Passive and Active Remote Sensing Systems Passive remote sensing systems record electromagnetic energy that was reflected (e.g., blue, green, red, and.

Passive and Active Remote Passive and Active Remote Sensing SystemsSensing Systems

Passive and Active Remote Passive and Active Remote Sensing SystemsSensing Systems

PassivePassive remote sensing systems record electromagnetic energy that was remote sensing systems record electromagnetic energy that was reflected (e.g., blue, green, red, and near-infrared light) or emitted (e.g., reflected (e.g., blue, green, red, and near-infrared light) or emitted (e.g., thermal infrared energy) from the surface of the Earth. There are also active thermal infrared energy) from the surface of the Earth. There are also active remote sensing systems that are not dependent on the Sun’s electromagnetic remote sensing systems that are not dependent on the Sun’s electromagnetic energy or the thermal properties of the Earth. energy or the thermal properties of the Earth.

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

PassivePassive remote sensing systems record electromagnetic energy that was remote sensing systems record electromagnetic energy that was reflected (e.g., blue, green, red, and near-infrared light) or emitted (e.g., reflected (e.g., blue, green, red, and near-infrared light) or emitted (e.g., thermal infrared energy) from the surface of the Earth. There are also active thermal infrared energy) from the surface of the Earth. There are also active remote sensing systems that are not dependent on the Sun’s electromagnetic remote sensing systems that are not dependent on the Sun’s electromagnetic energy or the thermal properties of the Earth. energy or the thermal properties of the Earth.

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

Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

Active Remote Sensing SystemsActive Remote Sensing SystemsActive Remote Sensing SystemsActive Remote Sensing Systems

The most widely used The most widely used activeactive remote sensing systems include: remote sensing systems include:

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

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

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

The most widely used The most widely used activeactive remote sensing systems include: remote sensing systems include:

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

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

• • SONARSONAR, which is based on the transmission of sound waves through a , which is based on the transmission of sound waves through a water column and then recording the amount of energy back-scattered water column and then recording the amount of energy back-scattered from the bottom or from objects within the water column.from the bottom or from objects within the water column. Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

Sending and Receiving a Pulse of Microwave EMR - System Components

Sending and Receiving a Pulse of Microwave EMR - System Components

The discussion is based initially on the system components and The discussion is based initially on the system components and functions of a functions of a real aperture side-looking airborne radar (SLAR).real aperture side-looking airborne radar (SLAR). The The discussion then expands to include discussion then expands to include synthetic aperture radars (SAR)synthetic aperture radars (SAR) that have improved capabilities.that have improved capabilities.

The discussion is based initially on the system components and The discussion is based initially on the system components and functions of a functions of a real aperture side-looking airborne radar (SLAR).real aperture side-looking airborne radar (SLAR). The The discussion then expands to include discussion then expands to include synthetic aperture radars (SAR)synthetic aperture radars (SAR) that have improved capabilities.that have improved capabilities.

JensenJensen, 2000, 2000JensenJensen, 2000, 2000

Side-looking Airborne RADAR (SLAR) System

Side-looking Airborne RADAR (SLAR) System

transmitted pulse

backscattered pulse

antenna

TransmitterDuplexer

• sends and receives

Pulse Generator

CRT Display or Digital Recorder

Receiver

b.

a. antenna

transmitted pulse

backscattered pulse

antenna

TransmitterDuplexer

• sends and receives

Pulse Generator

CRT Display or Digital Recorder

Receiver

b.

a. antenna


Sending and Receiving a Pulse of Microwave Sending and Receiving a Pulse of Microwave EMR - EMR - System ComponentsSystem Components

Sending and Receiving a Pulse of Microwave Sending and Receiving a Pulse of Microwave EMR - EMR - System ComponentsSystem Components

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

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

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

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

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

•• The unusual names associated with the radar wavelengths (e.g., The unusual names associated with the radar wavelengths (e.g., K, Ka, K, Ka, Ku, X, C, S, L,Ku, X, C, S, L, and and PP) are an artifact of the original secret work on radar ) are an artifact of the original secret work on radar remote sensing when it was customary to use the alphabetic descriptor remote sensing when it was customary to use the alphabetic descriptor instead of the actual wavelength or frequency. instead of the actual wavelength or frequency. Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

Active Microwave (RADAR)Commonly Use Frequencies

Active Microwave (RADAR)Commonly Use Frequencies


K K X C S L P

Microwave Bands

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

Visible Middle-IR Thermal infrared

UV Near-infrared

10 GHz 1 GHz

a u K K X C S L P

Microwave Bands

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

Visible Middle-IR Thermal infrared

UV Near-infrared

10 GHz 1 GHz

a u

RADAR Wavelengths and Frequencies used in Active Microwave Remote Sensing Investigations

RADAR Wavelengths and Frequencies used in Active Microwave Remote Sensing Investigations

Band DesignationsBand Designations(common wavelengths (common wavelengths Wavelength (Wavelength () ) Frequency (Frequency ())shown in parentheses)shown in parentheses) in cmin cm in GHz in GHz______________________________________________________________________________________________KK 1.18 - 1.671.18 - 1.67 26.5 to 18.026.5 to 18.0KKaa (0.86 cm) (0.86 cm) 0.75 - 1.180.75 - 1.18 40.0 to 26.540.0 to 26.5

KKu u 1.67 - 2.41.67 - 2.4 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 - 3.82.4 - 3.8 12.5 - 8.012.5 - 8.0C (7.5, 6.0 cm)C (7.5, 6.0 cm) 3.8 - 7.53.8 - 7.5 8.0 - 4.0 8.0 - 4.0S (8.0, 9.6, 12.6 cm)S (8.0, 9.6, 12.6 cm) 7.5 - 15.07.5 - 15.0 4.0 - 2.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm)L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 15.0 - 30.0 2.0 - 1.0 2.0 - 1.0P (68.0 cm)P (68.0 cm) 30.0 - 10030.0 - 100 1.0 - 0.3 1.0 - 0.3

