Active Microwave Remote Sensing

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Active Microwave Remote Sensing

• Lecture 8
• Oct 22, 2007

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Recap 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)
• Advantages all weathers and all times

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Widely used active RS systems

• RADAR RAdio Detection And Ranging
• Long-wavelength microwaves (1 100 cm)
• LIDAR LIght Detection And Ranging
• Short-wavelength laser light (UV, visible, near
  IR)
• SONAR SOund Navigation And Ranging (very long
  wave, low Hz)
• Sound can not travel through vacuum
• Earth and water absorb acoustic energy far less
  than EMR energy
• Seismic survey use small explosions, record the
  reflected sound
• Medical imaging using ultrasound
• Sound waves through a water column.
• Sound waves are extremely slow (300 m/s in air,
  1,530 m/s in sea-water)
• Bathymetric sonar (measure water depths and
  changes in bottom topography )
• Imaging sonar or sidescan imaging sonar (imaging
  the bottom topography and bottom roughness)

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Types of radar

• Nonimaging radar
• Traffic police use handheld Doppler radar system
  determine the speed by measuring frequency shift
  between transmitted and return microwave signal
• Plan position indicator (PPI) radars use a
  rotating antenna to detect targets over a
  circular area, such as NEXRDA
• Satellite-based radar altimeters (low spatial
  resolution but high vertical resolution)
• Imaging radar
• Usually high spatial resolution,
• Consists of a transmitter, a receiver, one or
  more antennas, GPS, computers

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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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Two imaging radar systems

• In World War II, ground based radar was used to
  detect incoming planes and ships (non-imaging
  radar).
• Imaging RADAR was not developed until the 1950s
  (after World War II). Since then, side-looking
  airborne radar (SLAR) has been used to get
  detailed images of enemy sites along the edge of
  the flight field. SLAR is usually a real aperture
  radar. The longer the antenna (but there is
  limitation), the better the spatial resolution
• Real aperture radar (RAR)
• Aperture means antenna
• A fixed length (for example 1 - 15m)
• 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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1. RAR Principle of SLAR
waveform
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Radar Nomenclature and Geometry
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
Azimuth flight direction
Look/Range direction
?
Flightline groundtrack
?
Near range
Far range
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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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2. 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.
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Azimuth resolution is constant D/2, it is
independent of the slant range distance, ? , and
the platform altitude. So the same SAR system in
a aircraft and in a spacecraft should have the
same resolution. There is no other remote sensing
system with this capability.
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(No Transcript)
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Animation of the Doppler Effect
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Animation of the Doppler Effect
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Animation of the Doppler Effect
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Animation of the Doppler Effect
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Animation of the Doppler Effect
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Animation of the Doppler Effect
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Animation of the Doppler Effect
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Animation of the Doppler Effect
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At time n3, the shortest distance and area of
zero Doppler shift
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Speckle noise

• Using SAR, we can get high spatial resolution in
  the azimuth dimension (direction). But the
  coherently recording returned echoes (SAR) also
  causes speckle noise.
• For one-single channel SAR system, the speckle
  noise has a multiplicative nature for the
  amplitude and an additive nature for the phase.
• For multi-dimensional (or polarimetric) SAR (or
  PolSAR) system, speckle noise is even
  complicated.
• There are two ways to remove speckle noise
• Using several looks, i.e., averaging takes place,
  usually 4 or 16 looks (N). But lose resolution
  Azimuth resolution N(D/2)
• Modeling the noise, then remove them.

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Backscatter

• The portion of the outgoing radar signal that the
  target redirects directly back towards the radar
  antenna.
• When a radar system transmits a pulse of energy
  to the ground (A), it scatters off the ground in
  all directions (C). A portion of the scattered
  energy is directed back toward the radar receiver
  (B), and this portion is referred to as
  "backscatter".

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Amount of backscatter per unit area
http//earth.esa.int/applications/data_util/SARDOC
S/spaceborne/Radar_Courses/Radar_Course_III/parame
ters_affecting.htm
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Fundamental radar equation
t
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wrong
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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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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Penetration ability into subsurface
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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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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
  (ESA)
• ASAR on EnviSat, 2002-now, C band
  (ESA)
• 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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Active Radar Systems for Mars

• MARSIS (Mars advanced radar for subsurface and
  ionosphere sounding) of Mras Express, 2003, 1.8-5
  MHz, up to 5 KM deep (ESA)
• http//sci.esa.int/science-e/www/object/index.cfm
  ?fobjectid34826fbodylongid1601
• SHARAD (shallow subsurface radar) of MRO, 2005,
  15-25 MHz, up to 1Km deep (ISA-NASA)
• http//mars.jpl.nasa.gov/mro/mission/sc_instru_sha
  rad.html