introduction to sar interferometry
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Introduction to SAR interferometry
Rüdiger Gens
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Outline
! Relevant terms! Geometry! Why does InSAR work?! Techniques! Products! Applications! Error sources! Trends and challenges
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Relevant terms
! amplitude" measure of the
strength of the signal ! phase
" angle of a complex number
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Relevant terms
! baseline" separation between the two antenna positions either
mounted on an aircraft or realized by two repeating satellite orbits
Source: Gens and van Genderen, IJRS, 1996
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Relevant terms
! fringe" represents the whole
range of the phase in an interferogram from 0 to 2π in a full colour cycle
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Geometry of SAR interferometry
z
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Why does InSAR work?
! coherent signal" single frequency
! same geometry covering the same area from slightly different position in space
Does optical interferometry work?
! coherent light source: ! application: holography
laser
yes
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Interferometric techniques
! across-track interferometry" regular airborne geometry
! along-track interferometry" airborne geometry" monitoring ocean currents or other moving objects
! repeat-pass interferometry" usually spaceborne
! differential interferometry" change detection
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Across-track interferometry
! presently only employed on airborne system
! two SAR antenna systems to be mounted simultaneously on the platform perpendicular to the flight direction
! distinction between errors caused by the aircraft roll and the influence of the terrain slope is not possible.
hx
r2
r1
z r21
θ21
θlook
H
flight path
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Along-track interferometry
! only applicable to airborne SAR systems
! yaw and pitch cause baseline components in y- and z-direction which produce additional phase differences.
! calibration of the phase difference is necessary before absolute velocity measurements are possible
x
hy
r2
r1
z r21
θ21
θlook
H
flight path
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Repeat-pass interferometry
! requires only one antenna
! most suited to spaceborne sensors
! precise location of the flight path is required
! satellite passing nearly the same orbit to cover an area twice with a slightly different viewing geometry.
flight pathsz O2
O1
r2
r1
h
yx
H
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Interferogram
complex multiplication of the two images; i.e. the correspondingamplitudes have to be averaged and the corresponding phases have to be differenced at each point in the image
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Coherence image
"measure for the correlation of corresponding signals"ranges from 0 to 1
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Digital elevation model
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Digital elevation model
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Differential interferogram
! change detection: measurement of small-scale movements in the vertical direction
! displacement measured is not vertical, but along the viewing direction
! relative accuracy of the order of a few centimetres or even less vs. absolute accuracy of digital elevation models of about 10-15 meters (for ERS data)
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Interferometric applications
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Seismic events
Source: Massonnet et al. (1993)
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Volcanic hazards
Source: Massonnet (1997)
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Glacier research
Image Credits:Received by CCRSPre-processed by RSI
Interpretation:Laurence Gray, CCRSKarim Mattar, IntermapParis Vachon, CCRS
© CSA, 1996
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Land subsidence
Source: http://www.gamma-rs.ch/research/mexico.html
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Forestry
Source: Strozzi and Wegmüller (1997)
interferometric correlation
backscatter intensity
backscatter change
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Error sources
! atmospheric effects" maybe most limiting factor for differential InSAR" delays signal return" can be heterogeneous and very local
→ difficult to determine" change the tropospheric water vapor content" new research field for studying of tropospheric
turbulences and ionospheric phenomena
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Error sources
! orbits" predicted vs. restituted vs. precise orbits
! temporal decorrelation" caused by physical changes in the surface over the
time period between observations
! baseline decorrelation" higher noise level with increasing length of baseline
→ decorrelation of radar signal
! processing
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Trends and challenges
! Shuttle Radar Topography Mission (SRTM)" flown in February 2000
! swath processing" data volume" Doppler frequency issues
! DEM production for larger areas" SRTM" AKDEM project
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Questions