InSAR Principles: Guidelines for SAR Interferometry


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TM-19
February 2007
InSAR Principles: Guidelines for SAR Interferometry
Processing and Interpretation

TM-19 _________________________________________________________________________ InSAR Principles

Acknowledgements
Authors: Alessandro Ferretti, Andrea Monti-Guarnieri, Claudio Prati, Fabio Rocca Dipartimento di Elettronica ed Informazione, Politecnico di Milano, Italy Didier Massonnet CNES, Toulouse, France
Technical coordination: Juerg Lichtenegger ESA/ESRIN (retired), Frascati, Italy

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InSAR Principles: Guidelines for SAR Interferometry Processing and Interpretation (TM-19, February 2007)
Karen Fletcher
ESA Publications ESTEC Postbus 299 2200 AG Noordwijk The Netherlands Tel: +31 71 565 3400 Fax: +31 71 565 5433
The Netherlands
€40
92-9092-233-8
1013-7076
© 2007 European Space Agency

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______________________________________________________________________________ Table of Contents
Table of Contents
Scope ............................................................................................. viii
Part A Interferometric SAR image processing and interpretation
1. Synthetic Aperture Radar basics......................................................... A-3 1.1 Introduction................................................................................. A-3 1.1.1 Introduction to ERS ....................................................... A-3 1.1.2 Introduction to Envisat................................................... A-4 1.2 SAR images of the Earth’s surface ............................................. A-5 1.2.1 What is a strip-map SAR imaging system?.................... A-5 1.2.2 What is a complex SAR image?..................................... A-6 1.2.3 SAR resolution cell projection on the ground.............. A-11
2. SAR interferometry: applications and limits .................................... A-17 2.1 Introduction............................................................................... A-17 2.2 Terrain altitude measurement through the interferometric phase ......................................................................................... A-18 2.2.1 Interferogram flattening ............................................... A-19 2.2.2 Altitude of ambiguity ................................................... A-20 2.2.3 Phase unwrapping and DEM generation...................... A-20 2.3 Terrain motion measurement: Differential Interferometry ....... A-23 2.4 The atmospheric contribution to the interferometric phase ...... A-24 2.5 Other phase noise sources ......................................................... A-25 2.6 Coherence maps ........................................................................ A-26
3. SAR Differential Interferometry basics and examples ..................... A-31 3.1 Introduction............................................................................... A-31 3.2 Landers co-seismic deformation ............................................... A-31 3.3 Small earthquake modelling ..................................................... A-33 3.4 The quiet but complicated deformation after an earthquake..... A-35 3.5 A case of coherence loss: India................................................. A-37 3.6 A case of damaged raw data, studying a large earthquake in Chile.......................................................................................... A-38
Part B InSAR processing: a practical approach
1. Selecting ERS images for InSAR processing ..................................... B-3 1.1 Introduction................................................................................. B-3 1.2 Available information about ERS images................................... B-3 1.2.1 The ESA on-line multi-mission catalogue ..................... B-3 1.2.2 DESCW.......................................................................... B-4 1.2.3 Expected coherence (prototype)..................................... B-6 1.3 Selecting images for InSAR DEM generation ............................ B-8 1.4 Selecting images for Differential InSAR applications................ B-9
2. Interferogram generation .................................................................. B-11
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TM-19 _________________________________________________________________________ InSAR Principles
2.1 Introduction................................................................................B-11 2.2 Generation of synthetic fringes..................................................B-12 2.3 Co-registering ............................................................................B-13
2.3.1 Co-registering coefficients............................................B-14 2.3.2 Co-registering parameter estimation .............................B-16 2.3.3 Implementation of resampling ......................................B-17 2.4 Master and slave oversampling..................................................B-17 2.5 Range spectral shift & azimuth common bandwidth filtering ...B-18 2.5.1 Range spectral shift filtering .........................................B-18 2.5.2 Azimuth common band filtering ...................................B-20 2.6 Interferogram computation ........................................................B-22 2.6.1 Complex multi-looking .................................................B-24 2.6.2 Generation of coherence maps......................................B-26 2.7 Applications of coherence ........................................................B-27 2.8 Interferogram geocoding & mosaicking ....................................B-29 3. InSAR DEM reconstruction .............................................................B-31 3.1 Introduction................................................................................B-31 3.2 Processing chain and data selection...........................................B-31 3.3 Phase unwrapping techniques for InSAR DEM reconstruction.B-33 3.3.1 What are we looking for?..............................................B-34 3.3.2 Case p=2, Unweighted Least Mean Squares method ...B-37 3.3.3 Case p=2, Weighted Least Mean Squares method .......B-38 3.3.4 Case p=1, Minimum Cost Flow method.......................B-38 3.3.5 Case p=0, Branch-Cut and other minimum L0
