introduction to sar systems and differential sar ... · sar systems and differential sar...
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Tele-Rilevamento Europa − T.R.E. s.r.l.Tele-Rilevamento Europa − T.R.E. s.r.l.
Introduction toSAR systems and
Differential SAR Interferometry
Introduction toSAR systems and
Differential SAR Interferometry
TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July
Outline of the presentationOutline of the presentation
• Satellite SAR systems (in particular ESA ERS)
• SAR images: amplitude and phase data
• Basic of SAR interferometry
• Introduction to DInSAR analysis
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The Synthetic Aperture Radar mounted on the ESA-ERS satellitesThe Synthetic Aperture Radar mounted on the ESA-ERS satellites
λ = 5.66 cm (f = 5.3 GHz)
H = 780 Km
θ = 23 deg
dr = 7.91 m Slant Range Resolution
da = 3.98 m Azimuth Resolution
S ≈ 100 Km
Revisiting Time = 35 days
λ = 5.66 cm (f = 5.3 GHz)
H = 780 Km
θ = 23 deg
dr = 7.91 m Slant Range Resolution
da = 3.98 m Azimuth Resolution
S ≈ 100 Km
Revisiting Time = 35 days
ERS ParametersERS Parameters
The SAR is an active, coherent acquisition
system: both amplitude and phase informationphase information
are recorded. Phase values contains information
about the distance between the sensordistance between the sensor
and the targeand the target on ground. Data are available since 1992since 1992
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SAR can be mounted on different platformsSAR can be mounted on different platforms
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•Today 3 SAR platforms are available for civil applications.
ERS-2 - Gyro failures, reduced performances in attitude control high DC values
Envisat - Multi-mode acquisitions, slightlydifferent wavelength with respect to ERS
Radarsat - Reduced performances inattitude control (no yaw-steering)No systematic acquisition plans
Data availabilityData availability
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Satellite orbitPlane perpendicularto the orbit
Antenna footprint
Slant rangeSlant range
AzimuthAzimuth
Ground rangeGround range
Strip-map
Off-nadirOff-nadir
Acquisition geometry (ESA-ERS)Acquisition geometry (ESA-ERS)
Azimuth Direction
Range
H=780 km
23o
v=7.5 km/s
Antenna dimensions: 1 m (cross range) x 10 m (azimuth)
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R
Earth
Radar satellite
R (sensor-target distance)=
T (delay) . C (light speed)
Range measurementsRange measurements
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What is Synthetic Aperture?What is Synthetic Aperture?
High spatial resolution usually calls for large antenna, however there are practical limits to the dimensions of antenna that can be deployed in space.
Basic idea: the radar can look at the same area from different angles while moving through the platform’s trajectory and can synthetise a larger antenna by properly combining the different echoes…
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SAR amplitude data vs optical imagesSAR amplitude data vs optical images
Optical Image (SPIN-2 Data) Multi-Image SAR (41 Records)
SAR amplitude data are not affected by sun illuminationand/or meteo conditions.
Satellite SAR sensors now available can only detect a single frequency (and polarization). Hence they see a grey-scale image.
Pomona - Los Angeles, CaliforniaPomona - Los Angeles, California
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The detected SAR image contains a measurement of the amplitude of the radiation backscattered toward the radar by the objects (scatterers) contained in each SAR resolution cell.
Typically, exposed rocks and urban areas show strong amplitude ( bright pixel) whereas smooth flat surface,like quiet water basins show low amplitude (dark pixels) since the radiation is mainly measured away from the radar.
Amplitude dataAmplitude data
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There are several differences between radar imagesand optical images such as you get from the Landsator SPOT series of satellites.
Sees the color of the tops of the trees
Sometimes radio waves can penetrate vegetation (dependingon operating frequency)
sees how well different colors of light are reflected
"sees" how well radio waves reflectand scatter off structures
Wavelength : really small!Wavelength : 2-22 cm (X, C, L)
Illumination from SunActive sensor
Can't see through cloudsSee through clouds
OpticalRadar
http://trfic.jpl.nasa.gov/GRFM/cdrom/samerica/DOCS/HTML/TUTORIAL.HTM
About amplitude dataAbout amplitude data
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There are two important things to keep in mind:
1) There are significant variations of reflectivity values bychainging the incidence angle of the illuminating wave;
2) the radio waves interact with the surface - sometimes penetrating, sometimes scattering, sometimes reflecting off more than one target - how they interact determines whatthe image looks like.
http://trfic.jpl.nasa.gov/GRFM/cdrom/samerica/DOCS/HTML/TUTORIAL.HTM
About amplitude dataAbout amplitude data
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How does it look like?How does it look like?
