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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

    = 5.66 cm (f = 5.3 GHz)

    H = 780 Km

    = 23 deg

    dr = 7.91 m Slant Range Resolution

    da = 3.98 m Azimuth Resolution

    S 100 Km

    Revisiting Time = 35 days

    = 5.66 cm (f = 5.3 GHz)

    H = 780 Km

    = 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 platforms 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

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

    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

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

    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

    r

    Gr()

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

    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

    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

    cohe

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