2009-10 CEGEG046 / GEOG3051
Principles & Practice of Remote Sensing (PPRS)
8: RADAR 1
Dr. Mathias (Mat) Disney
UCL Geography
Office: 113, Pearson Building
Tel: 7670 05921
Email: mdisney@ucl.geog.ac.uk
www.geog.ucl.ac.uk/~mdisney
OVERVIEW FOR LECTURES 8-10
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•
•
•
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Principles of RADAR, SLAR and SAR
Characteristics of RADAR
SAR interferometry
Applications of SAR
Summaries
2
LECTURE 8
PRINCIPLES AND CHARACTERISTICS OF
RADAR, SLAR AND SAR
•
•
•
•
•
•
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Examples
Definitions
Principles of RADAR and SAR
Resolution
Frequency
Geometry
Radiometry: the RADAR equation(s)
3
References
• Jensen, J. R. (2000) Remote sensing of the Environment, Chapter 9.
• Henderson and Lewis, Principles and Applications of Imaging Radar,
John Wiley and Sons
• Allan T D (ed) Satellite microwave remote sensing, Ellis Horwood,
1983
• F. Ulaby, R. Moore and A. Fung, Microwave Remote Sensing: Active
and Passive (3 vols), 1981, 1982, 1986
• S. Kingsley and S. Quegan, Understanding Radar Systems, SciTech
Publishing.
• C. Oliver and S. Quegan, Understanding Synthetic Aperture Radar
Images, Artech House, 1998.
• Woodhouse I H (2000) Tutorial review. Stop, look and listen: auditory
perception analogies for radar remote sensing, International Journal of
Remote Sensing 21 (15), 2901-2913.
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Web resources, tutorials
Canada
• http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/chapter3/01_e.php
•
http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/pdf/fundamentals_e.pdf
ESA
• http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar
_Courses/
Miscellaneous:
•
http://www.radartutorial.eu/index.en.html
Infoterra TERRASAR-X
•
http://www.infoterra.de/image-gallery/images.html
5
9/8/91
ERS-1 (11.25 am), Landsat (10.43 am)
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© Infoterra Gmbh 2009: 12/1/09 1m resolution
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Ice
8
Oil slick
Galicia, Spain
9
Nicobar Islands
December
2004
tsunami
flooding in
red
10
Paris
11
Definitions
• Radar - an acronym for Radio Detection And Ranging
• SLAR – Sideways Looking Airborne Radar
– Measures range to scattering targets on the ground, can be used
to form a low resolution image.
• SAR Synthetic Aperture Radar
– Same principle as SLAR, but uses image processing to create
high resolution images
• IfSAR Interferometric SAR
– Generates X, Y, Z from two SAR images using principles of
interferometry (phase difference)
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What is RADAR?
• Radio Detection and Ranging
• Radar is a ranging instrument
• (range) distances inferred from time elapsed between
transmission of a signal and reception of the returned
signal
• imaging radars (side-looking) used to acquire images
(~10m - 1km)
• altimeters (nadir-looking) to derive surface height
variations
• scatterometers to derive reflectivity as a function of
incident angle, illumination direction, polarisation, etc
13
What is RADAR?
• A Radar system has three primary functions:
- It transmits microwave (radio) signals towards a
scene
- It receives the portion of the transmitted energy
backscattered from the scene
- It observes the strength (detection) and the time
delay (ranging) of the return signals.
• Radar is an active remote sensing system & can
operate day/night
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Principle of RADAR
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Principle of
ranging and
imaging
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17
ERS 1 and 2
geometry
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Radar wavelength
• Most remote sensing radar wavelengths 0.5-75 cm:
X-band: from 2.4 to 3.75 cm (12.5 to 8 GHz).
C-band: from 3.75 to 7.5 cm (8 to 4 GHz).
S-band: from 7.5 to 15 cm (4 to 2 GHz).
L-band: from 15 to 30 cm (2 to 1 GHz).
P-band: from 30 to 100 cm (1 to 0.3 GHz).