Band DesignationsBand Designations(common wavelengths (common wavelengths Wavelength (Wavelength () ) Frequency (Frequency ())shown in parentheses)shown in parentheses) in cmin cm in GHz in GHz______________________________________________________________________________________________KK 1.18 - 1.671.18 - 1.67 26.5 to 18.026.5 to 18.0KKaa (0.86 cm) (0.86 cm) 0.75 - 1.180.75 - 1.18 40.0 to 26.540.0 to 26.5

KKu u 1.67 - 2.41.67 - 2.4 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 - 3.82.4 - 3.8 12.5 - 8.012.5 - 8.0C (7.5, 6.0 cm)C (7.5, 6.0 cm) 3.8 - 7.53.8 - 7.5 8.0 - 4.0 8.0 - 4.0S (8.0, 9.6, 12.6 cm)S (8.0, 9.6, 12.6 cm) 7.5 - 15.07.5 - 15.0 4.0 - 2.0 4.0 - 2.0L (23.5, 24.0, 25.0 cm)L (23.5, 24.0, 25.0 cm) 15.0 - 30.0 15.0 - 30.0 2.0 - 1.0 2.0 - 1.0P (68.0 cm)P (68.0 cm) 30.0 - 10030.0 - 100 1.0 - 0.3 1.0 - 0.3 Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

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

Remote Sensing of the EnvironmentRemote Sensing of the Environment

•• Active microwave energy penetrates clouds and can be anActive microwave energy penetrates clouds and can be an all-weatherall-weather remote sensing system. remote sensing system. •• Synoptic viewsSynoptic views of large areas, for mapping at 1:25,000 to of large areas, for mapping at 1:25,000 to 1:400,000; cloud-shrouded countries may be imaged.1:400,000; cloud-shrouded countries may be imaged.•• Coverage can be obtained at Coverage can be obtained at user-specified times, even at nightuser-specified times, even at night..• • Permits imaging at Permits imaging at shallow look anglesshallow look angles, resulting in different , resulting in different perspectives that cannot always be obtained using aerial perspectives that cannot always be obtained using aerial photography.photography.•• Senses in Senses in wavelengths outside the visible and infrared regions wavelengths outside the visible and infrared regions of the electromagnetic spectrumof the electromagnetic spectrum, providing information on , providing information on surface roughness, dielectric properties, and moisture content.surface roughness, dielectric properties, and moisture content.

•• Active microwave energy penetrates clouds and can be anActive microwave energy penetrates clouds and can be an all-weatherall-weather remote sensing system. remote sensing system. •• Synoptic viewsSynoptic views of large areas, for mapping at 1:25,000 to of large areas, for mapping at 1:25,000 to 1:400,000; cloud-shrouded countries may be imaged.1:400,000; cloud-shrouded countries may be imaged.•• Coverage can be obtained at Coverage can be obtained at user-specified times, even at nightuser-specified times, even at night..• • Permits imaging at Permits imaging at shallow look anglesshallow look angles, resulting in different , resulting in different perspectives that cannot always be obtained using aerial perspectives that cannot always be obtained using aerial photography.photography.•• Senses in Senses in wavelengths outside the visible and infrared regions wavelengths outside the visible and infrared regions of the electromagnetic spectrumof the electromagnetic spectrum, providing information on , providing information on surface roughness, dielectric properties, and moisture content.surface roughness, dielectric properties, and moisture content.


•• May May penetratepenetrate vegetation, sand, and surface layers of snow. vegetation, sand, and surface layers of snow.•• Has its Has its own illuminationown illumination, and the , and the angle of illuminationangle of illumination can be controlled. can be controlled.•• Enables Enables resolution to be independent of distance to the objectresolution to be independent of distance to the object, with the , with the size size of a resolution cell being as small as 1 x 1 m.of a resolution cell being as small as 1 x 1 m.•• Images can be produced from Images can be produced from different types of polarized energydifferent types of polarized energy (HH, (HH, HV, VV, VH).HV, VV, VH).•• May operate simultaneously in several wavelengths (frequencies) and thus May operate simultaneously in several wavelengths (frequencies) and thus has has multi-frequency potentialmulti-frequency 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 viewing and Can produce overlapping images suitable for stereoscopic viewing and radargrammetryradargrammetry..•• Supports interferometric operation using two antennas for 3-D mapping, Supports interferometric operation using two antennas for 3-D mapping, and analysis of incident-angle signatures of objects.and analysis of incident-angle signatures of objects.

•• May May penetratepenetrate vegetation, sand, and surface layers of snow. vegetation, sand, and surface layers of snow.•• Has its Has its own illuminationown illumination, and the , and the angle of illuminationangle of illumination can be controlled. can be controlled.•• Enables Enables resolution to be independent of distance to the objectresolution to be independent of distance to the object, with the , with the size size of a resolution cell being as small as 1 x 1 m.of a resolution cell being as small as 1 x 1 m.•• Images can be produced from Images can be produced from different types of polarized energydifferent types of polarized energy (HH, (HH, HV, VV, VH).HV, VV, VH).•• May operate simultaneously in several wavelengths (frequencies) and thus May operate simultaneously in several wavelengths (frequencies) and thus has has multi-frequency potentialmulti-frequency 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 viewing and Can produce overlapping images suitable for stereoscopic viewing and radargrammetryradargrammetry..•• Supports interferometric operation using two antennas for 3-D mapping, Supports interferometric operation using two antennas for 3-D mapping, and analysis of incident-angle signatures of objects.and analysis of incident-angle signatures of objects.


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

Radar NomenclatureRadar Nomenclature

• • nadirnadir•• azimuth flight directionazimuth flight direction•• look directionlook direction•• range (near and far)range (near and far)•• depression angle (depression angle ())•• incidence angle (incidence angle ())•• altitude above-ground-level, altitude above-ground-level, HH•• polarizationpolarization

Radar NomenclatureRadar Nomenclature

• • nadirnadir•• azimuth flight directionazimuth flight direction•• look directionlook direction•• range (near and far)range (near and far)•• depression angle (depression angle ())•• incidence angle (incidence angle ())•• altitude above-ground-level, altitude above-ground-level, HH•• polarizationpolarization


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

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

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

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

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

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


Azimuth DirectionAzimuth DirectionAzimuth DirectionAzimuth Direction

The The rangerange or or look directionlook direction for any radar image is the direction for any radar image is the direction of the radar illumination that is at right angles to the direction of the radar illumination that is at right angles to the direction the aircraft or spacecraft is traveling. the aircraft or spacecraft is traveling.

