GEOGG141/GEOG3051 Principles & Practice of Remote Sensing
(PPRS) Active Remote Sensing: RADAR I Dr. Mathias (Mat) Disney UCL
Geography Office: 113, Pearson Building Tel: 7670 05921 Email:
[email protected] www.geog.ucl.ac.uk/~mdisney
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2 OVERVIEW OF NEXT 2 LECTURES Principles of RADAR, SLAR and SAR
Characteristics of RADAR SAR interferometry Applications of SAR
Summaries
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3 PRINCIPLES AND CHARACTERISTICS OF RADAR, SLAR AND SAR
Examples Definitions Principles of RADAR and SAR Resolution
Frequency Geometry Radiometry: the RADAR equation(s)
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4 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 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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5 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/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 Free
data archive: http://www.infoterra.de/terrasar-x-archive/
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6 9/8/91 ERS-1 (11.25 am), Landsat (10.43 am)
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7 Infoterra Gmbh 2009: 12/1/09 1m resolution
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8 Ice
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9 Oil slick Galicia, Spain
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10 Nicobar Islands December 2004 tsunami flooding in red
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11 Paris
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12 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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13 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
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14 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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15 Principle of RADAR
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16 Principle of ranging and imaging
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17
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18 ERS 1 and 2 geometry
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19 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 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 P t is
transmitted by an antenna with gain G t, the power per unit solid
angle in the direction of the scatterer is P t G t, where the value
of G t in that direction is used. READ:
http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses
/Radar_Course_III/radar_equation.htm and Jensen Chapter 9
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21 The Radar Equation The cross-section 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. f
a is absorption A rs is effective area of incident beam received by
scatterer G ts is gain of the scatterer in the direction of the
receiver 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 Where: The Radar scattering cross section R = range P = power
G = gain of antenna A = area of the antenna Because READ:
http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar
_Course_III/radar_equation.htm and Jensen Chapter 9
http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Courses/Radar
_Course_III/radar_equation.htm
23 The Radar Equation: Point targets Power received G t is the
transmitter gain, A r is the effective area of receiving antenna
and the effective area of the target. Assuming same transmitter and
receiver, A/G= 2 /4
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24
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25
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26 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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27 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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28
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29 Principle of Synthetic Aperture Radar SAR Doppler frequency
shift f D due to sensor movement As target gets closer
http://www.radartutorial.eu/11.coherent/co06.en.html
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30 Azimuth resolution (along track): RAR Target time in beam =
arc length / v = S /v = S /vL a so resolution = S /L a v S Arc = S
LaLa = beamwidth = /L a
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31 Range resolution (across track): RAR i.e. A-B is < PL/2
cannot resolve A & B
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32 Range and azimuth resolution (RAR) Range resolution (across
track) L S R a L =antenna length S = slant range = height H/sin
=wavelength Azimuth resolution (along track) cos : inverse
relationship with angle L sin H Pulse length typically 0.4-1 s i.e.
8-200m Short pulse == higher R r BUT lower signal T = pulse length
c = speed of light = depression angle (deg)
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33 Azimuth resolution: SAR
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34 Azimuth resolution (along track): SAR LaLa S RaRa
Previously, azimuth resolution R a = S/L = H/Lsin where H = height
So, for synthetic aperture of 2Ra & nominal slant range S
(H/sin ) we see R a, SAR = S/2R a = L/2 So R a, SAR independent of
H, and improves (goes down) as L goes down See:
http://facility.unavco.org/insar-class/sar_summary.pdf
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35 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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36 Some Spaceborne Systems
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37 ERS 1 and 2 Specifications Geometric specifications Spatial
resolution: along track /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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55 Oil slick Galicia, Spain
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56 Los Angeles
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57 Response to soil moisture Source: Graham 2001
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58 Crop moisture SAR image In situ irrigation Source: Graham
2001
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59 Types of scattering of radar from different surfaces
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60 Scattering
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61 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.