methods .........................................................................B-39 3.3.6 Outlook .........................................................................B-41 3.4 From phase to elevation .............................................................B-42 3.4.1 Polynomial approximation of satellite orbits, point
localisation and data geocoding ....................................B-42 3.4.2 Data resampling ............................................................B-45 3.4.3 Impact of baseline errors on the estimated
topography ....................................................................B-45 3.4.4 Precise orbit determination ..........................................B-47 3.5 Error sources, multi-baseline strategies and data fusion............B-48 3.5.1 Multi-interferogram InSAR DEM reconstruction.........B-50 3.6 Combination of ascending and descending passes ....................B-53 3.7 Conclusions................................................................................B-55 4. Differential Inteferometry (DInSAR) ................................................B-57 4.1 Examples of differential interferometry on land........................B-57 4.1.1 Physical changes ...........................................................B-57 4.1.2 Volcano: Okmok ...........................................................B-57 4.1.3 Surface rupture: Superstition Hill .................................B-58 4.1.4 Subsidence: East Mesa..................................................B-60 4.2 Example of differential interferometry on ice ...........................B-62 4.3 Review of various criteria for data selection .............................B-63
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______________________________________________________________________________ Table of Contents
4.4 Interferometric interpretation.................................................... B-63 4.4.1 Interferometry phase signal ruggedness....................... B-64 4.4.2 Fictitious example interferograms for analysis ............ B-65 4.4.3 Analysis of fictitious situations.................................... B-67
Part C InSAR processing: a mathematical approach
1. Statistics of SAR and InSAR images.................................................. C-3 1.1 The backscattering process ......................................................... C-3 1.1.1 Introduction.................................................................... C-3 1.1.2 Artificial backscatterers ................................................. C-3 1.1.3 Natural backscatterers: the spectral shift principle ........ C-4 1.1.4 Statistics of the return .................................................... C-7 1.2 Interferometric images: coherence.............................................. C-8 1.2.1 Statistics of coherence estimators .................................. C-9 1.2.2 Impact of the baseline on coherence ............................ C-12 1.3 Power spectrum of interferometric images ............................... C-13 1.4 Causes of coherence loss .......................................................... C-13 1.4.1 Noise, temporal change................................................ C-13 1.4.2 Volumetric effects........................................................ C-13
2. Focusing, interferometry and slope estimate .................................... C-15 2.1 SAR model: acquisition and focusing....................................... C-15 2.1.1 Phase preserving focusing............................................ C-15 2.1.2 CEOS offset processing test......................................... C-18 2.2 Interferometric SAR processing ............................................... C-18 2.2.1 Spectral shift and common band filtering (revisited)... C-19 2.3 DEM generation: optimal slope estimate.................................. C-21 2.4 Noise sources ............................................................................ C-24 2.5 Processing decorrelation artefacts............................................. C-25 2.5.1 Examples of decorrelation sources............................... C-25
3. Advances in phase unwrapping ........................................................ C-29 3.1 Introduction............................................................................... C-29 3.2 Residues and charges ................................................................ C-31 3.2.1 Effects of noise: pairs of residues, undefined positions of the ‘ghost lines’ ........................................ C-33 3.2.2 Effects of alias: unknown position of the ghost lines... C-36 3.3 Optimal topographies under the Lp norm.................................. C-37 3.3.1 L2, L1, L0 optimal topographies ................................... C-37 3.3.2 Slope estimates............................................................. C-40 3.3.3 Removal of low resolution estimates of the topography ................................................................... C-41 3.3.4 Bias of the slope estimate............................................. C-41 3.4 Analysis in the wave-number domain....................................... C-42 3.4.1 L2 optimisation in the wave-number domain ............... C-42 3.5 Weighting factors in the optimisation....................................... C-43