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

The The rangerange or or look directionlook direction for any radar image is the direction for any radar image is the direction of the radar illumination that is at right angles to the direction of the radar illumination that is at right angles to the direction the aircraft or spacecraft is traveling. the aircraft or spacecraft is traveling.

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


Range DirectionRange DirectionRange DirectionRange Direction

Look DirectionLook DirectionLook DirectionLook Direction

a.

b.look direction

X - band, HH polarization look direction

sX - band, HH polarization

a.

b.look direction

X - band, HH polarization look direction

sX - band, HH polarizationJensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

The The depression angledepression angle ( () is the angle between a horizontal ) is the angle between a horizontal plane extending out from the aircraft fuselage and the plane extending out from the aircraft fuselage and the electromagnetic pulse of energy from the antenna to a specific electromagnetic pulse of energy from the antenna to a specific point on the ground. point on the ground.

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

The The depression angledepression angle ( () is the angle between a horizontal ) is the angle between a horizontal plane extending out from the aircraft fuselage and the plane extending out from the aircraft fuselage and the electromagnetic pulse of energy from the antenna to a specific electromagnetic pulse of energy from the antenna to a specific point on the ground. point on the ground.

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


Depression AngleDepression AngleDepression AngleDepression Angle

The The incident angleincident angle ( () is the angle between the radar pulse of ) is the angle between the radar pulse of EMR and a line perpendicular to the Earth’s surface where it EMR and a line perpendicular to the Earth’s surface where it makes contact. When the terrain is flat, the incident angle (makes contact. When the terrain is flat, the incident angle () is ) is the complement (the complement ( == 9090 -- ) of the depression angle () of the depression angle (). If the ). If the terrain is sloped, there is no relationship between depression terrain is sloped, there is no relationship between depression angle and incident angle. The incident angle best describes the angle and incident angle. The incident angle best describes the relationship between the radar beam and surface slope. relationship between the radar beam and surface slope.

•• Many mathematical radar studies assume the terrain surface is Many mathematical radar studies assume the terrain surface is flat (horizontal) therefore, the incident angle is assumed to be flat (horizontal) therefore, the incident angle is assumed to be the complement of the depression angle.the complement of the depression angle.

The The incident angleincident angle ( () is the angle between the radar pulse of ) is the angle between the radar pulse of EMR and a line perpendicular to the Earth’s surface where it EMR and a line perpendicular to the Earth’s surface where it makes contact. When the terrain is flat, the incident angle (makes contact. When the terrain is flat, the incident angle () is ) is the complement (the complement ( == 9090 -- ) of the depression angle () of the depression angle (). If the ). If the terrain is sloped, there is no relationship between depression terrain is sloped, there is no relationship between depression angle and incident angle. The incident angle best describes the angle and incident angle. The incident angle best describes the relationship between the radar beam and surface slope. relationship between the radar beam and surface slope.

•• Many mathematical radar studies assume the terrain surface is Many mathematical radar studies assume the terrain surface is flat (horizontal) therefore, the incident angle is assumed to be flat (horizontal) therefore, the incident angle is assumed to be the complement of the depression angle.the complement of the depression angle.


Incident AngleIncident AngleIncident AngleIncident Angle

UnpolarizedUnpolarized energy vibrates in all possible directions energy vibrates in all possible directions perpendicular to the direction of travel. perpendicular to the direction of travel.

• • Radar antennas send and receive Radar antennas send and receive polarizedpolarized energy. This energy. This means that the pulse of energy is filtered so that its electrical means that the pulse of energy is filtered so that its electrical wave vibrations are only in a single plane that is perpendicular wave vibrations are only in a single plane that is perpendicular to the direction of travel. The pulse of electromagnetic energy to the direction of travel. The pulse of electromagnetic energy sent out by the antenna may be sent out by the antenna may be vertically vertically oror horizontally horizontally polarizedpolarized. .

UnpolarizedUnpolarized energy vibrates in all possible directions energy vibrates in all possible directions perpendicular to the direction of travel. perpendicular to the direction of travel.

• • Radar antennas send and receive Radar antennas send and receive polarizedpolarized energy. This energy. This means that the pulse of energy is filtered so that its electrical means that the pulse of energy is filtered so that its electrical wave vibrations are only in a single plane that is perpendicular wave vibrations are only in a single plane that is perpendicular to the direction of travel. The pulse of electromagnetic energy to the direction of travel. The pulse of electromagnetic energy sent out by the antenna may be sent out by the antenna may be vertically vertically oror horizontally horizontally polarizedpolarized. .


PolarizationPolarizationPolarizationPolarization




a.

b.

look direction

N

Ka - band, HH polarization

Ka - band, HV polarization

a.

b.

look direction

N

Ka - band, HH polarization

Ka - band, HV polarizationJensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

The transmitted pulse of electromagnetic energy interacts with The transmitted pulse of electromagnetic energy interacts with the terrain and some of it is back-scattered at the speed of light the terrain and some of it is back-scattered at the speed of light toward the aircraft or spacecraft where it once again must pass toward the aircraft or spacecraft where it once again must pass through a filter. If the antenna accepts the back-scattered through a filter. If the antenna accepts the back-scattered energy, it is recorded. Various types of back-scattered polarized energy, it is recorded. Various types of back-scattered polarized energy may be recorded by the radar. energy may be recorded by the radar.

The transmitted pulse of electromagnetic energy interacts with The transmitted pulse of electromagnetic energy interacts with the terrain and some of it is back-scattered at the speed of light the terrain and some of it is back-scattered at the speed of light toward the aircraft or spacecraft where it once again must pass toward the aircraft or spacecraft where it once again must pass through a filter. If the antenna accepts the back-scattered through a filter. If the antenna accepts the back-scattered energy, it is recorded. Various types of back-scattered polarized energy, it is recorded. Various types of back-scattered polarized energy may be recorded by the radar. energy may be recorded by the radar.



