07 Microwave Rs

1

CHAPTER 9CHAPTER 9CHAPTER 9:CHAPTER 9:Active and PassiveActive and Passive

Microwave RSMicrowave RSREFERENCE: Remote Sensing REFERENCE: Remote Sensing of the Environment of the Environment John R. Jensen (2007)John R. Jensen (2007)Second EditionSecond EditionPearson Prentice HallPearson Prentice Hall

Passive Remote SensingPassive Remote Sensing

Passive remote sensing systems record electromagnetic energy that was reflected (e.g., blue, green, red, and near-infrared light) or emitted (e.g., thermal infrared energy) from the surface of the E hEarth.

2

SSM/I Passive Microwave Radiometer Image of SSM/I Passive Microwave Radiometer Image of the Amazon Barin Obtained at a Frequency of the Amazon Barin Obtained at a Frequency of

85 GHz with Vertical Polarization85 GHz with Vertical Polarization

Active Remote SensingActive Remote Sensing

There are also active remote sensing systems that are not dependent on the Sun’s electromagnetic energy or the thermal properties of the Earth.

Active remote sensors create their own electromagnetic Active remote sensors create their own electromagnetic energy that 1) is transmitted from the sensor toward the terrain

(and is largely unaffected by the atmosphere), 2) interacts with the terrain producing a backscatter of

energy, and 3) is recorded by the remote sensor’s receiver.

3

The most widely used active remote sensing systems include:

• acti e mic o a e (RADAR) hich is based on the

Active Remote SensingActive Remote Sensing

• active microwave (RADAR), which is based on the transmission of long-wavelength microwaves (e.g., 3 –25 cm) through the atmosphere and then recording the amount of energy back-scattered from the terrain;

• LIDAR, which is based on the transmission of relatively short-wavelength laser light (e.g., 0.90 μm) and then recording the amount of light back-scattered g gfrom the terrain; and

• SONAR, which is based on the transmission of sound waves through a water column and then recording the amount of energy back-scattered from the bottom or from objects within the water column.

Sending and Receiving a Sending and Receiving a Pulse of Microwave EMR Pulse of Microwave EMR --

System ComponentsSystem Components

The discussion is based initially on the system components and functions of a real aperture side-looking airborne radar (SLAR).

Th di i th d t The discussion then expands to include synthetic aperture radars (SAR) that have improved capabilities.

4

SideSide--looking Airborne looking Airborne RADAR (SLAR) SystemRADAR (SLAR) System

a. antenna

transmitted pulse

backscattered pulse

antenna

TransmitterDuplexer

• sends and receives

Pulse Generator

CRT Display or Digital Recorder

Receiver

b.

SLAR Data of Puerto RicoSLAR Data of Puerto Rico

http://pubs.usgs.gov/of/of00-006/htm/slar.htm

5

Sending and Receiving a Sending and Receiving a Pulse of Microwave EMR Pulse of Microwave EMR --

System ComponentsSystem Components

The pulse of electromagnetic radiation sent out by the transmitter through the antenna is of a specific wavelength and duration (i.e., it has a pulse length measured in microseconds, msec).

• The wavelengths are much longer than visible, near-infrared, mid-infrared, or thermal infrared energy used in other remote sensing systems. Therefore, microwave energy is usually measured in centimeters rather than micrometers is usually measured in centimeters rather than micrometers.

• The unusual names associated with the radar wavelengths (e.g., K, Ka, Ku, X, C, S, L, and P) are an artifact of the original secret work on radar remote sensing when it was customary to use the alphabetic descriptor instead of the actual wavelength or frequency.

6

Active Microwave (RADAR)Active Microwave (RADAR)Commonly Use FrequenciesCommonly Use Frequencies

K K X C S L P

Microwave Bands

0.2 μm 1.0 μm 10 μm 1 mm 1 cm 10 cm 1 m

Visible Middle-IR Thermal infrared

10 GHz 1 GHz

K K X C S L P UV Near-infrared a u

RADAR Wavelengths and Frequencies RADAR Wavelengths and Frequencies used in Active Microwave Remote used in Active Microwave Remote

Sensing InvestigationsSensing Investigations

Band Designations(common wavelengths Wavelength Frequencyshown in parentheses) in cm in GHz_______________________________________________K 1.18 - 1.67 26.5 to 18.0Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5Ku 1.67 - 2.4 18.0 to 12.5X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0C (7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0S (8.0, 9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0P (68.0 cm) 30.0 - 100 1.0 - 0.3

7

SIRSIR--C/XC/X--SAR SAR Images of a Images of a Portion of Portion of Rondonia, Rondonia,

Brazil, Brazil, Obtained on Obtained on

April 10, 1994April 10, 1994

Primary Advantages of RADAR Primary Advantages of RADAR Remote Sensing of the EnvironmentRemote Sensing of the Environment

• Active microwave energy penetrates clouds and can be anall-weather remote sensing system all-weather remote sensing system.

