Fundamentals of Radar

Passive and Active Passive and Active Remote Sensing Remote Sensing

SystemsSystems

Passive and Active Passive and Active Remote Sensing Remote Sensing

SystemsSystems

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

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

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

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

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

Active Remote Sensing Active Remote Sensing SystemsSystems

Active Remote Sensing Active Remote Sensing SystemsSystems

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

• • active microwave (RADARactive microwave (RADAR),), which is based on the transmission of which is based on the transmission of long-wavelength microwaves (e.g., 1 – 100 cm) through the atmosphere long-wavelength microwaves (e.g., 1 – 100 cm) through the atmosphere and then recording the amount of energy back-scattered from the and then recording the amount of energy back-scattered from the terrain;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 (RADARactive microwave (RADAR),), which is based on the transmission of which is based on the transmission of long-wavelength microwaves (e.g., 1 – 100 cm) through the atmosphere long-wavelength microwaves (e.g., 1 – 100 cm) through the atmosphere and then recording the amount of energy back-scattered from the and then recording the amount of energy back-scattered from the terrain;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.

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 atuser-specified times, even at nightnight..• • 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 atuser-specified times, even at nightnight..• • 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.

Radar and photographic views of erupting volcano

SIR-C radar

Shuttle photo

Mauna Loa,Hawaii

Shuttle color photo (left)

Shuttle radar (right)

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

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

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

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

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

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

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

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

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

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

Brazil, Obtained on April Brazil, Obtained on April 10, 199410, 1994

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

Brazil, Obtained on April Brazil, Obtained on April 10, 199410, 1994

•• 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 can be controlled.controlled.•• SAR provides SAR provides resolution that is independent of distance to the objectresolution that is independent of distance to the object, , with the size of a resolution cell being as small as 1 x 1 m.with the size 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).•• Can operate simultaneously in several wavelengths (frequencies) and Can operate simultaneously in several wavelengths (frequencies) and thus has thus 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 Supports interferometric operation using two antennasinterferometric operation using two antennas for topographic for topographic mapping, and analysis of incident-angle signatures of objects.mapping, 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 can be controlled.controlled.•• SAR provides SAR provides resolution that is independent of distance to the objectresolution that is independent of distance to the object, , with the size of a resolution cell being as small as 1 x 1 m.with the size 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).•• Can operate simultaneously in several wavelengths (frequencies) and Can operate simultaneously in several wavelengths (frequencies) and thus has thus 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 Supports interferometric operation using two antennasinterferometric operation using two antennas for topographic for topographic mapping, and analysis of incident-angle signatures of objects.mapping, and analysis of incident-angle signatures of objects.

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

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

Nile River Nile River SudanSudan

Nile River Nile River SudanSudan

SIR-C Color SIR-C Color Composite:Composite: •• Red = C-band Red = C-band HVHV •• Green = L-band Green = L-band HVHV •• Blue = L-band Blue = L-band HHHH

SIR-C Color SIR-C Color Composite:Composite: •• Red = C-band Red = C-band HVHV •• Green = L-band Green = L-band HVHV •• Blue = L-band Blue = L-band HHHH

Space Space Shuttle Shuttle Color-Color-

Infrared Infrared PhotographPhotograph

Space Space Shuttle Shuttle Color-Color-

Infrared Infrared PhotographPhotograph

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

PolarizationPolarizationPolarizationPolarization

PolarizationPolarizationPolarizationPolarization

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 (polarized energy (designated 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 (polarized energy (designated 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). ).

PolarizationPolarizationPolarizationPolarization

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

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

PolarizationPolarizationPolarizationPolarization

PolarizationPolarizationPolarizationPolarizationa.b.look directionNKa - band, HH polarizationKa - band, HV polarizationa.b.look directionNKa - band, HH polarizationKa - band, HV polarization

Radar jargon

Radar jargon

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

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

• • The terrain illuminated nearest the aircraft in the line of The terrain illuminated nearest the aircraft in the line of sight is called the sight 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 flight directionazimuth flight direction. .