It is possible to:It is possible to:

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

•• send horizontal and receive horizontally polarized energy (send horizontal and receive horizontally polarized energy (HHHH),), •• send horizontal and receive vertically polarized energy (send horizontal and receive vertically polarized energy (HVHV), or), or

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

It is possible to:It is possible to:

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

•• send horizontal and receive horizontally polarized energy (send horizontal and receive horizontally polarized energy (HHHH),), •• send horizontal and receive vertically polarized energy (send horizontal and receive vertically polarized energy (HVHV), or), or

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



•• HHHH and and VVVV configurations produce configurations produce like-polarizedlike-polarized radar imagery. radar imagery.

• • HVHV and and VHVH configurations produce configurations produce cross-polarizedcross-polarized imagery. imagery.

•• HHHH and and VVVV configurations produce configurations produce like-polarizedlike-polarized radar imagery. radar imagery.

• • HVHV and and VHVH configurations produce configurations produce cross-polarizedcross-polarized imagery. imagery.



Slant-range versus Ground-Range GeometrySlant-range versus Ground-Range Geometry

Radar imagery has a Radar imagery has a different geometrydifferent geometry than that produced by most than that produced by most conventional remote sensor systems, such as cameras, multispectral conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements. attempting to make radargrammetric measurements.

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

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

Radar imagery has a Radar imagery has a different geometrydifferent geometry than that produced by most than that produced by most conventional remote sensor systems, such as cameras, multispectral conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements. attempting to make radargrammetric measurements.

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

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



RADAR ResolutionRADAR Resolution

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

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


Range ResolutionRange ResolutionRange ResolutionRange Resolution

The The range resolutionrange resolution ( (RRrr) at any point between the near and far-range of the ) at any point between the near and far-range of the

illuminated strip can be computed if the depression angle (illuminated strip can be computed if the depression angle () of the sensor at ) of the sensor at that location and the pulse length (that location and the pulse length () are known. It is possible to convert ) are known. It is possible to convert pulse length into distance by multiplying it times the speed of light (pulse length into distance by multiplying it times the speed of light (cc = 3 x = 3 x 101088 m sec m sec-1-1). The resulting distance is measured in the slant-range previously ). The resulting distance is measured in the slant-range previously discussed. Because we want to know the range resolution in the ground-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 (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 depression range by dividing the slant-range distance by the cosine of the depression angle (angle (). Therefore, the equation for computing the ). Therefore, the equation for computing the range resolutionrange resolution is:is:

x x ccRRrr = __________ = __________

2 cos 2 cos

The The range resolutionrange resolution ( (RRrr) at any point between the near and far-range of the ) at any point between the near and far-range of the

illuminated strip can be computed if the depression angle (illuminated strip can be computed if the depression angle () of the sensor at ) of the sensor at that location and the pulse length (that location and the pulse length () are known. It is possible to convert ) are known. It is possible to convert pulse length into distance by multiplying it times the speed of light (pulse length into distance by multiplying it times the speed of light (cc = 3 x = 3 x 101088 m sec m sec-1-1). The resulting distance is measured in the slant-range previously ). The resulting distance is measured in the slant-range previously discussed. Because we want to know the range resolution in the ground-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 (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 depression range by dividing the slant-range distance by the cosine of the depression angle (angle (). Therefore, the equation for computing the ). Therefore, the equation for computing the range resolutionrange resolution is:is:

x x ccRRrr = __________ = __________

2 cos 2 cos Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

Range Range ResolutionResolution

Range Range ResolutionResolution


Azimuth ResolutionAzimuth Resolution

Thus far we have only identified the length in meters of an Thus far we have only identified the length in meters of an active microwave resolution element at a specific depression active microwave resolution element at a specific depression angle and pulse length in the range (across-track) direction. To angle and pulse length in the range (across-track) direction. To know both the length and width of the resolution element, we know both the length and width of the resolution element, we must also compute the width of the resolution element in the must also compute the width of the resolution element in the direction the aircraft or spacecraft is flying — the direction the aircraft or spacecraft is flying — the azimuth azimuth directiondirection. .

Thus far we have only identified the length in meters of an Thus far we have only identified the length in meters of an active microwave resolution element at a specific depression active microwave resolution element at a specific depression angle and pulse length in the range (across-track) direction. To angle and pulse length in the range (across-track) direction. To know both the length and width of the resolution element, we know both the length and width of the resolution element, we must also compute the width of the resolution element in the must also compute the width of the resolution element in the direction the aircraft or spacecraft is flying — the direction the aircraft or spacecraft is flying — the azimuth azimuth directiondirection. .


Azimuth resolutionAzimuth resolution ( (RRaa) is determined by computing the ) is determined by computing the width of the terrain width of the terrain

strip that is illuminated by the radar beamstrip that is illuminated by the radar beam. .

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

Azimuth resolutionAzimuth resolution ( (RRaa) is determined by computing the ) is determined by computing the width of the terrain width of the terrain

strip that is illuminated by the radar beamstrip that is illuminated by the radar beam. .

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



Fortunately, the beam width is also inversely proportional to antenna length Fortunately, the beam width is also inversely proportional to antenna length ((LL). This means that the ). This means that the longer the radar antennalonger the radar antenna, the , the narrower the beam narrower the beam widthwidth and the higher the azimuth resolution. The relationship between and the higher the azimuth resolution. The relationship between wavelength (wavelength () and antenna length () and antenna length (LL) is summarized below, which can be ) is summarized below, which can be used to compute the azimuth resolution:used to compute the azimuth resolution:

SS x x RRaa = ___________ = ___________

LL

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

Fortunately, the beam width is also inversely proportional to antenna length Fortunately, the beam width is also inversely proportional to antenna length ((LL). This means that the ). This means that the longer the radar antennalonger the radar antenna, the , the narrower the beam narrower the beam widthwidth and the higher the azimuth resolution. The relationship between and the higher the azimuth resolution. The relationship between wavelength (wavelength () and antenna length () and antenna length (LL) is summarized below, which can be ) is summarized below, which can be used to compute the azimuth resolution:used to compute the azimuth resolution:

SS x x RRaa = ___________ = ___________

LL

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



Azimuth Azimuth ResolutionResolutionAzimuth Azimuth

ResolutionResolution


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

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

Geometric distortions exist in almost Geometric distortions exist in almost all radar imagery, including :all radar imagery, including :

• • foreshorteningforeshortening, ,

• • layoverlayover, and , and

• • shadowingshadowing. .