• Synoptic views of large areas, for mapping at 1:25,000 to 1:400,000; cloud-shrouded countries may be imaged.

• Coverage can be obtained at user-specified times, even at night.

• Permits imaging at shallow look angles, resulting in different e s ag g a s a o oo a g es, esu g d e eperspectives that cannot always be obtained using aerial photography.

• Senses in wavelengths outside the visible and infrared regions of the electromagnetic spectrum, providing information on surface roughness, dielectric properties, and moisture content.

8

• May penetrate vegetation, sand, and surface layers of snow.

• Has its own illumination, and the angle of illumination can be controlled.

Secondary Advantages of RADAR Secondary Advantages of RADAR Remote Sensing of the EnvironmentRemote Sensing of the Environment

, g

• Enables resolution to be independent of distance to the object, with the size of a resolution cell being as small as 1 x 1 m.

• Images can be produced from different types of polarized energy (HH, HV, VV, VH).

• May operate simultaneously in several wavelengths (frequencies) and thus has multi-frequency potential.

• Can measure ocean wave properties, even from orbital altitudes.

• Can produce overlapping images suitable for stereoscopic viewing and radargrammetry.

• Supports interferometric operation using two antennas for 3-D mapping, and analysis of incident-angle signatures of objects.

Radar Nomenclature

• nadir

• azimuth flight near and far range

depression

γ

range look direction

azimuth flight direction

γ

f

n

• azimuth flight direction

• look direction

• range (near and far)

• depression angle (γ)near and far range i id

pangles, γ

puls

e of

mic

row

ave

ener

gy

θ

altitude aboveground level, H

• incidence angle (θ)

• altitude above-ground-level, H

• polarization

incidenceangles, θ

θ

flightline ground track

near range

far range

nadir

θ n

f

9

RADARRADARlogiclogic

• The aircraft travels in a straight line that is called the azimuth flight direction

Azimuth DirectionAzimuth Direction

called the azimuth flight direction.

• Pulses of active microwave electromagnetic energy illuminate strips of the terrain at right angles (orthogonal) to the aircraft’s direction of travel, which is called the range or look direction. look direction.

• The terrain illuminated nearest the aircraft in the line of sight is called the near-range. The farthest point of terrain illuminated by the pulse of energy is called the far-range.

10

The range or look direction for any radar image is the direction of the radar illumination that is at

Range DirectionRange Direction

right angles to the direction the aircraft or spacecraft is traveling.

• Generally, objects that trend (or strike) in a direction that is orthogonal (perpendicular) to the range or look direction are enhanced much more than those objects in the terrain that lie parallel than those objects in the terrain that lie parallel to the look direction. Consequently, linear features that appear dark or are imperceptible in a radar image using one look direction may appear bright in another radar image with a different look direction.

Look Look DirectionDirection

a.X - band, HH polarization look direction

b.look directionsX - band, HH polarization

11

The depression angle (γ) is the angle between a horizontal plane extending out from the aircraft

Depression AngleDepression Angle

fuselage and the electromagnetic pulse of energy from the antenna to a specific point on the ground.

• The depression angle within a strip of illuminated terrain varies from the near-rangedepression angle to the far range depression depression angle to the far-range depression angle. The average depression angle of a radar image is computed by selecting a point midway between the near and far-range in the image strip. Summaries of radar systems often only report the average depression angle.

The incident angle (θ) is the angle between the radar pulse of EMR and a line perpendicular to the Earth’s surface where it makes contact. When the

Incident AngleIncident Angle

terrain is flat, the incident angle (θ) is the complement (θ = 90 - g) 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

f l 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.

12

Unpolarized energy vibrates in all possible

PolarizationPolarization

directions perpendicular to the direction of travel.

• Radar antennas send and receive polarizedenergy. This means that the pulse of energy is filtered so that its electrical wave vibrations are only in a single plane that is 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 horizontallypolarized.


13


a.look directionKa - band, HH polarization

b.

NKa - band, HV polarization

Th t itt d l f l t ti


The transmitted pulse of electromagneticenergy interacts with the terrain and someof it is back-scattered at the speed of lighttoward the aircraft or spacecraft where itonce again must pass through a filter. If theantenna accepts the back-scattered energy,it is recorded Various types of back-it is recorded. Various types of back-scattered polarized energy may be recordedby the radar.