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SAR coordinates: geometric distortionSAR coordinates: geometric distortion
Regular sampling in range
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SAR COORDINATES
Range (westing)
GEOGRAPHIC COORDINATES
Longitude
Slope + Slope +Slope -Slope -
Geographic coordinates vs. SAR coordinates (1/2)Geographic coordinates vs. SAR coordinates (1/2)La
titu
de
Azim
uth
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SAR COORDINATESGEOGRAPHIC COORDINATES
Azim
uth
Range
Geographic coordinates vs. SAR coordinates (2/2)Geographic coordinates vs. SAR coordinates (2/2)
SPOT ERS
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First summaryFirst summary
SAR amplitude data, related to the RCS of the targets,are (almost) not affected by:
- sun illumination and/or - meteo conditions
Two scatterers having the same range distance to the sensorbelong to the same resolution cell and cannot be resolved.SAR imaging geometry has cylindrical symmetry:No way to measure the elevation angle of a radar targetfrom a single SAR image:
impossible to recover the local topography(using just one acquisition).
H θ
r
Gr(θ)
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Q1: Is it possible to pass from SAR to Geographic coordinates?
A1: Yes, if we know the elevation of the radar target.
On the contrary, it is always possible to pass from GEO to SAR coordinates(once we know satellite state vectors)
Azimuth Direction
Range
h
Off-nadir
angle
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Q2: What do you mean with “ascending and descending orbits”?
A2: The combination of the motion of the satellite and the motion of the earth makes it possible to look at the same area of interest fromtwo opposite acquisition geometries.
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Descendin
g orbitA
scen
ding
orb
it
The AOI is then illuminated both from East and from West.
More on ascending and descending orbits
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E
N
U
λ λ
sDsA
descending
ascending
E
N
sA sD
asc desc
E
U asc desc
If we could measure possible displacements along LOS…
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Is it possible to combine ascending and descending orbits? YES!Is it possible to combine ascending and descending orbits? YES!
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Q3: Are “ascending and descending data” always available?
A3: For ERS data usually yes.
For Envisat and Radarsat it depends on the acquisition policy
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A synthetic aperture radar (SAR) works by illuminating the Earth with a beam of
coherent microwave radiation such as a laser. This radiation can be thought of as
an (almost) sinusoidal wave, such as a water wave or a sound wave.
A wave can be described by 3 properties: its wavelength, amplitude, and phase.
Basic ideasBasic ideas
http://www.asf.alaska.edu/apd/software/insar/phase.html
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In SAR, the phase of the echoing signal is compared to a referencewave, so the phase of a SAR image is actually the phase differencebetween the echo and this reference. The phase of the signalbackscattered from a radar target is then related to the sensor-targetdistance. A SAR image is actually a set of pixels characterized by bothamplitude and phase values:
Basic ideasBasic ideas
Amplitude Phase
KNOWN MODULO 2π
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Phase contributions of a single SAR acquisition:
ναλπψφ +++= r4
ψ * Reflectivity of the radar target
SAR data: phase contributionsSAR data: phase contributions
4πr/λ * Propagator. It depends on the sensor-target distance
α * Atmospheric Phase Contribution
ν * Noise
Amplitude
Phase
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SAR data: phase contributionsSAR data: phase contributions
Phase contributions in a SAR interferogram:
noiser +∆+∆+∆= αλπψφ 4
IF “nothing has changed” and for high SNR (∆ψ = ∆α = 0) :
r∆=λπφ 4
∆ψ can not be considered zero whenever:• the look angle changes “too much” and/or• the temporal baseline of the two SAR acquisitions is too high
In any case, ∆α = 0 only for two images gathered simultaneouslyIt should be noted that phase values are known only modulo-2π
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SAR interferometry: Digital Elevation Model ReconstructionSAR interferometry: Digital Elevation Model Reconstruction
ERS-1
ERS-2Bn
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A first exampleA first example
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B
r1
r2
AzimuthGround range
Bn
SAR interferometry: Topographic FringesSAR interferometry: Topographic FringesVesuvius Volcano, ItalyVesuvius Volcano, Italy
After removal of phase components
due to the local ellipsoid
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Φ1
1’ acquisition
T0time
Repeat-Pass SAR interferometry: Target Motion DetectionRepeat-Pass SAR interferometry: Target Motion Detection
Φ2 = Φ1
2’ acquisition
∆
T0+∆ttime
Atmospheric Effects
Different acquisition geometry
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dntdisplaceme λπφ 4=
If a scatterer on the ground slightly changes its relative position in the time interval between two SAR acquisitions (e.g. subsidence, landslide, earthquake …), an additive phase term, independent of the baseline, appears.