Geometric distortions exist in almost Geometric distortions exist in almost all radar imagery, including :all radar imagery, including :

• • foreshorteningforeshortening, ,

• • layoverlayover, and , and

• • shadowingshadowing. .


RADAR Relief Displacement: RADAR Relief Displacement: Foreshortening Foreshortening andand Layover Layover

RADAR Relief Displacement: RADAR Relief Displacement: Foreshortening Foreshortening andand Layover Layover

When the terrain is flat, it is a easy to use the appropriate equation to When the terrain is flat, it is a easy to use the appropriate equation to convert a slant-range radar image into a ground-range radar image that is convert a slant-range radar image into a ground-range radar image that is planimetrically correct in x,y. However, when trees, tall buildings, or planimetrically correct in x,y. However, when trees, tall buildings, or mountains are present in the scene, mountains are present in the scene, radar relief displacementradar relief displacement occurs. occurs.

•• In In radar relief displacementradar relief displacement, the horizontal displacement of an object in , the horizontal displacement of an object in the image caused by the object’s elevation is in a direction toward the radar the image caused by the object’s elevation is in a direction toward the radar antenna. Because the radar image is formed in the range (cross-track) antenna. Because the radar image is formed in the range (cross-track) direction, the higher the object, the closer it is to the radar antenna, and direction, the higher the object, the closer it is to the radar antenna, and therefore the sooner (in time) it is detected on the radar image. This therefore the sooner (in time) it is detected on the radar image. This contrasts sharply with relief displacement in optical aerial photography contrasts sharply with relief displacement in optical aerial photography where the relief displacement is radially outward from the principal point where the relief displacement is radially outward from the principal point (center) of a photograph. The elevation-induced distortions in radar imagery (center) of a photograph. The elevation-induced distortions in radar imagery are referred to as are referred to as foreshorteningforeshortening and and layoverlayover..

When the terrain is flat, it is a easy to use the appropriate equation to When the terrain is flat, it is a easy to use the appropriate equation to convert a slant-range radar image into a ground-range radar image that is convert a slant-range radar image into a ground-range radar image that is planimetrically correct in x,y. However, when trees, tall buildings, or planimetrically correct in x,y. However, when trees, tall buildings, or mountains are present in the scene, mountains are present in the scene, radar relief displacementradar relief displacement occurs. occurs.

•• In In radar relief displacementradar relief displacement, the horizontal displacement of an object in , the horizontal displacement of an object in the image caused by the object’s elevation is in a direction toward the radar the image caused by the object’s elevation is in a direction toward the radar antenna. Because the radar image is formed in the range (cross-track) antenna. Because the radar image is formed in the range (cross-track) direction, the higher the object, the closer it is to the radar antenna, and direction, the higher the object, the closer it is to the radar antenna, and therefore the sooner (in time) it is detected on the radar image. This therefore the sooner (in time) it is detected on the radar image. This contrasts sharply with relief displacement in optical aerial photography contrasts sharply with relief displacement in optical aerial photography where the relief displacement is radially outward from the principal point where the relief displacement is radially outward from the principal point (center) of a photograph. The elevation-induced distortions in radar imagery (center) of a photograph. The elevation-induced distortions in radar imagery are referred to as are referred to as foreshorteningforeshortening and and layoverlayover..


RADAR Foreshortening is Influenced by:RADAR Foreshortening is Influenced by:RADAR Foreshortening is Influenced by:RADAR Foreshortening is Influenced by:

• • object heightobject height: The greater the height of the object above local : The greater the height of the object above local datum, the greater the foreshortening. datum, the greater the foreshortening. • • depression angledepression angle (or incident angle): The greater the (or incident angle): The greater the depression angle (depression angle () or smaller the incident angle () or smaller the incident angle (), the ), the greater the foreshortening. greater the foreshortening. • • location of objects in the across-track rangelocation of objects in the across-track range: Features in the : Features in the near-range portion of the swath are generally foreshortened near-range portion of the swath are generally foreshortened more than identical features in the far-range. Foreshortening more than identical features in the far-range. Foreshortening causes features to appear to have steeper slopes than they causes features to appear to have steeper slopes than they actually have in the near-range of the radar image and to have actually have in the near-range of the radar image and to have shallower slopes than they actually have in the image far-range.shallower slopes than they actually have in the image far-range.

• • object heightobject height: The greater the height of the object above local : The greater the height of the object above local datum, the greater the foreshortening. datum, the greater the foreshortening. • • depression angledepression angle (or incident angle): The greater the (or incident angle): The greater the depression angle (depression angle () or smaller the incident angle () or smaller the incident angle (), the ), the greater the foreshortening. greater the foreshortening. • • location of objects in the across-track rangelocation of objects in the across-track range: Features in the : Features in the near-range portion of the swath are generally foreshortened near-range portion of the swath are generally foreshortened more than identical features in the far-range. Foreshortening more than identical features in the far-range. Foreshortening causes features to appear to have steeper slopes than they causes features to appear to have steeper slopes than they actually have in the near-range of the radar image and to have actually have in the near-range of the radar image and to have shallower slopes than they actually have in the image far-range.shallower slopes than they actually have in the image far-range.


Forshortening, Forshortening, Layover, and Layover, and

ShadowShadow

Forshortening, Forshortening, Layover, and Layover, and

ShadowShadow


ForeshorteningForeshortening

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

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 Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

RADAR Relief Displacement: RADAR Relief Displacement: Image LayoverImage Layover

RADAR Relief Displacement: RADAR Relief Displacement: Image LayoverImage Layover

Image layover Image layover is an extreme case of image foreshortening. It is an extreme case of image foreshortening. It occurs when the incident angle (occurs when the incident angle () is smaller than the foreslope ) is smaller than the foreslope ((++) i.e., ) i.e., < < ++. .

