1. SAR Interferometry (InSAR): principles
INGV
SAR Interferometry (InSAR): principles
Salvatore Stramondo
Antonio Montuori
Istituto Nazionale di Geofisica e Vulcanologia
Corso presso Univ. della Calabria
10/04/2017

2. Summary Interferometry SAR Interferometry (InSAR) technique: theory.
Differential InSAR (DInSAR): general aspects.
Multi-Temporal DInSAR techniques: SBAS & PSI
Corso presso Univ. della Calabria
10/04/2017

3. Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are coherently combined in order to extract information about the waves.
An instrument used to interfere waves is called an interferometer.
Interferometry is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, quantum mechanics, nuclear and particle physics, plasma physics, remote sensing and biomolecular interactions.
Corso presso Univ. della Calabria
10/04/2017

4. SAR Interferometry (InSAR)
InSAR technique is a powerful tool to retrieve the position and/or the displacement of surface point scatterers through the pixel-to-pixel phase difference processing of couple SAR images acquired over the same scene viewed from comparable well-known acquisition geometries.
We call interferogram the image of the pixel to pixel phase differences. An interferogram is a complex image with (a) magnitude given by the product of the SAR amplitudes and (b) phase (the InSAR phase) given by the path length difference, as well as variations of the scattering properties and the medium conditions.
Corso presso Univ. della Calabria
10/04/2017

5. SAR Interferometry (InSAR)
The recorded phase is composed by two terms:
the propagation phase, relevant to the sensor-to-target distance
the backscattered phase, due to the surface backscattering
The backscattered phase is obtained as the sum of the phase contribution of each scatterer within the resolution cell on the plane perpendicular to the satellite LOS
Corso presso Univ. della Calabria
10/04/2017

6. SAR Interferometry (InSAR)
baseline Bn Bp
Two radar antennas mounted on board of two satellite-based SARs (1 and 2) observe a target with a slight separation in space (the interferometric spatial baseline).
Bp is the parallel spatial baseline
Bn is the parallel spatial baseline
If the two images are acquired simultaneously  single-pass interferometry
If the two images are acquired at different times  repeat-pass interferometry
Corso presso Univ. della Calabria
10/04/2017

7. SAR Interferometry (InSAR)
Along-track InSAR mode:
The two SAR antennas are along-track aligned and acquire the scattered electromagnetic field at slightly different times. This mode is exploited to estimate the sea surface spectrum. This mode is obviously operated in a single-pass.
Across-track InSAR mode:
The two SAR antennas are aligned across-track. This mode can be operated both in single-pass and multi-pass configurations. This mode is used to estimate the Digital Elevation Model (DEM).
Corso presso Univ. della Calabria
10/04/2017

8. SAR Interferometry (InSAR)
SAR vs InSAR

9. SAR Interferometry (InSAR)
Whenever the surface backscattering is unchanged (high coherence degree), the signal (S) received by the SAR from a target at distance R has an amplitude (A) related to the scattering strength of the target and a phase () related to the two-way traveling wave path between the radar and the target:

10. SAR Interferometry (InSAR)
Be S1 and S2 the received signals at two satellite positions:
The interferogram is the map of the pixel-to-pixel phase differences between S1 and S2:
baseline Bn Bp
Corso presso Univ. della Calabria
10/04/2017

11. SAR Interferometry (InSAR)
The two complex SAR images must be coregistered by interpolating one image (the slave image) to generate imagery at the same pixel locations as the second (the master image).
After registration, the two complex SAR images are multiplied, and the interferometric phase is obtained.
Corso presso Univ. della Calabria
10/04/2017

12. SAR Interferometry (InSAR)
The interferometric phase contains some distinct contributions:
flat Earth
topographic phase
deformation phase
atmospheric phase
noise (error phase)
Corso presso Univ. della Calabria
10/04/2017

13. SAR Interferometry (InSAR)
Flat Earth
Raw interferogram includes a quasi-linear phase trend caused by tilt of terrain surface relative to the baseline
Flattening removes interferometric phase component using a sphere with radius of curvature derived from the ellipsoid.
Corso presso Univ. della Calabria
10/04/2017

14. SAR Interferometry (InSAR) Flat Earth
Unflattened interferogram
Flattened interferogram
Corso presso Univ. della Calabria
10/04/2017

15. SAR Interferometry (InSAR) Flat Earth
Unflattened interferogram
Flattened interferogram

16. SAR Interferometry (InSAR)
Topographic phase
The topographic phase contains the information relative to the relief. The spacing between the fringes depends on the perpendicular baseline: the longer the perpendicular baseline, the narrower the fringes
B=174 m
B=40 m
Corso presso Univ. della Calabria
10/04/2017