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

• • The terrain illuminated nearest the aircraft in the line of The terrain illuminated nearest the aircraft in the line of sight is called the sight 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 Azimuth DirectionDirectionAzimuth Azimuth DirectionDirection

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

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

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

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

Range DirectionRange DirectionRange DirectionRange Direction

Look Look DirectionDirection

Look Look DirectionDirection

a.b.look directionX - band, HH polarizationlook directionsX - band, HH polarizationa.b.look directionX - band, HH polarizationlook directionsX - band, HH polarization

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

RADAR Resolution

RADAR 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 effect a ranging device that measures the distance to objects in the terrain by means of sending out and objects in the terrain by means of sending out and receiving pulses of active microwave energy. The receiving pulses of active microwave energy. The range range resolutionresolution in the across-track direction is proportional to in the across-track direction is proportional to the length of the microwave pulse. the length of the microwave pulse. The shorter the pulse The shorter the pulse length, the finer the range resolutionlength, the finer the range resolution.. Pulse length is a Pulse length is a function of the speed of light function of the speed of light ((cc)) multiplied by the duration multiplied by the duration of the transmission 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 effect a ranging device that measures the distance to objects in the terrain by means of sending out and objects in the terrain by means of sending out and receiving pulses of active microwave energy. The receiving pulses of active microwave energy. The range range resolutionresolution in the across-track direction is proportional to in the across-track direction is proportional to the length of the microwave pulse. the length of the microwave pulse. The shorter the pulse The shorter the pulse length, the finer the range resolutionlength, the finer the range resolution.. Pulse length is a Pulse length is a function of the speed of light function of the speed of light ((cc)) multiplied by the duration multiplied by the duration of the transmission of the transmission (().).

Resolution in slant range equals

half the radar pulse width

Resolution in slant range equals

half the radar pulse width

Range ResolutionRange ResolutionRange ResolutionRange Resolution

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

range of the illuminated strip can be computed if the depression range of the illuminated strip can be computed if the depression angle angle (()) of the sensor at that location and the pulse length of the sensor at that location and the pulse length (()) are are known. It is possible to convert pulse length into distance by known. It is possible to convert pulse length into distance by multiplying it times the speed of light (multiplying it times the speed of light (cc = 3 x 10 = 3 x 1088 m sec m sec-1-1). The ). The resulting distance is measured in the slant-range previously resulting distance is measured in the slant-range previously discussed. Because we want to know the range resolution in the discussed. Because we want to know the range resolution in the ground-range (not the slant-range) it is necessary to convert slant-ground-range (not the slant-range) it is necessary to convert slant-range to ground-range by dividing the slant-range distance by the range to ground-range by dividing the slant-range distance by the cosine of the depression angle cosine of the depression angle (().). Therefore, the equation for Therefore, the equation for computing the computing the range resolutionrange resolution is:is:

. . ccRRrr = __________ = __________

2 cos 2 cos

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

range of the illuminated strip can be computed if the depression range of the illuminated strip can be computed if the depression angle angle (()) of the sensor at that location and the pulse length of the sensor at that location and the pulse length (()) are are known. It is possible to convert pulse length into distance by known. It is possible to convert pulse length into distance by multiplying it times the speed of light (multiplying it times the speed of light (cc = 3 x 10 = 3 x 1088 m sec m sec-1-1). The ). The resulting distance is measured in the slant-range previously resulting distance is measured in the slant-range previously discussed. Because we want to know the range resolution in the discussed. Because we want to know the range resolution in the ground-range (not the slant-range) it is necessary to convert slant-ground-range (not the slant-range) it is necessary to convert slant-range to ground-range by dividing the slant-range distance by the range to ground-range by dividing the slant-range distance by the cosine of the depression angle cosine of the depression angle (().). Therefore, the equation for Therefore, the equation for computing the computing the range resolutionrange resolution is:is:

. . ccRRrr = __________ = __________

2 cos 2 cos

Range Range ResolutionResolution

Range Range ResolutionResolution

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

the terrain strip that is illuminated by the radar beamthe terrain strip that is illuminated by the radar beam..

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

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

the terrain strip that is illuminated by the radar beamthe terrain strip that is illuminated by the radar beam..

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

Azimuth Resolution

Azimuth Resolution

Real vs. synthetic aperture radar

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

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

SS . . 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 Resolution

Azimuth Resolution

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 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 to convert a slant-range radar image into a ground-range radar image that is planimetrically correct in x,y. However, when trees, image that is planimetrically correct in x,y. However, when trees, tall buildings, or mountains are present in the scene, tall buildings, or mountains are present in the scene, radar relief radar relief displacementdisplacement occurs occurs. .