• • This distortion cannot be corrected even when the surface This distortion cannot be corrected even when the surface topography is known. Great care must be exercised when topography is known. Great care must be exercised when interpreting radar images of mountainous areas where the interpreting radar images of mountainous areas where the thresholds for image layover exist. thresholds for image layover exist.

Image layover Image layover is an extreme case of image foreshortening. It is an extreme case of image foreshortening. It occurs when the incident angle (occurs when the incident angle () is smaller than the foreslope ) is smaller than the foreslope ((++) i.e., ) i.e., < < ++. .

• • This distortion cannot be corrected even when the surface This distortion cannot be corrected even when the surface topography is known. Great care must be exercised when topography is known. Great care must be exercised when interpreting radar images of mountainous areas where the interpreting radar images of mountainous areas where the thresholds for image layover exist. thresholds for image layover exist.


LayoverLayoverLayoverLayover

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

look directionN

Pasadena

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

look directionN

Pasadena


RADAR ShadowsRADAR ShadowsRADAR ShadowsRADAR Shadows

ShadowsShadows in radar images can enhance the geomorphology and in radar images can enhance the geomorphology and texture of the terrain. Shadows can also obscure the most texture of the terrain. Shadows can also obscure the most important features in a radar image, such as the information important features in a radar image, such as the information behind tall buildings or land use in deep valleys. If certain behind tall buildings or land use in deep valleys. If certain conditions are met, any feature protruding above the local conditions are met, any feature protruding above the local datum can cause the incident pulse of microwave energy to datum can cause the incident pulse of microwave energy to reflect all of its energy on the foreslope of the object and reflect all of its energy on the foreslope of the object and produce a black shadow for the backslope. produce a black shadow for the backslope.

ShadowsShadows in radar images can enhance the geomorphology and in radar images can enhance the geomorphology and texture of the terrain. Shadows can also obscure the most texture of the terrain. Shadows can also obscure the most important features in a radar image, such as the information important features in a radar image, such as the information behind tall buildings or land use in deep valleys. If certain behind tall buildings or land use in deep valleys. If certain conditions are met, any feature protruding above the local conditions are met, any feature protruding above the local datum can cause the incident pulse of microwave energy to datum can cause the incident pulse of microwave energy to reflect all of its energy on the foreslope of the object and reflect all of its energy on the foreslope of the object and produce a black shadow for the backslope. produce a black shadow for the backslope.


RADAR ShadowsRADAR ShadowsRADAR ShadowsRADAR Shadows

• • A backslope is in A backslope is in radar shadowradar shadow when its angle when its angle is steeper is steeper than the depression angle (than the depression angle (), i.e., ), i.e., >> . .

• • If the backslope equals the depression angle, If the backslope equals the depression angle, == , then the , then the backslope is just barely illuminated by the incident energy. backslope is just barely illuminated by the incident energy. This is called This is called grazing illuminationgrazing illumination because the radar pulse just because the radar pulse just grazes the backslope. grazes the backslope.

• • The backslope is fully illuminated when it is less than the The backslope is fully illuminated when it is less than the depression angle (depression angle ( << . ). )

• • A backslope is in A backslope is in radar shadowradar shadow when its angle when its angle is steeper is steeper than the depression angle (than the depression angle (), i.e., ), i.e., >> . .

• • If the backslope equals the depression angle, If the backslope equals the depression angle, == , then the , then the backslope is just barely illuminated by the incident energy. backslope is just barely illuminated by the incident energy. This is called This is called grazing illuminationgrazing illumination because the radar pulse just because the radar pulse just grazes the backslope. grazes the backslope.

• • The backslope is fully illuminated when it is less than the The backslope is fully illuminated when it is less than the depression angle (depression angle ( << . ). )


RADAR Shadow CharacteristicsRADAR Shadow CharacteristicsRADAR Shadow CharacteristicsRADAR Shadow Characteristics

•• Unlike airphotos, where light may be scattered into the shadow area and Unlike airphotos, where light may be scattered into the shadow area and then recorded on film, then recorded on film, there is no information within the radar shadow there is no information within the radar shadow areaarea. It is . It is blackblack..

•• Two terrain features (e.g., mountains) with identical heights and fore- and Two terrain features (e.g., mountains) with identical heights and fore- and backslopes may be recorded with entirely different shadows, backslopes may be recorded with entirely different shadows, depending depending upon where they are in the across-trackupon where they are in the across-track. A feature that casts an extensive . A feature that casts an extensive shadow in the far-range might have its backslope completely illuminated in shadow in the far-range might have its backslope completely illuminated in the near-range.the near-range.

•• Radar shadows occur only in the cross-track dimensionRadar shadows occur only in the cross-track dimension . Therefore, the . Therefore, the orientation of shadows in a radar image provides information about the look orientation of shadows in a radar image provides information about the look direction and the location of the near- and far-range.direction and the location of the near- and far-range.

•• Unlike airphotos, where light may be scattered into the shadow area and Unlike airphotos, where light may be scattered into the shadow area and then recorded on film, then recorded on film, there is no information within the radar shadow there is no information within the radar shadow areaarea. It is . It is blackblack..

•• Two terrain features (e.g., mountains) with identical heights and fore- and Two terrain features (e.g., mountains) with identical heights and fore- and backslopes may be recorded with entirely different shadows, backslopes may be recorded with entirely different shadows, depending depending upon where they are in the across-trackupon where they are in the across-track. A feature that casts an extensive . A feature that casts an extensive shadow in the far-range might have its backslope completely illuminated in shadow in the far-range might have its backslope completely illuminated in the near-range.the near-range.

•• Radar shadows occur only in the cross-track dimensionRadar shadows occur only in the cross-track dimension . Therefore, the . Therefore, the orientation of shadows in a radar image provides information about the look orientation of shadows in a radar image provides information about the look direction and the location of the near- and far-range.direction and the location of the near- and far-range.


