Active and Passive Microwave Remote Sensing

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Active and Passive Microwave Remote Sensing

• Lecture 7
• Oct 6, 2004

Reading materials Chapter 9
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Basics of passive and active RS

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

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Widely used active RS systems

• Active microwave (RADAR RAdio Detection And
  Ranging, read p285 for an explanation)
• Long-wavelength microwaves (1 100 cm)
• LIDAR (LIght Detection And Ranging)
• Short-wavelength laser light (UV, visible, near
  IR)
• SONAR (SOund Navigation And Ranging)
• Sound waves through a water column.
• Sound waves extremely slow (300 m/s in air, 1,530
  m/s in sea-water)
• Bathymetric sonar (measure water depths and,
  hence changes in bottom topography )
• Imaging sonar or sidescan imaging sonar (imaging
  the bottom topography and bottom roughness)
• It is not our focus in this remote sensing class.

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Microwaves
Band Designations (common wavelengths
Wavelength (?) Frequency (?) shown in
parentheses) in cm in
GHz ______________________________________________
_ Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5 K 1.18
- 1.67 26.5 to 18.0 Ku 1.67 - 2.4 18.0 to
12.5 X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 8.0 C
(7.5, 6.0 cm) 3.8 - 7.5 8.0 - 4.0 S (8.0,
9.6, 12.6 cm) 7.5 - 15.0 4.0 - 2.0 L (23.5,
24.0, 25.0 cm) 15.0 - 30.0 2.0 - 1.0 P (68.0
cm) 30.0 - 100 1.0 - 0.3
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1. Active microwave remote sensing
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Two active radar imaging systems

• In world war II, ground based radar was used to
  detect incoming planes and ships.
• Imaging RADAR was not developed until the 1950s
  (after the world war II). Since then, the
  side-looking airborne radar (SLAR) has been used
  to get detail image of enemy sites along the edge
  of the fight field.
• Real aperture radar
• Aperture means antenna
• A fixed length (for example 1 - 11m)
• Synthetic aperture radar (SAR)
• 1m (11m) antenna can be synthesized
  electronically into a 600m (15 km) synthetic
  length.
• Most (air-, space-borne) radar systems now use
  SAR.

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Principle of SLAR
A CRT (cathode ray tube) shows a quick-look
display
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Radar Nomenclature nadir azimuth (or flight)
direction look (or range) direction range
(near, middle, and far) depression angle (?)
incidence angle (?) altitude above-ground-level,
H polarization
?
?
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Polarization

• Unpolarized energy vibrates in all possible
  directions perpendicular to the direction of
  travel.
• The pulse of electromagnetic energy is filtered
  and sent out by the antenna may be vertically or
  horizontally polarized.
• The pulse of energy received by the antenna may
  be vertically or horizontally polarized
• VV, HH like-polarized imagery
• VH, HV- cross-polarized imagery

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• Lava flow in north-center Arizona

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Slant-range vs. Ground-range geometry
Radar imagery has a different geometry than that
produced by most conventional remote sensor
systems, such as cameras, multispectral scanners
or area-array detectors. Therefore, one must be
very careful when attempting to make
radargrammetric measurements. Uncorrected
radar imagery is displayed in what is called
slant-range geometry, i.e., it is based on the
actual distance from the radar to each of the
respective features in the scene. It is
possible to convert the slant-range display into
the true ground-range display on the x-axis so
that features in the scene are in their proper
planimetric (x,y) position relative to one
another in the final radar image.
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• Most radar systems and data providers now provide
  the data in ground-range geometry

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Range (or across-track) Resolution
Pulse duration (t) 0.1 x 10 -6 sec

• t.c called pulse length. It seems the short pulse
  length will lead fine range resolution.
• However, the shorter the pulse length, the less
  the total amount of energy that illuminates the
  target.

t.c/2
t.c/2
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Azimuth (or along-track) Resolution

• The shorter wavelength and longer antenna will
  improve azimuth resolution.
• The shorter the wavelength, the poorer the
  atmospheric and vegetation penetration capability
• There is practical limitation to the antenna
  length, while SAR will solve this problem.