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TM-19 _________________________________________________________________________ InSAR Principles
4. Multiple image combination for DEM generation and ground motion estimation ..............................................................................C-45 4.1 Multi-baseline phase unwrapping for InSAR topography estimation...................................................................................C-45 4.2 Applications to repeat-pass interferometry ................................C-48 4.2.1 Example 1: the Vesuvius data set .................................C-50 4.2.2 Example 2: The Etna data set........................................C-53 4.3 The ‘Permanent Scatterers’ technique .......................................C-56 4.3.1 Space-time estimation ...................................................C-58 4.3.2 Subsidence in Pomona .................................................C-59 4.3.3 Ground slip along the Hayward fault ............................C-62 4.3.4 Seasonal deformation in the Santa Clara Valley...........C-63
5. Applications based on spectral shift .................................................C-65 5.1 Introduction to spectral shift ......................................................C-65 5.2 Interferometric quick look (IQL) ...............................................C-67 5.3 Super-resolution.........................................................................C-69
6. Differential interferometry ................................................................C-71 6.1 Introduction................................................................................C-71 6.2 Differential interferometry using an available DEM .................C-72 6.3 Differential interferometry with three or more combined images ........................................................................................C-77 6.4 Techniques to avoid phase unwrapping.....................................C-79 6.4.1 Integer combination ......................................................C-79 6.4.2 Interferogram stacking ..................................................C-82 6.5 Information contained in interferometric measurements ...........C-83 6.5.1 Residual orbital fringes .................................................C-83 6.5.2 Uncorrected topography................................................C-86 6.5.3 Heterogeneous troposphere...........................................C-86 6.5.4 Heterogeneous ionosphere ............................................C-87 6.5.5 Static atmosphere ..........................................................C-88 6.5.6 Radar clock drift ...........................................................C-88
7. Envisat-ASAR interferometric techniques and applications .............C-91 7.1 Introduction................................................................................C-91 7.2 ScanSAR: an introduction .........................................................C-92 7.2.1 Acquisition....................................................................C-93 7.2.2 Focusing........................................................................C-94 7.3 ScanSAR interferometry............................................................C-96 7.3.1 Common band (CB) filtering ........................................C-97 7.4 Multi-mode SAR interferometry................................................C-98 7.4.1 Multi-mode interferometric combination......................C-98 7.5 Applications .............................................................................C-101 7.5.1 AP/AP/IM interferometry ...........................................C-101 7.5.2 WSM/WSM and WSM/IM interferometry .................C-102
8. ERS-Envisat interferometry ............................................................C-107 8.1 Introduction..............................................................................C-107
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______________________________________________________________________________ Table of Contents 8.2 ERS-Envisat interferometric combination .............................. C-107 8.3 Frequency gap compensation.................................................. C-108 8.4 Vertical accuracy .................................................................... C-108 8.5 Altitude of ambiguity.............................................................. C-109 8.6 Effect of volume scattering ..................................................... C-110 8.7 Experimental results................................................................ C-110
References ............................................................................................ I
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TM-19 _________________________________________________________________________ InSAR Principles
Scope
This manual has been produced as a text book to introduce radar interferometry to remote sensing specialists. It consists of three parts. Part A is meant for readers who already have a good knowledge of optical and microwave remote sensing, to acquaint them with interferometric SAR image processing and interpretation. Part B provides a practical approach and the technical background for people who are starting up with InSAR processing. In Part C a more mathematical approach can be found, for a deeper understanding of the interferometric process. There, the manual also includes an appreciation of themes such as super resolution and ERS/Envisat interferometry.
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Part A
Interferometric SAR image processing and interpretation

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InSAR Principles: Guidelines for SAR Interferometry