RADAR Image SpeckleRADAR Image SpeckleRADAR Image SpeckleRADAR Image Speckle

SpeckleSpeckle is a grainy salt-and-pepper pattern in radar imagery is a grainy salt-and-pepper pattern in radar imagery present due to the coherent nature of the radar wave, which present due to the coherent nature of the radar wave, which causes random constructive and destructive interference, and causes random constructive and destructive interference, and hence random bright and dark areas in a radar image. The hence random bright and dark areas in a radar image. The speckle can be reduced by processing separate portions of an speckle can be reduced by processing separate portions of an aperture and recombining these portions so that interference aperture and recombining these portions so that interference does not occur. This process, called does not occur. This process, called multiple looksmultiple looks or non- or non-coherent integration, produces a more pleasing appearance, and coherent integration, produces a more pleasing appearance, and in some cases may aid in interpretation of the image but at a in some cases may aid in interpretation of the image but at a cost of degraded resolution. cost of degraded resolution.

SpeckleSpeckle is a grainy salt-and-pepper pattern in radar imagery is a grainy salt-and-pepper pattern in radar imagery present due to the coherent nature of the radar wave, which present due to the coherent nature of the radar wave, which causes random constructive and destructive interference, and causes random constructive and destructive interference, and hence random bright and dark areas in a radar image. The hence random bright and dark areas in a radar image. The speckle can be reduced by processing separate portions of an speckle can be reduced by processing separate portions of an aperture and recombining these portions so that interference aperture and recombining these portions so that interference does not occur. This process, called does not occur. This process, called multiple looksmultiple looks or non- or non-coherent integration, produces a more pleasing appearance, and coherent integration, produces a more pleasing appearance, and in some cases may aid in interpretation of the image but at a in some cases may aid in interpretation of the image but at a cost of degraded resolution. cost of degraded resolution.


Number Number of Looksof LooksNumber Number of Looksof Looks

a.

b.

c.

1 - Look radar image



a.

b.

c.



16 - Look radar image Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

Synthetic Aperture Radar SystemsSynthetic Aperture Radar SystemsSynthetic Aperture Radar SystemsSynthetic Aperture Radar Systems

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

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


Synthetic Aperture Radar SystemsSynthetic Aperture Radar SystemsSynthetic Aperture Radar SystemsSynthetic Aperture Radar Systems

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

•• An approaching train whistle will have an increasingly higher frequency An approaching train whistle will have an increasingly higher frequency pitch as it approaches. This pitch will be highest when it is directly pitch as it approaches. This pitch will be highest when it is directly perpendicular to the listener (receiver). This is called the point of zero perpendicular to the listener (receiver). This is called the point of zero Doppler. As the train passes by, its pitch will decrease in frequency in Doppler. As the train passes by, its pitch will decrease in frequency in proportion to the distance it is from the listener (receiver). This principle is proportion to the distance it is from the listener (receiver). This principle is applicable to all harmonic wave motion, including the microwaves used in applicable to all harmonic wave motion, including the microwaves used in radar systems.radar systems.

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

•• An approaching train whistle will have an increasingly higher frequency An approaching train whistle will have an increasingly higher frequency pitch as it approaches. This pitch will be highest when it is directly pitch as it approaches. This pitch will be highest when it is directly perpendicular to the listener (receiver). This is called the point of zero perpendicular to the listener (receiver). This is called the point of zero Doppler. As the train passes by, its pitch will decrease in frequency in Doppler. As the train passes by, its pitch will decrease in frequency in proportion to the distance it is from the listener (receiver). This principle is proportion to the distance it is from the listener (receiver). This principle is applicable to all harmonic wave motion, including the microwaves used in applicable to all harmonic wave motion, including the microwaves used in radar systems.radar systems.


Synthetic Aperture RadarSynthetic Aperture RadarSynthetic Aperture RadarSynthetic Aperture Radar


Surface RoughnessSurface RoughnessSurface RoughnessSurface Roughness

• • Surface roughnessSurface roughness is the terrain property that most strongly influences is the terrain property that most strongly influences the strength of the radar backscatter. When interpreting aerial the strength of the radar backscatter. When interpreting aerial photography we often use the terminology - rough (coarse), intermediate, photography we often use the terminology - rough (coarse), intermediate, or smooth (fine) - to describe the surface texture characteristics. It is or smooth (fine) - to describe the surface texture characteristics. It is possible to extend this analogy to the interpretation of radar imagery if possible to extend this analogy to the interpretation of radar imagery if we keep in mind that the surface roughness we are talking about is we keep in mind that the surface roughness we are talking about is usually measured in centimeters (i.e. the height of stones, size of leaves, 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 or length of branches in a tree) and not thousands of meters as with mountains. mountains.

•• In radar imagery we are actually talking about In radar imagery we are actually talking about micro-relief surface micro-relief surface roughnessroughness characteristics rather than topographic relief.characteristics rather than topographic relief.

• • Surface roughnessSurface roughness is the terrain property that most strongly influences is the terrain property that most strongly influences the strength of the radar backscatter. When interpreting aerial the strength of the radar backscatter. When interpreting aerial photography we often use the terminology - rough (coarse), intermediate, photography we often use the terminology - rough (coarse), intermediate, or smooth (fine) - to describe the surface texture characteristics. It is or smooth (fine) - to describe the surface texture characteristics. It is possible to extend this analogy to the interpretation of radar imagery if possible to extend this analogy to the interpretation of radar imagery if we keep in mind that the surface roughness we are talking about is we keep in mind that the surface roughness we are talking about is usually measured in centimeters (i.e. the height of stones, size of leaves, 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 or length of branches in a tree) and not thousands of meters as with mountains. mountains.

•• In radar imagery we are actually talking about In radar imagery we are actually talking about micro-relief surface micro-relief surface roughnessroughness characteristics rather than topographic relief.characteristics rather than topographic relief.


• • There is a relationship between the wavelength of the radar There is a relationship between the wavelength of the radar ((), the depression angle (), the depression angle (), and the local height of objects (), and the local height of objects (hh in cm) found within the resolution cell being illuminated by in cm) found within the resolution cell being illuminated by microwave energy. It is called the microwave energy. It is called the modified Rayleigh criteriamodified Rayleigh criteria and can be used to predict what the earth's surface will look and can be used to predict what the earth's surface will look like in a radar image if we know the surface roughness like in a radar image if we know the surface roughness characteristics and the radar system parameters (characteristics and the radar system parameters ( , , ,h ,h) ) mentioned. mentioned.