14

It is possible to:

• send vertically polarized energy and receive


• send vertically polarized energy and receive only vertically polarized energy (designated VV),

• send horizontal and receive horizontally polarized energy (HH),

• send horizontal and receive vertically polarized • send horizontal and receive vertically polarized energy (HV), or

• send vertical and receive horizontally polarized energy (VH).


• HH and VV configurations produce like-polarized radar imagery.

• HV and VH configurations produce cross-polarized imagery.

15

SlantSlant--range versus range versus GroundGround--Range GeometryRange Geometry

Radar imagery has a different geometry than that d d b t ti l t t produced by most conventional remote sensor systems,

such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements.

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

16

RADAR ResolutionRADAR Resolution

To determine the spatial resolution at any point in a radar image, it is necessary to compute the in a radar image, it is necessary to compute the resolution in two dimensions: the range and azimuth resolutions. Radar is in effect a ranging device that measures the distance to objects in the terrain by means of sending out and receiving pulses of active microwave energy. The range resolution in the across-track direction is proportional to the length of the microwave pulse. The shorter the pulse length, the finer the range resolution. Pulse length is a function of the speed of light (c) multiplied by the duration of the transmission (t).

Range ResolutionRange ResolutionThe range resolution (Rr) at any point between the near and far-range of the illuminated strip can be computed if the depression angle (γ) of the sensor at that location

d th l l th ( ) k It i ibl t and the pulse length (τ) are known. It is possible to convert pulse length into distance by multiplying it times the speed of light (c = 3 x 108 m sec-1). The resulting distance is measured in the slant-range previously discussed. Because we want to know the range resolution in the ground-range (not the slant-range) it is necessary to convert slant-range to ground-range by dividing the slant-range distance by the cosine of the dividing the slant range distance by the cosine of the depression angle (γ). Therefore, the equation for computing the range resolution is:

τ x cRr = __________

2 cos γ

17

Range Range ResolutionResolution

τ x cRr = __________

2 cos γ

Azimuth ResolutionAzimuth Resolution

Thus far we have only identified the length inmeters of an active microwave resolutionelement at a specific depression angle andpulse length in the range (across-track)direction. To know both the length and widthof the resolution element, we must alsocompute the width of the resolution elementcompute the width of the resolution elementin the direction the aircraft or spacecraft isflying — the azimuth direction.

18

Azimuth resolution (Ra) is determined by computing thewidth of the terrain strip that is illuminated by the radar


width of the terrain strip that is illuminated by the radarbeam.

• Real aperture active microwave radars produce a lobe-shaped beam which is narrower in the near-range andspreads out in the far-range. Basically, the angular beamwidth is directly proportional to the wavelength of thetransmitted pulse of energy, i.e., the longer thewavelength, the wider the beam width, and the shorter thewavelength, the narrower the beam width. Therefore, inreal aperture (brute force) radars a shorter wavelengthpulse will result in improved azimuth resolution.Unfortunately, the shorter the wavelength, the poorer theatmospheric and vegetation penetration capability.

Fortunately, the beam width is also inversely proportional to antenna length (L) This means that the


proportional to antenna length (L). This means that the longer the radar antenna, the narrower the beam widthand the higher the azimuth resolution. The relationship between wavelength (λ) and antenna length (L) is summarized below, which can be used to compute the azimuth resolution:

S x λRa = ___________

L

where S is the slant-range distance to the point of interest.

19

Azimuth Azimuth ResolutionResolution

S x λRa = ___________

L

RADAR Relief Displacement, Image RADAR Relief Displacement, Image Foreshortening, and ShadowingForeshortening, and Shadowing

Geometric distortions exist in almost all radar imagery, including :

• foreshortening,

• layover and • layover, and

• shadowing.

20

ForshorteningForshortening

ForshorteningForshortening

a. b.C-band ERS-1 depression angle =67Þ

look angle = 23Þ

L-band JERS-1 depression angle =54Þ

look angle = 36Þ

look direction

c. d.X - band Aerial Photographlook direction N

21

LayoverLayover

LayoverLayover

L-band SIR-C (HH) July 20, 1995

look directionN

Pasadena

22

ShadowShadow

Shuttle Imaging Radar Shuttle Imaging Radar (SIR(SIR--C) Image of MauiC) Image of Maui

23

RADAR Image SpeckleRADAR Image SpeckleSpeckle is a grainy salt-and-pepper pattern inradar imagery present due to the coherentnature of the radar wave which causesnature of the radar wave, which causesrandom constructive and destructiveinterference, and hence random bright anddark areas in a radar image. The speckle canbe reduced by processing separate portions ofan aperture and recombining these portions sothat interference does not occur. This process,t at te e e ce does ot occu s p ocess,called multiple looks or non-coherentintegration, produces a more pleasingappearance, and in some cases may aid ininterpretation of the image but at a cost ofdegraded resolution.