• The capability to penetrate through precipitation or
into a surface layer is increased with longer
wavelengths. Radars operating at wavelengths > 4 cm
are not significantly affected by cloud cover
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20
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Choice of wave length
• Radar wavelength should be matched to the size of
the surface features that we wish to discriminate
• – e.g. Ice discrimination, small features, use X-band
• – e.g. Geology mapping, large features, use L-band
• – e.g. Foliage penetration, better at low frequencies,
use P-band, but……
• In general, C-band is a good compromise
• New airborne systems combine X and P band to give
optimum measurement of vegetation
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Synthetic Aperture Radar (SAR)
• Imaging side-looking accumulates data along path –
ground surface “illuminated” parallel and to one side
of the flight direction. Data processing needed to
produce radar images.
• Motion of platform used to synthesise larger antenna
• The across-track dimension is the “range”. Near range
edge is closest to nadir; far range edge is farthest
from the radar.
• The along-track dimension is referred to as “azimuth”.
• Resolution is defined for both the range and azimuth
directions.
• Digital signal processing is used to focus the image
and obtain a higher resolution than achieved by
conventional radar
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24
Principle of
Synthetic
Aperture Radar
SAR
Doppler frequency shift fD due
to sensor movement
As target gets closer
http://www.radartutorial.eu/11.coherent/co06.en.html
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Azimuth resolution (along track): RAR
v
La

= beamwidth
= /La
S
Arc = S
Target
time in beam = arc length / v = S/v =
S/vLa so resolution = S/La
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Range resolution (across track): RAR
τ
i.e. A-B is < PL/2 cannot resolve A & B
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Range and azimuth resolution (RAR)
Range resolution (across track)
Azimuth resolution (along track)
S
H
Tc
Rr 
2 cos 
Ra 
T = duration of the radar pulse
c = speed of light
γ = depression angle
L = antenna length
S = slant range = height H/sin
λ = wavelength
L

L sinγ
Pulse length typically 0.4-1s i.e. 8-200m
Short pulse == higher Rr BUT lower signal
cos : inverse relationship with angle
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Azimuth resolution: SAR
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Azimuth resolution (along track): SAR
See: http://facility.unavco.org/insar-class/sar_summary.pdf
La
S
Ra
Previously, azimuth resolution Ra = S/L = H/Lsin where H = height
So, for synthetic aperture of 2Ra & nominal slant range S (H/sin) we see
Ra, SAR = S/2Ra = L/2
So Ra, SAR independent of H, and improves (goes down) as L goes down
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Important point
• Resolution cell (i.e. the cell defined by the resolutions
in the range and azimuth directions) does NOT mean
the same thing as pixel. Pixel sizes need not be the
same thing. This is important since (i) the
independent elements in the scene are resolutions
cells, (ii) neighbouring pixels may exhibit some
correlation.
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Some Spaceborne Systems
Launch
Agency
properties
ERS-1
ERS-2
Radarsat
1991 (-1997)
1995
1995
ESA
C-VV
CSA
C-HH
JERS
1992-1998
NASDA
L-HH
NASA
DARA / ASI
L,C, X
polarimetric
SIR-C/X-SAR 1994 (2x10 days)
resolution
swath
25 m
100 km
10-100 m
40-500 km
18 m
76 km
30 m
15-90 km
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ERS 1 and 2 Specifications
Geometric specifications
Spatial resolution:
along track <=30 m
across-track <=26.3 m
Swath width:
102.5 km (telemetered)
80.4 km (full performance)
Swath standoff: 250 km to the right of the satellite
track
Localisation accuracy:
along track <=1 km;
across-track <=0.9 km
Incidence angle: near swath 20.1deg.
mid swath 23deg.
far swath 25.9deg
Incidence angle tolerance:
<=0.5 deg.
Radiometric specifications:
Frequency:
5.3 GHz (C-band)
Wave length:
5.6 cm
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Speckle
• Speckle appears as
“noisy” fluctuations in
brightness
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Speckle
• Fading / speckle are inherent “noise-like” processes in a
coherent imaging system.