17. SAR Interferometry (InSAR)
Topographic phase
Baseline doubling
Corso presso Univ. della Calabria
10/04/2017

18. SAR Interferometry (InSAR)
Topographic phase
The ambiguity height is the elevation difference corresponding to a full phase cycle (2):
ERS satellites:
Corso presso Univ. della Calabria
10/04/2017

19. SAR Interferometry (InSAR)
Phase ambiguity
The interferometric phase is generally modulus 2p.
A phase unwrapping method can be then applied to calculate the exact phase value in order to extract correct information about the scene (the elevation).
Corso presso Univ. della Calabria
10/04/2017

20. SAR Interferometry (InSAR)
Phase ambiguity
The interferometric phase component is known save for 2Np:
Phase unwrapping algorithms can be applied to retrieve from the “wrapped phase” :
Corso presso Univ. della Calabria
10/04/2017

21. SAR Interferometry (InSAR)
Phase ambiguity
Branch-cut region growing algorithm: it is based on the identification of branches linking the areas with phase continuities. It is typically applied to the filtered interferograms. Critical areas, such as areas of very low coherence or residues, are identified and avoided in the phase unwrapping.
Minimum Cost Flow (MCF) techniques and Triangular Irregular Network (TIN): global optimization technique to the phase unwrapping problem (for example at locations of very low coherence) which provides high density of unwrapped points together with an efficient and robust unwrapping
Corso presso Univ. della Calabria
10/04/2017

22. SAR Interferometry (InSAR)
Phase ambiguity
Corso presso Univ. della Calabria
10/04/2017

23. SAR Interferometry (InSAR) From SAR raw data to Interferogram
Corso presso Univ. della Calabria
10/04/2017

24. SAR Interferometry (InSAR)
Atmospheric phase
Corso presso Univ. della Calabria
10/04/2017

25. SAR Interferometry (InSAR)
Atmospheric phase
The phase due to atmospheric artifacts does not depend on the baseline and its sensitivity to the atmosphere is related to the wavelength (Longer wavelengths are less sensitive to atmospheric distortions).
A propagation delay (Dl) of 2cm would result in an additional phase of an almost full fringe at C-band but only of 1/6th of a fringe at L-band.
In case of stronger delays, the spatial phase variations might be so large that at higher frequencies phase unwrapping could fail because of more cycles being wrapped.
Corso presso Univ. della Calabria
10/04/2017

26. SAR Interferometry (InSAR)
Tropospheric effects
Interferogram
B perp. =5 m B temp.=55gg
SAR ERS intensity image of the Apennine
Corso presso Univ. della Calabria
10/04/2017

27. SAR Interferometry (InSAR)
Complex coherence
The degree of correlation between two SAR images is measured by the coherence parameter.
The amplitude is the degree of coherence, the phase is the interferometric phase.
Coherence is a measure of the phase noise or fringe visibility
Corso presso Univ. della Calabria
10/04/2017

28. SAR Interferometry (InSAR)
Complex coherence
Interferometric phase
Coherence
Corso presso Univ. della Calabria
10/04/2017

29. SAR Interferometry (InSAR)
Complex coherence
The spatial coherence can be filtered out. A part remains if we have a volume (forest, snow, city). This is called volume decorrelation and increases with the spatial baseline.
The temporal coherence can NOT be filtered out, it is a property of the image. This is also referred to as temporal decorrelation term and depends on the stability of the objects between the two acquisitions.
Corso presso Univ. della Calabria
10/04/2017

30. SAR Interferometry (InSAR)
Complex coherence
Geometric decorrelation: it relies on the sight angle differences of the two SAR scenes part of the interferogram. The critical value of the baseline at which complete decorrelation occurs is given by:
For ERS and ENVISAT the critical baseline is about 1100 m (R=850 km, =23º, Lc=25m, =5.6 cm).
For JERS-1 and PALSAR the critical baseline is about 4 km (R=730 km, =35º, Lc=25m, =23 cm).
Corso presso Univ. della Calabria
10/04/2017

31. SAR Interferometry (InSAR)
Complex coherence
Corso presso Univ. della Calabria
10/04/2017

32. SAR Interferometry (InSAR)
Complex coherence
The two SAR antennas see the scene under slightly different angles, hence they record different parts of the image spectrum shifted by an amount f.
Spectrum1
Spectrum2
If the spectrum shif is equal to the critical one (see the formula above), they automatically loose part of the correlation they have (spatial decorrelation). Spectral shift filtering removes the effect of spatial decorrelation for level surfaces. There is a proportional loss of range resolution.
Corso presso Univ. della Calabria
10/04/2017