Number Number of Looksof Looks

a.

b.

1 - Look radar image

4 - Look radar image

c. 16 - Look radar image

24

Synthetic Aperture Radar (SAR) Synthetic Aperture Radar (SAR) SystemsSystems

A major advance in radar remote sensing has been the improvement in azimuth resolution through the development of synthetic aperture radar (SAR) systems development of synthetic aperture radar (SAR) systems. Remember, in a real aperture radar system that the size of the antenna (L) is inversely proportional to the size of the angular beam width. Great improvement in azimuth resolution could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object. Doppler principles are then used to monitor the returns from all these additional microwave pulses to synthesize the azimuth resolution to become one very narrow beam.

Synthetic Aperture Synthetic Aperture Radar SystemsRadar Systems

The Doppler principle states that the frequencyThe Doppler principle states that the frequency(pitch) of a sound changes if the listener and/orsource are in motion relative to one another.

• An approaching train whistle will have anincreasingly higher frequency pitch as itapproaches. This pitch will be highest when it isdirectly perpendicular to the listener (receiver).Thi i ll d th i t f D l A th t iThis is called the point of zero Doppler. As the trainpasses by, its pitch will decrease in frequency inproportion to the distance it is from the listener(receiver). This principle is applicable to allharmonic wave motion, including the microwavesused in radar systems.

25

Synthetic Synthetic Aperture Aperture

RadarRadar(SAR)(SAR)(SAR)(SAR)

9 8 7 6 5 4 3 2 1

time n time n+2

pulses of microwave energy

a.b. c.

8 7

time n+1

object is a constant distance from the flightline

time n+4ti +3

interference signal

radar hologram

6.5 7

9 9 8 9 8 7

Synthetic Synthetic time n+4time n+3

d. e.

78 9 78 9 6.5 6.5 7

Synthetic Synthetic Aperture Aperture

RadarRadar(SAR)(SAR)

26

Creation Creation of the of the RADAR RADAR ImageImage

coherent lightradar

hologram 9 8 7 6.5 7 8 etc.

9 8 7

etc.

image of object

Optical Image CorrelationOptical Image Correlation

27

Fundamental Radar EquationFundamental Radar Equation

The fundamental radar equation is:

1 1Pr = Pt x Gt ____ σ ____ Ar

4πR2 4πR2

where Pr is power received, Pt is the power transmitted toward the target, Gt is 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.

Fundamental Radar Fundamental Radar EquationEquation

The modified fundamental radar equation is:

Pt x G2 x σ x λ2

Pr = ________________(4π)3 x R4( )

28

Radar Backscatter Coefficient, Radar Backscatter Coefficient, σσ˚ ˚

Finally, it is the effects of terrain on the radar signal thatwe are most interested in, i.e. the amount of radar cross-section σ reflected back to the receiver per unit area asection, σ , reflected back to the receiver, per unit area aon the ground. This is called the radar backscattercoeffieient (σ ˚) and is computed as :

σ˚ = σa

• The radar backscatter coefficient determines thepercentage of electromagnetic energy reflected back tothe radar from within a resolution cell, e.g. 10 x 10 m.The actual σ˚ for a surface depends on a number ofterrain parameters like geometry, surface roughness,moisture content, and the radar system parameters(wavelength, depression angle, polarization, etc.).

Surface RoughnessSurface Roughness

• Surface roughness is the terrain property that moststrongly influences the strength of the radar backscatter.Wh i t ti i l h t h ft thWhen interpreting aerial photography we often use theterminology - rough (coarse), intermediate, or smooth(fine) - to describe the surface texture characteristics. Itis possible to extend this analogy to the interpretation ofradar imagery if we keep in mind that the surfaceroughness we are talking about is usually measured incentimeters (i.e. the height of stones, size of leaves, orlength of branches in a tree) and not thousands of meterslength of branches in a tree) and not thousands of metersas with mountains.

• In radar imagery we are actually talking about micro-relief surface roughness characteristics rather thantopographic relief.

