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Terrafirma Terrafirma User Group Workshop, 22 User Group Workshop, 22 - - 23 July 23 July Tele-Rilevamento Europa T.R.E. s.r.l. Tele-Rilevamento Europa T.R.E. s.r.l. Introduction to SAR systems and Differential SAR Interferometry Introduction to SAR systems and Differential SAR Interferometry

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TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

SAR can be mounted on different platformsSAR can be mounted on different platforms

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

•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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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)

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

R

Earth

Radar satellite

R (sensor-target distance)=

T (delay) . C (light speed)

Range measurementsRange measurements

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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…

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

Amplitude

data

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

How does it look like?How does it look like?

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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.

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

Descendin

g orbitA

scen

ding

orb

it

The AOI is then illuminated both from East and from West.

More on ascending and descending orbits

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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…

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

Is it possible to combine ascending and descending orbits? YES!Is it possible to combine ascending and descending orbits? YES!

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

Q3: Are “ascending and descending data” always available?

A3: For ERS data usually yes.

For Envisat and Radarsat it depends on the acquisition policy

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

Phase

Data

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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)

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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)

TerrafirmaTerrafirma User Group Workshop, 22User Group Workshop, 22--23 July23 July

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)

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End of Part AEnd of Part A