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

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

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

Shadowing, foreshortening &

layover

ForeshorteningForeshortening

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

look angle = 23˚

L-band JERS-1depression angle =54˚

look angle = 36˚

look directionc.d.X - bandAerial Photographlook directionNa. b. C-band ERS-1depression angle =67˚

look angle = 23˚

L-band JERS-1depression angle =54˚

look angle = 36˚

look directionc.d.X - bandAerial Photographlook directionN

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

Real vs. synthetic aperture radar

Synthetic Aperture Radar Synthetic Aperture Radar SystemsSystems

Synthetic Aperture Radar Synthetic Aperture Radar SystemsSystems

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

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

Synthetic Aperture Radar Synthetic Aperture Radar SystemsSystems

Synthetic Aperture Radar Synthetic Aperture Radar SystemsSystems

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

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

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

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

Synthetic Aperture Radar (SAR)

Synthetic Aperture RadarSynthetic Aperture RadarSynthetic Aperture RadarSynthetic Aperture Radar

9 8 7 6 5 4 3 2 1 time ntime n+4time n+3time n+2pulses of microwave energyinterference signalradar holograma.b.c.d.e. 8 7 6.5 7 9 9 8 9 8778 9 78 9 6.56.57time n+1object is a constant distance from the flightline

9 8 7 6 5 4 3 2 1 time ntime n+4time n+3time n+2pulses of microwave energyinterference signalradar holograma.b.c.d.e. 8 7 6.5 7 9 9 8 9 8778 9 78 9 6.56.57time n+1object is a constant distance from the flightline

Synthetic Synthetic ApertureApertureRADARRADAR

Synthetic Synthetic ApertureApertureRADARRADAR

Fundamental Radar Fundamental Radar EquationEquation

Fundamental Radar Fundamental Radar EquationEquation

The The fundamental radar equation fundamental radar equation is: is:

1 11 1PPrr = P = Ptt . G . Gtt ____ ____ AArr

44RR2 2 44RR22

where where PPrr is power received, is power received, PPtt is the power transmitted toward is the power transmitted toward

the target, the target, GGtt is the gain of the antenna in the direction of the is the gain of the antenna in the direction of the

target,target, RR is the range distance from the transmitter to the target, is the range distance from the transmitter to the target, is the effective backscatter area of the target, and is the effective backscatter area of the target, and AArr is the is the

area of the receiving antenna.area of the receiving antenna.

The The fundamental radar equation fundamental radar equation is: is:

1 11 1PPrr = P = Ptt . G . Gtt ____ ____ AArr

44RR2 2 44RR22

where where PPrr is power received, is power received, PPtt is the power transmitted toward is the power transmitted toward

the target, the target, GGtt is the gain of the antenna in the direction of the is the gain of the antenna in the direction of the

target,target, RR is the range distance from the transmitter to the target, is the range distance from the transmitter to the target, is the effective backscatter area of the target, and is the effective backscatter area of the target, and AArr is the is the

area of the receiving antenna.area of the receiving antenna.

Radar Backscatter Radar Backscatter Coefficient, Coefficient, ˚̊

Radar Backscatter Radar Backscatter Coefficient, Coefficient, ˚̊

Finally, it is the effects of terrain on the radar signal that we are Finally, it is the effects of terrain on the radar signal that we are most interested in, i.e. the amount of most interested in, i.e. the amount of radar cross-sectionradar cross-section, , , , reflected back to the receiver, per unit area reflected back to the receiver, per unit area aa on the ground. This is on the ground. This is called the called the radar backscatter coeffieient radar backscatter coeffieient (( ˚ ˚)) and is computed as : and is computed as :

˚ = ˚ = aa

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

Finally, it is the effects of terrain on the radar signal that we are Finally, it is the effects of terrain on the radar signal that we are most interested in, i.e. the amount of most interested in, i.e. the amount of radar cross-sectionradar cross-section, , , , reflected back to the receiver, per unit area reflected back to the receiver, per unit area aa on the ground. This is on the ground. This is called the called the radar backscatter coeffieient radar backscatter coeffieient (( ˚ ˚)) and is computed as : and is computed as :

˚ = ˚ = aa

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

Surface Surface RoughnessRoughnessSurface Surface

RoughnessRoughness

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

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

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

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

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 from terrain illuminated with 3 cm illuminated with 3 cm

wavelength microwave wavelength microwave energy with a energy with a

depression angle of depression angle of 45˚.45˚.

Expected surface Expected surface roughness back-scatter roughness back-scatter

from terrain from terrain illuminated with 3 cm illuminated with 3 cm

wavelength microwave wavelength microwave energy with a energy with a

depression angle of depression angle of 45˚.45˚.

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

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

Surface Surface RoughnessRoughnessSurface Surface

RoughnessRoughness

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

Backscatter from bare surfaces

Backscatter from rough, bare surfaces

a) VV, b) VH; c) HV; d) HH (Johnson et al., 1996)

Backscatter from man-made objects

Backscatter from vegetation

Soil Moisture -- Dielectric properties of surface materials

Shuttle imaging radar (SIR-C) and Landsat over archeological site in Oman