarized..

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Polarization

It is possible to:It is possible to:•• send and receive vertically send and receive vertically

((VVVV) ) •• send and receive horizontally send and receive horizontally

((HHHH))•• send horizontal and receive send horizontal and receive

vertically (vertically (HVHV))•• send vertical and receive send vertical and receive

horizontally (horizontally (VHVH) ) •• HHHH and and VVVV produce produce likelike--

polarizedpolarized imagery. imagery. •• HVHV and and VHVH produce produce crosscross--

polarizedpolarized imagery. imagery. .

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near and far range incidenceangles, θ

near and far range

depressionangles, γ

θ

γ

range look direction

azimuth flight direction

flightline ground track

near range

far range

puls

e of

mic

row

ave

ener

gy

γ

f

n

nadir

θ

altitude aboveground level, H

n

f

•• azimuth flight direction azimuth flight direction •• look directionlook direction•• range (near and far)range (near and far)•• depression angle (depression angle (γγ))•• incidence angle (incidence angle (θθ=90=90--γγ))•• altitude abovealtitude above--groundground--level, level, HH

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SAR geometry

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•• Radar imagery has a Radar imagery has a different geometrydifferent geometry than than that produced by most that produced by most conventional remote conventional remote sensor systems.sensor systems.

•• Uncorrected radar Uncorrected radar imagery is displayed in imagery is displayed in what is called what is called slantslant--range range geometrygeometry, i.e., it is based , i.e., it is based on the actual distance on the actual distance from the radar to each of from the radar to each of the respective features in the respective features in the scene. the scene.

•• It is possible to convert the It is possible to convert the slantslant--range display into range display into the true the true groundground--range range displaydisplay on the xon the x--axis so axis so that features in the scene that features in the scene are in their proper (are in their proper (x,yx,y) ) position relative to one position relative to one another in the final image.another in the final image.

Slant range vs ground range

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Slant range vs ground range