ESM 266: Active microwave remote sensing

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ESM 266 Active microwave remote sensing

• Jeff Dozier

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Active and passive remote sensing

• Passive uses natural energy, either reflected
  sunlight or emitted thermal or microwave
  radiation
• Active sensor creates its own energy
• Transmitted toward Earth
• Interacts with atmosphere and/or surface
• Reflects back toward sensor (backscatter)

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Widely used active remote sensing systems

• Active microwave (radar)
• long-wavelength microwaves (1-100cm)
• recording the amount of energy back-scattered
  from the terrain
• Lidar
• short-wavelength laser light (e.g., 0.90 µm)
• recording the light back-scattered from the
  terrain or atmosphere
• Sonar
• sound waves through a water column
• recording the amount of energy back-scattered
  from the water column or the bottom

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Frequency-wavelength relation

• Generally in the microwave part of the spectrum
  we use frequency instead of wavelength
• Typically measured in s1, called Hertz (Hz)
• Most often Gigahertz (GHz) 109Hz

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Microwave band codes
Unusual names are an artifact of the original
secret work on radar remote sensing in World War
II
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Sending and receiving a pulse of microwave
radiation
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SIR-C/X-SAR images of Rondonia, Brazil
April 10, 1994
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Advantages of radar

• All weather, day or night
• Some areas of Earth are persistently cloud
  covered
• Penetrates clouds, vegetation, dry soil, dry snow
• Sensitive to water content, surface roughness
• Can measure waves in water
• Sensitive to polarization and frequency
• Interferometry (later) using 2 receiving antennas

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Disadvantages of radar

• Penetrates clouds, vegetation, dry soil, dry snow
• Signal is integrated over a depth range and a
  variety of materials
• Sensitive to water content, surface roughness
• Small amounts of water affect signal
• Hard to separate the volume response from the
  surface response
• Sensitive to polarization and frequency
• Many choices for instrument, expensive to cover
  range of possibilities
• The math can be formidable

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How it works

• Pulses of active microwave electromagnetic energy
  illuminate strips of the terrain at right angles
  (orthogonal) to the direction of travel
• called the range or look direction
• The terrain illuminated nearest the aircraft is
  the near-range
• The farthest point of terrain illuminated is the
  far-range

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How it works (cont.)

• Aircraft or satellite travels in a straight line
  the azimuth direction
• Pulses of microwave electromagnetic energy
  illuminate strips of the terrain orthogonal to
  direction of travel the range or look direction
• Terrain illuminated nearest the sensor in the
  line of sight is the near-range
• The farthest point of terrain illuminated by the
  pulse of energy is the far-range
• Generally, objects that trend (or strike) in a
  direction orthogonal (perpendicular) to the range
  or look direction are enhanced much more than
  those objects in the terrain that lie parallel to
  the look direction
• Consequently, linear features that are
  imperceptible in a radar image using one look
  direction may appear bright in another radar
  image with a different look direction.

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Nomenclature

• nadir
• azimuth flight direction
• look direction
• range (near and far)
• depression angle (?)
• incidence angle (?)
• altitude above-ground-level, H
• polarization

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Variability with look direction
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Depression angles and incidence angles

• Depression angle (g) between a horizontal plane
  extending out from the sensor and the
  electromagnetic pulse of energy from the antenna
  to a specific point on the ground
• Incidence angle (q) between the radar pulse and
  the normal to Earths surface
• When surface is flat, q 90g

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Polarization

• 1st letter is transmitted polarization, 2nd is
  received
• Can have VV, HH (like)
• HV, VH (cross)

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Polarization with visible light

• In this case, incoming radiation (sunlight) is
  not polarized (or is polarized in both
  directions)
• Vertically polarized light is reflected from
  surface
• At this view angle, horizontally polarized light
  is not
• So horizontal filter allows us to see the bottom

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Polarization with radar
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Radar geometry

• is weird, not like cameras or multispectral
  sensors
• Uncorrected radar imagery is displayed in
  slant-range geometry, based on the distance from
  the radar to each of the respective features in
  the scene
• But can also display in ground-range geometry, so
  that features in the scene are in their proper
  planimetric (x,y) positions
• Radar resolution has 2 dimensions, range and
  azimuth

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Range resolution
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Azimuth resolution
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Foreshortening, layover, shadow
Geometric distortions in all radar imagery
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Foreshortening

• In flat terrain, easy to convert a slant-range
  radar image into a ground-range radar image
• but with trees, tall buildings, or mountains,
  you get radar relief displacement
• 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
• Terrain that slopes toward the radar will appear
  compressed or foreshortened compared to slopes
  away from the radar

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Foreshortening
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Layover

• Extreme case of foreshortening, when incidence
  angle is less than slope angle toward radar (i.e.
  ?lta)
• cannot be corrected
• got to be careful in the mountains

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Shadow

• When slope away from radar is steeper than the
  depression angle, i.e. a gt ?

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Speckle

• Grainy salt-and-pepper pattern in radar imagery
• Caused by coherent nature of the radar wave,
  which causes random constructive and destructive
  interference, and hence random bright and dark
  areas in a radar image
• Reduced by multiple looks
• processing separate portions of an aperture and
  recombining these portions so that interference
  does not occur

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Synthetic aperture radar (SAR)

• Major advance in radar remote sensing to improve
  azimuth resolution by synthesizing a long antenna

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Synthetic aperture radar (SAR)
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Based on Doppler principle

• Frequency (pitch) of a wave changes if the
  receiver and/or source are in motion relative to
  one another
• Train whistle has a increasing pitch as it
  approaches, highest when it is directly
  perpendicular to the listener (receiver)
• Point of zero Doppler
• After train passes by, its pitch will decrease in
  frequency in proportion to the distance it is
  from the listener (receiver)
• This principle is applicable to all harmonic wave
  motion, including the microwaves used in radar
  systems

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Synthetic aperture radar
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Creation of SAR image
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Radar equation
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Radar backscatter coefficient

• Primary signal of interest
• Percentage of electromagnetic energy reflected
  back to the radar from within a resolution cell
• Depends on terrain parameters like
• geometry, surface roughness, moisture content,
  and
• radar system parameters (wavelength, depression
  angle, polarization, etc.)

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Roughness
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Nile River, Sudan
Space shuttle color VNIR
SIR-C Color Composite Red C-band HV Green
L-band HV Blue L-band HH
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Sources of radar backscattering from a vegetation
canopy

• Subscripts
• t trunk
• s soil
• c leaves
• m multiple

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Types of scattering from a pine stand
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Strength of scattering from a pine stand depends
on frequency
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RADARSAT-2 (launched Dec 2007)

• C-band radar (5.4 GHz) with HH, VV, HV, and VH
  polarizations

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SIR-C/X-SAR web site at JPL

• SIR-C
• Spaceborne Imaging Radar-C (following SIR-A in
  1981 and SIR-B in 1984)
• X-SAR
• X-band Synthetic Aperture Radar (built by
  Germans)
• Flew on Shuttle, 2 10-day missions in 1994

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