Here, d is the relative scattererdisplacement projected on the slant-range directionP P’
S 1
S 2
r
d
The interferometric phase is sensitive only to the ground motion components along the line of sight (LOS). Ground motion components normal to the LOS (e.g. along the azimuth direction) are invisible to the interferometric SAR.
Repeat-Pass SAR interferometry: Target MotionRepeat-Pass SAR interferometry: Target Motion
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The sensitivity of the interferometric phase to the ground motion is much larger than that to the relative elevation.
In the ERS case, assuming a perpendicular baseline of 150m, the following expression of the interferometric phase (after interferogramflattening) holds:
dqdqBntdisplacemeelevation
⋅+=
=⋅+⋅⋅⋅=
=+=
−
22210
222107.6 4
φ
φφφ
d = λ/2 = 2.8 cm φ = 2π
Phase Sensitivity to Target MotionPhase Sensitivity to Target Motion
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φφφφφφ noiseatmospherentdisplacemeelevationflat++++=
θλπ
sin4
0
qRB n ∆
⋅⋅ dλπ4
θλπ
tan4 r
RBn
atmospherenoiseDEMerrormotiontopo +++=−=∆ φφφ ˆ
Differential Interferometric SAR phase
Phase contributionsPhase contributions
Geometricaland/or temporaldecorrelation
ψ∆
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Interferogram Synthetic Interferogramgenerated from a DEM Differential Interferogram- =
φφφφφφ noiseatmospherentdisplacemeelevationflat++++=
Differential Interferogram Generation (2/2)Differential Interferogram Generation (2/2)
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DInSARapplications:
some examples
DInSARapplications:
some examples
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ERS differential interferogram of the Landers (N-E to Los Angeles) earthquake occurred on June 18, 1992.
Topography compensated with the fringes generated by means of an ERS Tandem pair.
The differential interferogram has been geocoded.
Co-seismic deformation patternCo-seismic deformation pattern
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The geocoded differential interferogram showing the eruption effects of the July 2001 eruption.
Volcano monitoringVolcano monitoring
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The landslide of St.Etienne de Tinee
Single ERS interferometric pairNormal baseline: 6 metersTime interval: 3 days
No need of topographic compensation
Detection of sliding areasDetection of sliding areas
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INTERESTING,BUT…
INTERESTING,BUT…
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Limits of conventional DInSAR analysis (1/2)Limits of conventional DInSAR analysis (1/2)
Since 1993 a growing galleryof examples of differential SAR interferometrystarted being available.
While more and more InSARwere generated, the presence of atmospheric artifacts and problemsdue to phase decorrelation(temporal and/or geometrical)became more and more evident and dampened somewhat the enthusiasm
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Limits of conventional DInSAR analysis (2/2)Limits of conventional DInSAR analysis (2/2)
15-months...
0≠∆ψ
1-day (Tandem)Interferogram
0=∆ψ
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φatmosphere
If the propagation medium changes the time interval between two SAR acquisitions (e.g. humidity, temperature, pressure …), an additive phase term, independent of the baseline, appears.
Atmospheric Phase Contributions (1/2)Atmospheric Phase Contributions (1/2)
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Atmospheric spatial inhomogeneities, due to variations of Pressure, Temperature and Humidity affects the propagation velocity. The different delay of the repeated observations results in a “phase screen ”.
The APS can generate phase variations up to two fringes in C band. This is converted in elevation errors (baseline dependent) or motion errors (baseline independent).
These errors cannot be estimated or recovered from a single interferometric pair.
Atmospheric phase screen over Paris reflectivity
The typical “Atmospheric phase screen” (APS) power spectrum is of fractal type:
hence it is correlated in space (~ hundred of meters). Coherence maps cannot measure it.
( ) 35
32 with <<∝ − ααffS
Atmospheric Phase Contributions (2/2)Atmospheric Phase Contributions (2/2)
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Despite its remarkable potential DInSAR analysis is still not
a “standard geodetic tool”.
Difficulties are related to: (1) phase decorrelation; (2)
atmospheric effects; (3) platform stability
In order to get reliable data ready to be used by final users
we need a more sophisticated processing chain:
we need a multi-interferogram framework
Conclusion (PART A)Conclusion (PART A)