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Synthetic Aperture Radar - SAR
A major advance in radar remote sensing has been
the improvement in azimuth resolution through the
development of synthetic aperture radar (SAR)
systems. Great improvement in azimuth resolution
could be realized if a longer antenna were used.
Engineers have developed procedures to synthesize
a very long antenna electronically. Like a brute
force or real aperture radar, a synthetic
aperture radar also uses a relatively small
antenna (e.g., 1 m) that sends out a relatively
broad beam perpendicular to the aircraft. The
major difference is that a greater number of
additional beams are sent toward the object.
Doppler principles are then used to monitor the
returns from all these additional microwave
pulses to synthesize the azimuth resolution to
become one very narrow beam.
Azimuth resolution is constant D/2, it
is independent of the slant range distance, ? ,
and the platform altitude.
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Fundamental radar equation
t
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Amount of backscatter per unit area
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Intermediate
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Penetration ability to forest
Response of A Pine Forest Stand to X-, C- and
L-band Microwave Energy
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Penetration ability to subsurface
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Roughness and Penetration ability
to subsurface
Railway
hw90
Radar Image
ETM Image
Xie et al., 2004
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Penetration ability to heavy rainfall
SIR-C/X-SAR Images of a Portion of Rondonia,
Brazil, Obtained on April 10, 1994
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Radar Shadow

• Shadows in radar images can enhance the
  geomorphology and texture of the terrain. Shadows
  can also obscure the most important features in a
  radar image, such as the information behind tall
  buildings or land use in deep valleys. If certain
  conditions are met, any feature protruding above
  the local datum can cause the incident pulse of
  microwave energy to reflect all of its energy on
  the foreslope of the object and produce a black
  shadow for the backslope
• Unlike airphotos, where light may be scattered
  into the shadow area and then recorded on film,
  there is no information within the radar shadow
  area. It is black.
• Two terrain features (e.g., mountains) with
  identical heights and fore- and backslopes may be
  recorded with entirely different shadows,
  depending upon where they are in the
  across-track. A feature that casts an extensive
  shadow in the far-range might have its backslope
  completely illuminated in the near-range.
• Radar shadows occur only in the cross-track
  dimension. Therefore, the orientation of shadows
  in a radar image provides information about the
  look direction and the location of the near- and
  far-range

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Shadows and look direction
Shuttle Imaging Radar (SIR-C) Image of Maui
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Radar Noise Speckle

• Speckle is a grainy salt-and-pepper pattern in
  radar imagery present due to the coherent nature
  of the radar wave, which causes random
  constructive and destructive interference, and
  hence random bright and dark areas in a radar
  image. The speckle can be reduced by processing
  separate portions of an aperture and recombining
  these portions so that interference does not
  occur. This process, called multiple looks or
  non-coherent integration, produces a more
  pleasing appearance, and in some cases may aid in
  interpretation of the image but at a cost of
  degraded resolution. N (D/2)

N, number of looks D, antenna length
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Another way to remove speckle noise
Blurred objects and boundary
Statistical algorithms Geometric algorithms
G-MAP
Gamma Maximum A Posteriori Filter
Xie et al., 2004
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Striping Noise and Removal
CPCA
Combined Principle Component Analysis
Xie et al., 2004
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Major Active Radar Systems

• Seasat, June 1978, 105 days mission, L-HH band,
  25 m resolution
• SIR-A, Nov. 1981, 2.5 days mission, L-HH band, 40
  m resolution
• SIR-B, Oct. 1984, 8 days mission, L-HH band,
  about 25 m resolution
• SIR-C, April and Sept. 1994, 10 days each. X-,
  C-, L- bands multipolarization (HH, VV, HV, VH),
  10-30 m resolution,
• JERS-1, 1992-1998, L-band, 15-30 m resolution,
  (Japan)
• RADARSAT, Jan. 1995-now, C-HH band, 10, 50,
  and 100 m, (Canada)
• ERS-1, 2, July 1991-now, C-VV band, 20-30 m,
  (European)
• AIRSAR/TOPSAR, 1998-now, C,L,P bands with full
  polarization, 10m,
• NEXRAD, 1988-now, S-band, 1-4 km,
  
• TRMM precipitation radar, 1997, Ku-band, 4km,
  vertical 250m, (USA and
• 
  
  Japan)

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Advantages of active 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 (soil moisture),
  roughness
• Can measure waves
• Sensitive to polarization
• Interferometry

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2. Passive microwave remote sensing
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Principals

• While dominate wavelength of Earth is 9.7 um, a
  continuum of energy is emitted from Earth to the
  atmosphere. In fact, the Earth passively emits a
  steady stream of microwave energy, though it is
  relatively weak in intensity.
• A suit of radiometers developed can record it.
  They measure the brightness temperature of the
  terrain or the atmosphere. This is much like the
  thermal infrared radiometer for temperature.
• A matrix of brightness temperature values can
  then be used to construct a passive microwave
  image.

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Jeff Dozier
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Some important passive microwave radiometers

• Special Sensor Mirowave/Imager (SSM/I)
• It was onboard the Defense Meterorological
  Satellite Program (DMSP) since 1987
• It measure the microwave brightness temperatures
  of atmosphere, ocean, and terrain at 19.35,
  22.23, 37, and 85.5 GHz.
• TRMM microwave imager (TMI)
• It is based on SSM/I, and added one more
  frequency of 10.7 GHz.

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