A Basic Introduction to
Radar Remote Sensing
~~~~~~~~~~
Rev. Ronald J. Wasowski, C.S.C.
Associate Professor of Environmental Science
University of Portland
Portland, Oregon
3 November 2015
Radar Imaging Wavelengths
• Remote sensing wavelength regions and bands
– Band name
• Gamma rays
• X-rays
• Ultraviolet (UV)
– Photographic UV
• Visible
• Infrared (IR)
– Reflected (RIR)
– Thermal (TIR)
»
• Radio
– Microwave
– Radar
– Radio
Wavelengths
Notes
< 0.03 nm
0.03 to 30 nm
0.03 to 0.4 mm
0.3 to 0.4 mm
0.4 to 0.7 mm
0.7 to 100 mm
0.7 to 3.0 mm
3.0 to 14.0 mm
Film
Small !
Sunlight
?!?!?
0.1 to 100 cm
0.1 to 100 cm
> 100 cm
Passive
Active
Passive
Radar Imaging System Components
• Seven basic system components
– Pulse generator
• Generate a signal of specified frequency / wavelength
– Signal transmitter
• Amplify and send the outgoing signal
– Duplexer
• Two-way switch
– Alternate between outgoing & incoming radar pulse
– Radar antenna
• Broadcast the outgoing pulse & accept the return pulse
– Receiver
• Amplify the return pulse amplitude to an acceptable level
– Recorder
• Permanent record of the return pulse:
– Image generator
• Conversion of return pulses into images
Film or digital
Radar Imaging System Components
Important Radar Terms
• Two basic types of radar imaging systems
– SLAR: Side-Looking Airborne Radar
– SAR: Synthetic Aperture Radar
• Azimuth direction
– Flight direction
• Look
direction
– Perpendicular to the azimuth direction
• Range
– Near range
– Far range
• Depression angle
– Angle below horizontal to any feature of interest
• 0°: The horizon
• 90°: Nadir
Important Radar Terms Illustrated
A Typical Radar Image: Columbia R.
Radar Wavelengths & Frequencies
Band
Designation
K
X
C
S
L
P
Wavelength
(cm)
0.8 to 2.4
2.4 to 3.8
3.8 to 7.5
7.5 to 15.0
15.0 to 30.0
30.0 to 100.0
Frequency
(GHz)
40.0 to 12.5
12.5 to 8.0
8.0 to 4.0
4.0 to 2.0
2.0 to 1.0
1.0 to 0.3
Depression & Incidence Angles
• Depression angle g
– Horizontal surface:
Incidence angle
g + Q = 90°
Q = 90° – g
Q
Depression & Incidence Angles
• Depression angle g
– Horizontal surface:
Incidence angle
g + Q = 90°
Q = 90° – g – s
Q
s = Slope
> 0° : Toward
< 0° : Away
Range Resolution of Radar Images
• Range resolution
– Increases from near range to far range
Radar Image Azimuth Resolution
• Azimuth resolution
– Decreases from near- to far-range as beam widens
Radar Displacement & Layover
• Radar image displacement
– Pixel placement determined by straight-line distance
• Near range has more displacement than far range
• Slope effects
– Slopes facing
toward
the radar are smaller than actual
– Slopes facing away from the radar are larger than actual
• Brightness effects
– Slopes facing
toward
the radar are too bright
– Slopes facing away from the radar are too dark
• Radar image
layover
– An extreme form of displacement
• An object’s top is displaced past its bottom
• The object’s top is closer
than its bottom
Radar Image Shadow
EMR Polarization
Radar Image Polarization
• Primary
polarization
Transmitted signal
– Horizontal
• Near range: Electric vector
• Far range: Electric vector
parallel
parallel
to hor surfaces
to hor surfaces
– Vertical
• Near range: Electric vector
parallel
to hor surfaces
• Far range: Electric vector perpendicular to hor surfaces
– Circular
• Secondary polarization
Returned
signal
– Horizontal
• HH:
• HV:
Non-depolarized return
Depolarized
return
Diagonal features
Non-depolarized return
Depolarized
return
Diagonal features
– Vertical
• VV:
• VH:
– Circular
Radar System Properties
• Wavelength
– Short l’s
• Many surfaces are rough
• Insignificant feature penetration
– Long l’s
• Few surfaces are rough
• Significant feature penetration
– Still not forest cover or even grass cover ! ! !
• Depression angle
– Small g’s
• Relatively dark signatures
– Large g’s
• Relatively bright signatures
• Polarization
– Horizontal transmitted: Consistent with terrain
– Vertical transmitted Inconsistent with terrain
Radar Terrain Properties
• Dielectric properties
Electrical conductivity
– Dry rock / soil: 3 < Dielectric constant < 8
– Water:
Dielectric constant = 80
Dark
Bright
• Geometry
– Micro- geometry: Surface texture = Surf. roughness
• Smooth
• Intermediate
• Rough
– Macro-geometry: Features parallel or perpendicular
• Specular reflectors
– One surface oriented nearly perpendicular to the look direction
• Two-sided reflectors
– Two perpendicular surfaces w/join line parallel to flightline
• Corner reflectors
– Three perpendicular surfaces open to the incident radar signal
A Typical Radar Image: Denver
Shuttle Imaging Radar: San Francisco
Radar Roughness: Smooth Texture
• L-band (23.5 cm) wavelength
– Radar-smooth surface:
• Specular reflection
0.0 cm < h < 1.0 cm
Total forescatter
Radar Roughness: Intermed. Texture
• L-band (23.5 cm) wavelength
– Radar-intermediate surface: 1.0 cm < h < 5.7 cm
• Composite specular/diffuse scattering Much forescatter
Radar Roughness: Rough Texture
• L-band (23.5 cm) wavelength
– Radar-rough surface:
• Diffuse scattering
5.7 cm < h
Uniform in all directions
Return Intensity & Depression Angle
• A continuum
– Smooth surface: Approximately specular at nadir
– Rough surface: Approximately uniform at all g’s
Shuttle Imaging Radar: Los Angeles
Radar Image Resolution Revisited
• Principal determining characteristics
– SLAR: Side Looking Airborne Radar Real aperture
• Directly proportional to l
– Shorter wavelengths are better
Rain may interfere
• Inversely proportional to antenna length
– Focus beam to preserve far range azimuth resolution
– Longer antennas are better
Flexing may interfere
– SAR:
Synthetic Aperture Radar
• Directly proportional to l
– Shorter wavelengths are better
• Inversely proportional to antenna length
– “Fake it” by “synthesizing” a very long antenna
» Use coherent radar signal
“Radar laser”
» Process Doppler shift data
» Illuminate each target multiple times
– Live with image speckle
A Typical Radar Image: Ice Floes
A Typical Radar Image: Indonesia
SIR L & C Radar Bands: Mt. Rainier
A Typical Radar Image: Clearcutting
Radar Relief Map: Death Valley CA
A Typical Radar Image: Taiwan
Shuttle Imaging Radar: San Andreas
Appalachians of Eastern Pennsylvania
Part of the Appalachian Mountains
Magellan Radar Mission to Venus