33. SAR Interferometry (InSAR)
Complex coherence
In the repeat-pass configuration the scatterers may move (e.g. water surfaces and tree canopies) or their dielectric properties may change (e.g. snow, wet soils) between observations.
The two SAR images are only partially correlated because of the temporal interval between the acquisitions. In general it is likely that the longer the time interval between acquisitions, the stronger the temporal decorrelation.
Taking into account that typically temporally unstable scatterers have dimensions of the order of a few centimeters or less (e.g. leaves, grass, snow grains etc.), temporal decorrelation is more pronounced at shorter wavelengths (e.g. at X- and C-band).
Corso presso Univ. della Calabria
10/04/2017

34. SAR Interferometry (InSAR) Spatial decorrelation effects
06/02/ /05/ Baseline ort. = 330 m
22/05/ /12/1997
Baseline ort. = 40 m
Corso presso Univ. della Calabria
10/04/2017

35. SAR Interferometry (InSAR) Temporal decorrelation effects
07/05/ /05/ Baseline temp. = 1 day
02/06/ /02/2002
Baseline temp. = 980 days
Baseline ort. = 3 m
Corso presso Univ. della Calabria
10/04/2017

36. Differential SAR Interferometry (DInSAR)
Differential SAR Interferometry (DInSAR) is an InSAR technique addressed to measure the Earth surface displacements with centimetric accuracy.
DInSAR is used in seismology, for instance, when an earthquake takes place. Two SAR images, one pre-seismic and one post-seismic, are acquired. The interferometric phase is computed.
Using a DEM, the topographic phase is canceled. The residual phase contains also the eventual surface deformation effect (differential interferogram). Each differential fringe corresponds to a full phase cycle (2p) and represents a sensor-to-target distance change (LOS change) of l/2. For C-Band sensors it is about 2.8 cm.
Corso presso Univ. della Calabria
10/04/2017

37. Differential SAR Interferometry (DInSAR)
Be S1 and S2 two SAR satellites. The interferometric phase is:
The residual phase contains, besides atmosphere and
Noise phase component, the displacement
projected onto the LOS.
Corso presso Univ. della Calabria
10/04/2017

38. Differential SAR Interferometry (DInSAR)
Objective of DInSAR: Separation of topo from total phase to determine displ
2-pass: Simulate topo based on existing DEM. Phase unwrapping not required for the simulated interferogram. High accuracy of DEM required.
3- and 4-pass: Derive topo from independent interferogram, no existing DEM is required but phase unwrapping required.
The combination of complex interferograms may be of interest
to do a kind of differential interferometry without phase unwrapping and geocoding requirement
to improve the sensitivity to topography.
Corso presso Univ. della Calabria
10/04/2017

39. Differential SAR Interferometry (DInSAR)
Consider a standard deviation on phase and displacement:
For ERS:
Corso presso Univ. della Calabria
10/04/2017

40. Differential SAR Interferometry (DInSAR) Multi-Temporal DInSAR
In conclusion...
InSAR Configurations
Along-Track
InSAR
(∆t = ms to s)
Across-Track InSAR
(∆θ)
DInSAR
(∆t = giorni - anni)
Multi-Temporal DInSAR
Field of Application
Oceanic current &
Target detection
Digital Elevation Model (DEM)
Surface deformation velocity maps
Surface time-series and deformation velocity maps
Corso presso Univ. della Calabria
10/04/2017

41. Principi di base – Interferometria SAR
Evolution of Satellite-based DInSAR: Multi-temporal DInSAR
Time series and deformation velocity maps of observed surface.
Main Multi-Temporal DInSAR Techniques
“Small BAseline Subset (SBAS)”
Super-Master reference SAR image
Different subsets of SAR images
“Small” spatial and temporal baselines
“Linking” among SAR subsets
Reference Scatter points with high interferometric coherence values
Poor spatial resolution

42. Principi di base – Interferometria SAR
Evolution of Satellite-based DInSAR: Multi-temporal DInSAR
Time series and deformation velocity maps of observed surface.
Main Multi-Temporal DInSAR Techniques
“Persistent Scatterers Interferometry (PSI)”
Unique “Reference master” SAR
“Large” spatial and temporal baselines
Scatter points smaller than resolution cell dimensions
Persistent scatter points (stability in terms of SAR amplitude)
High spatial resolution