29

• There is a relationship between the

Surface RoughnessSurface Roughness

• There is a relationship between thewavelength of the radar (λ), the depressionangle (γ), and the local height of objects (h incm) found within the resolution cell beingilluminated by microwave energy. It is calledthe modified Rayleigh criteria and can beused to predict what the earth's surface willplook like in a radar image if we know thesurface roughness characteristics and theradar system parameters (λ , γ ,h) mentioned.

Surface Roughness in

RADAR Imagery

Expected surface

roughness back-scatter from terrain illuminated with 3 cm

wavelength wavelength microwave

energy with a depression

angle of 45˚.

30

Nile River SudanNile River Sudan

Space Shuttle Color-Infrared

SIR-C Color Composite:

Color Infrared Photograph

SIR C Color Composite:• Red = C-band HV• Green = L-band HV• Blue = L-band HH

Types of Active Microwave Surface

and Volume Scattering that Take

Pl i surface scattering

f h Place in a Hypothetical Pine

Forest Stand

from the top of the canopy

volume scattering

surface and volume scattering from the ground

31

L-band 23.5 cm

C-band 5.8 cm

X-band 3 cm

Response of A Pine Forest Stand to XResponse of A Pine Forest Stand to X--, C, C-- and and LL--band Microwave Energyband Microwave Energy

a. b. c.

SIRSIR--C/XC/X--SAR SAR Images of a Images of a Portion of Portion of Rondonia, Rondonia,

Brazil, Brazil, Obtained on Obtained on

April 10, 1994April 10, 1994

32

The Cardinal The Cardinal Effect is Effect is

Responsible for Responsible for the Pronounced the Pronounced Bright Signature Bright Signature

of Portions of of Portions of

San Fernando

SIR-C/X-SAR Image of Greater Los Angeles, California

Santa Monica Santa Monica and San and San

Fernando in the Fernando in the Space Shuttle Space Shuttle SIRSIR--C/XC/X--SAR SAR Image of Los Image of Los

Angeles, CA on Angeles, CA on October 3 1994October 3 1994

San Diego Freeway

October 3, 1994.October 3, 1994.

Pacific Ocean

Santa Monica

Shuttle Imaging Radar Shuttle Imaging Radar (SIR(SIR--C) Image of Los AngelesC) Image of Los Angeles

33

Aerial Aerial Photography and Photography and

RADAR Imagery of RADAR Imagery of the Pentagon in the Pentagon in Washington, DCWashington, DC

a. Oblique Photograph of the Pentagon

b. Radar Image of the Pentagon

RADARSAT SARRADARSAT SAR

34

APPLICATIONS OFAPPLICATIONS OFSYNTHETIC APERTURE RADARSYNTHETIC APERTURE RADAR

XX--BAND ANTENNA AT UPRMBAND ANTENNA AT UPRM

RADARSATRADARSAT

35

Applications of SAR Data Applications of SAR Data in Puerto Ricoin Puerto Rico

Physical Oceanography: Internal Waves

36

MOSAIC OF SAR IMAGESMOSAIC OF SAR IMAGESOF PUERTO RICOOF PUERTO RICO

Synthetic Aperture Radar Synthetic Aperture Radar InterferometryInterferometry ((InSARInSAR))

• InSAR, or Synthetic Aperture Radar Interferometry is another highly active research area in radar remote sensing.

• Interferometry is the study of interference patterns created by combining two sets of radar signals.

• Two images are made of the same scene by two separated antennas, either on the same platform, or the same antenna may be utilized for repeat pass sensing.• Antennas separated in the “azimuth” direction (parallel p (p

to the path of flight), measure motions on the surface such as ocean currents.

• Antennas separated in the “range” direction (orthogonal to the path of flight) measure altitude differences.

• Both measurements require some re-sampling of one image to overlay the other accurately.

37

Geometric Geometric Relationship Relationship Between Two Between Two SAR Systems SAR Systems

Used for Used for

SAR #1

SAR #2

B (90 + θ - α)

α

Used for Used for Interferometry Interferometry

to Extract to Extract Topographic Topographic InformationInformation

r1

r2 h

θ

KARAKAX, TIBETKARAKAX, TIBET

38

• Interferometric map of the Hector Mine the Hector Mine earthquake, shows the ground displacement along the radar line of sight. A full color cycle represents 10 cm of range displacement. Gray areas show zones of low phase coherence that phase coherence that have been masked before unwrapping.

• thin solid lines within the zones of dense fringes are surface breaks from azimuth and range offsets

07 Microwave Rs

Documents

Transcript of 07 Microwave Rs