• Speckle = constructive / destructive interference
• Averaging independent samples can effectively reduce the
effects of speckle (~1/sqrt(N)) for N samples
• Multiple-look filtering
– separate maximum synthetic aperture into smaller sub-apertures to
generate independent views of target areas based on the angular
position of the targets. Looks are different Doppler frequency bands.
• Averaging (incoherently) adjacent pixels.
• Either approach
– enhances radiometric resolution at the expense of spatial resolution.
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Speckle
36
Speckle
• Radar images are formed coherently and
therefore inevitably have a “noise-like”
appearance
• Implies that a single pixel is not representative of
the backscattering
• “Averaging” needs to be done
37
Multi-looking
• Speckle can be suppressed by “averaging” several
intensity images
• This is often done in SAR processing
• Split the synthetic aperture into N separate parts
• Suppressing the speckle means decreasing the width
of the intensity distribution
• We also get a decrease in spatial resolution by the
same factor (N)
• Note this is in the azimuth direction (because it
relies on the motion of the sensor which is in this
direction)
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Speckle
39
Principle of
ranging and
imaging
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Geometric effects
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Shadow
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Foreshortening
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Layover
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Layover
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Radiometric aspects – the RADAR equation
• The brightness of features is combination of several
variables / characteristics
– Surface roughness of the target
– Radar viewing and surface geometry relationship
– Moisture content and electrical properties of the target
• http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/R
adar_Courses/Radar_Course_III/radar_equation.htm
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Returned energy
• Angle of the surface to the incident radar beam
– Strong from facing areas, weak from areas facing away
• Physical properties of the sensed surface
– Surface roughness
– Dielectric constant
Smooth
Rough
– Water content of the surface
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Roughness
Smooth, intermediate or rough?
• Peake and Oliver (1971) – surface height variation h
– Smooth: h < /25sin
– Rough: h > /4.4sin 
– Intermediate
–  is depression angle, so depends on  AND imaging geometry
http://rst.gsfc.nasa.gov/Sect8/Sect8_2.html
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Oil slick
Galicia, Spain
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Los
Angeles
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Source: Graham 2001
Response to soil moisture
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Crop moisture
SAR image
In situ irrigation
Source: Graham 2001
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Types of
scattering of
radar from
different
surfaces
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Scattering
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The Radar Equation
Relates characteristics of the radar, the target, and the received signal
The geometry of scattering from an isolated radar target (scatterer) is shown.
When a power Pt is transmitted by an antenna with gain Gt , the power per unit
solid angle in the direction of the scatterer is Pt Gt, where the value of Gt in that
direction is used.
READ:http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_C
ourses/Radar_Course_III/radar_equation.htm and Jensen Chapter 9
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The Radar Equation
Radar equation can be stated in 2 alternate forms: one in terms of
the antenna gain G and the other in terms of the antenna area
Because
R = range
P = power
G = gain of antenna
A = area of the antenna
Where:
The Radar scattering cross section
The cross-section σ is a function of the directions of the incident wave and the
wave toward the receiver, as well as that of the scatterer shape and dielectric
properties.
fa is absorption
Ars is effective area of incident beam received by scatterer
Gts is gain of the scatterer in the direction of the receiver
READ:
http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar
56
_Course_III/radar_equation.htm and Jensen Chapter 9
Measured quantities
s2
• Radar cross section [dBm2]
• Bistatic scattering coefficient [dB]
• Backscattering coefficient [dB]
|E |
lim
2
  r   4r
i2
|E |
s2
2
lim 4r |E |
0
 r
i2
A cos
i |E |
s2
2
lim 4r |E |
0
 r
i2
A
|E |
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The Radar Equation: Point targets
• Power received
P PG
r
t t
1
4R
2

1
4R
A
2 r
• Gt is the transmitter gain, Ar is the effective area of
receiving antenna and  the effective area of the target.
Assuming same transmitter and receiver, A/G=2/4
2 2
 G
P P

3 4
r
t
(4 ) R
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Calibration of SAR
• Emphasis is on radiometric calibration to
determine the radar cross section
• Calibration is done in the field, using test sites
with transponders.
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