• • There is a relationship between the wavelength of the radar There is a relationship between the wavelength of the radar ((), the depression angle (), the depression angle (), and the local height of objects (), and the local height of objects (hh in cm) found within the resolution cell being illuminated by in cm) found within the resolution cell being illuminated by microwave energy. It is called the microwave energy. It is called the modified Rayleigh criteriamodified Rayleigh criteria and can be used to predict what the earth's surface will look and can be used to predict what the earth's surface will look like in a radar image if we know the surface roughness like in a radar image if we know the surface roughness characteristics and the radar system parameters (characteristics and the radar system parameters ( , , ,h ,h) ) mentioned. mentioned.


Surface RoughnessSurface RoughnessSurface RoughnessSurface Roughness

Surface Surface Roughness Roughness in RADAR in RADAR

ImageryImagery

Surface Surface Roughness Roughness in RADAR in RADAR

ImageryImagery

Expected surface Expected surface roughness back-scatter roughness back-scatter from terrain illuminated from terrain illuminated with 3 cm wavelength with 3 cm wavelength

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

Expected surface Expected surface roughness back-scatter roughness back-scatter from terrain illuminated from terrain illuminated with 3 cm wavelength with 3 cm wavelength

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


Smooth and Rough Smooth and Rough Rayleigh CriteriaRayleigh Criteria

Smooth and Rough Smooth and Rough Rayleigh CriteriaRayleigh Criteria

• • The area with smooth surface roughness sends back very little backscatter The area with smooth surface roughness sends back very little backscatter toward the antenna, i.e. it acts like a specular reflecting surface where most toward the antenna, i.e. it acts like a specular reflecting surface where most of the energy bounces off the terrain away from the antenna. The small of the energy bounces off the terrain away from the antenna. The small amount of back-scattered energy returned to the antenna is recorded and amount of back-scattered energy returned to the antenna is recorded and shows up as a dark area on the radar image. The quantitative expression of shows up as a dark area on the radar image. The quantitative expression of thethe smooth criteriasmooth criteria is: is:

h < __h < ______ 25 sin 25 sin

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 > ______ 4.4 sin 4.4 sin

• • The area with smooth surface roughness sends back very little backscatter The area with smooth surface roughness sends back very little backscatter toward the antenna, i.e. it acts like a specular reflecting surface where most toward the antenna, i.e. it acts like a specular reflecting surface where most of the energy bounces off the terrain away from the antenna. The small of the energy bounces off the terrain away from the antenna. The small amount of back-scattered energy returned to the antenna is recorded and amount of back-scattered energy returned to the antenna is recorded and shows up as a dark area on the radar image. The quantitative expression of shows up as a dark area on the radar image. The quantitative expression of thethe smooth criteriasmooth criteria is: is:

h < __h < ______ 25 sin 25 sin

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 > ______ 4.4 sin 4.4 sin Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000

Nile River SudanNile River SudanNile River SudanNile River Sudan

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

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

Space Shuttle Space Shuttle Color-Infrared Color-Infrared

PhotographPhotograph

Space Shuttle Space Shuttle Color-Infrared Color-Infrared

PhotographPhotograph


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

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


surface scattering from the top of the canopy

volume scattering

surface and volume scattering from the ground

surface scattering from the top of the canopy

volume scattering

surface and volume scattering from the ground


L-band 23.5 cm

C-band 5.8 cm

X-band 3 cm

a. b. c.

L-band 23.5 cm

C-band 5.8 cm

X-band 3 cm

a. b. c.

Response of A Pine Forest Stand to X-, C- and L-band Microwave EnergyResponse of A Pine Forest Stand to X-, C- and L-band Microwave EnergyResponse of A Pine Forest Stand to X-, C- and L-band Microwave EnergyResponse of A Pine Forest Stand to X-, C- and L-band Microwave Energy

SIR-C/X-SAR SIR-C/X-SAR Images of a Portion Images of a Portion

of Rondonia, of Rondonia, Brazil, Obtained on Brazil, Obtained on

April 10, 1994April 10, 1994

SIR-C/X-SAR SIR-C/X-SAR Images of a Portion Images of a Portion

of Rondonia, of Rondonia, Brazil, Obtained on Brazil, Obtained on

April 10, 1994April 10, 1994


The Cardinal Effect The Cardinal Effect is Responsible for the is Responsible for the

Pronounced Bright Pronounced Bright Signature of Portions Signature of Portions of Santa Monica and of Santa Monica and San Fernando in the San Fernando in the Space Shuttle SIR-Space Shuttle SIR-C/X-SAR Image of C/X-SAR Image of Los Angeles, CA on Los Angeles, CA on

October 3, 1994.October 3, 1994.

The Cardinal Effect The Cardinal Effect is Responsible for the is Responsible for the

Pronounced Bright Pronounced Bright Signature of Portions Signature of Portions of Santa Monica and of Santa Monica and San Fernando in the San Fernando in the Space Shuttle SIR-Space Shuttle SIR-C/X-SAR Image of C/X-SAR Image of Los Angeles, CA on Los Angeles, CA on

October 3, 1994.October 3, 1994.

Jensen, 2000Jensen, 2000Jensen, 2000Jensen, 2000Pacific Ocean

San Diego Freeway

San Fernando

Santa Monica

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

Pacific Ocean

San Diego Freeway

San Fernando

Santa Monica

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

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

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


Intermap X-band Star 3Intermap X-band Star 3ii Orthorectified Image of Orthorectified Image of Bachelor Mountain, CA and Derived Digital Elevation ModelBachelor Mountain, CA and Derived Digital Elevation Model

Intermap X-band Star 3Intermap X-band Star 3ii Orthorectified Image of Orthorectified Image of Bachelor Mountain, CA and Derived Digital Elevation ModelBachelor Mountain, CA and Derived Digital Elevation Model


Passive and Active Remote Sensing Systems Passive remote sensing systems record electromagnetic energy that was reflected (e.g., blue, green, red, and.

Documents

active remote sensors

thermal infrared energy

radar wavelengths

backscatter of energy

remote sensors receiver

suns electromagnetic

infrared light

wavelength microwaves