This mid-term report gives a brief overview of the developments in radar interferometry since the IAG General Assembly 1999 in Birmingham, UK. The members of the Special Study Group on Spaceborne Interferometry Techniques have been working on a variety of topics related to the terms of reference. The objectives of the group are

to develop techniques and algorithms that allow extracting unambiguously topographic, deformation, and atmospheric signal from spaceborne repeat-pass radar interferometry,
to develop methods that allow describing the quality, in terms of accuracy and reliability, of the interferometric products taking the most significant error sources into account, and
to validate topographic and deformation maps for various applications and under various environmental conditions.


Falk Amelung (University of Hawaii, USA) amelung@pgd.hawaii.edu
Richard Bamler  (German Aerospace Center (DLR), Germany) Richard.Bamler@dlr.de
Alessandro Ferretti (Politecnico di Milano, Italy) aferre@elet.polimi.it
Satoshi Fujiwara  (Geographical Survey Institute, Japan)
Linlin GE (University of New South Wales, Australia)  l.ge@student.unsw.edu.au
Rick Guritz   (Alaska SAR Facility, USA) rguritz@images.alaska.edu
Ramon Hanssen (chair) (Delft University of Technology, The Netherlands) hanssen@geo.tudelft.nl
Johan Mohr (Technical University of Denmark) jm@emi.dtu.dk
David Sandwell (Scripps Institution of Oceanography, USA) sandwell@geosat.ucsd.edu
Andrew Wilkinson (University of Cape Town, South Africa)  ajw@eng.uct.ac.za
Howard Zebker (Stanford University, USA) zebker@jakey.stanford.edu


Progress in radar interferometry


 Spaceborne radar interferometry has developped considerably during the last two years. The experience with the repeat pass missions for topographic mapping, especially the problem of temporal decorrelation and atmospheric disturbances, culminated in the Shuttle Radar Topography Mission (SRTM) [17]. This Space Shuttle mission was performed between 11 and 23 February 2000 and used a single-pass configuration with a fixed 60 m boom to carry the two radar antennas. It mapped all land masses between 60°N and 58°S using C-band, and tiles of this area with a higher accuracy using X-band [5,22]. Currently, data calibration and processing is still in progress.

Progress in the field of the phase unwrapping problem focussed mainly on the application of network flow algorithms and the unwrapping of sparsely distributed, isolated resolution cells in interferograms [8,6,7].

An important development is the development of a procedure to use many or all available SAR images of a specific area in order to obtain topography or deformation measurements [10] [11] [9] [12] . This method, using Permanent Scatterers (image pixels which remain coherent over long time intervals and for wide baselines) is a successful attempt to overcome the temporal decorrelation problem as well as atmospheric disturbances. Single coherent pixels, which would not be identified using conventional methods are analyzed as time-series of deformation.

The influence of the atmospheric signal, which hampered quantitative quality statements of interferometric data, has been analyzed and modeled as a stochastic signal. [13] Scaling characteristics enable the construction of covariance functions to model the behavior of turbulent mixing in the atmosphere.

A new application of repeat-pass interferometry is atmospheric water vapor mapping [15,14]. If coherent interferograms are available over areas where topography is known, and surface deformation is absent, the phase variations mainly reflect the lateral atmospheric delay differences. For distances less than, say, 50 km these delay differences are mainly caused by water vapor heterogeneities, giving a new high-resolution perspective on radio propagation for space-geodetic methods.

To facilitate investigations, synthetic simulations of SAR data as well as of interferometric data were reported by [23].

[18] reported on the geometric calibration of ERS data. Using an algorithm which extracts SAR parameters and a method for raw data preparation, this calibration results in slant range images that have an accuracy that corresponds to 10 m horizontally on the ground. Their results also demonstrate the high stability of the ERS satellites.

Stacking methodologies as well as phase-gradient methods have been developped to improve DEM generation [19,20].

For the analysis of earthquakes using remote sensing, the combination of radar with optical imagery has been improved, using alternative (optical) correlation techniques.

The fast-track analysis of SAR data for e.g. earthquake research is demonstrated by [21], where ERS data of Hector Mine earthquake where received by a local groundstation and used to form an interferogram within 20 hours of the second overflight.

New insights in volcano monitoring were reported, see [24,4,1]. Volcanic uplift, caused by the accumulation of magma in subsurface reservoirs, is a common precursor to eruptions. But, for some volcanoes, uplift of metres or more has not yet led to an eruption. [3] present displacement maps of volcanoes in the Galápagos Islands, constructed using satellite radar interferometry, that might help explain this dichotomy. Subsidence studies have been reported by [2] and [16].



Members of the special studygroup are active in a wide range of research fields, spanning from electrotechnics to geodesy and geophysics. Due to this broad scale of interests, it is not feasible to arrange specific meetings where the entire group could come together. Nevertheless, several members meet regularly at a number of international symposia, allowing for interaction and cooperation. Progress in radar interferometry has been considerable in the last couple of years, and the members of the studygroup will continue to pursue the objectives mentioned above.


Publications by the members

  1. F Amelung, C Oppenheimer, P Segall, and H Zebker.
    Ground deformation near Gada 'Ale volcano, Afar, observed by radar interferometry.
    Geophysical Research Letters, 27(19):3093-3097, 2000.
  1. Falk Amelung, Devin L Galloway, John W Bell, Howard A Zebker, and Randell J Laczniak.
    Sensing the ups and downs of Las Vegas: InSAR reveals structural control of land subsidence and aquifer-system deformation.
    Geology, 27(6):483-486, June 1999.
  1. Falk Amelung, Sigurjón Jónssen, Howard Zebker, and Paul Segall.
    Widespread uplift and trap door faulting on Galápagos volcanoes observed with radar interferometry.
    Nature, 407(6807):993-996, October-26 2000.
  1. Falk Amelung, Sigurjon Jonsson, Howard Zebker, and Paul Segall.
    Prospects of volcano geodesy with ERS radar interferometry.
    In Second International Workshop on ERS SAR Interferometry, `FRINGE99', Ličge, Belgium, 10-12 Nov 1999, pages 1-9.
    ESA, 1999.
  1. R Bamler, M Eineder, and H Breit.
    The X-SAR single-pass interferometer on SRTM: Expected perfomance and processing concept.
    In EUSAR'96, Königswinter, Germany, pages 181-184, 1996.
  1. Curtis W Chen and Howard A Zebker.
    Network approaches to two-dimensional phase unwrapping: intractability and two new algorithms.
    Journal of the Optical Society of America A., 17(3):401-414, March 2000.
  1. Curtis W Chen and Howard A Zebker.
    Two-dimensional phase unwrapping using statistical models for cost functions in nonlinear optimization.
    Journal of the Optical Society of America A., in press, 2000.
  1. Mario Costantini.
    A novel phase unwrapping method based on network programming.
    IEEE Transactions on Geoscience and Remote Sensing, 36(3):813-821, May 1998.
  1. A Ferretti, C Prati, and F Rocca.
    Measuring subsidence with SAR interferometry: Applications of the permanent scatterers technique.
    In L Carbognin, G Gambolati, and A I Johnson, editors, Land subsidence; Proceedings of the Sixth International Symposium on Land Subsidence, volume 2, pages 67-79. CNR, 2000.
  1. Alessandro Ferretti, Claudio Prati, and Fabio Rocca.
    Multibaseline InSAR DEM reconstruction: The wavelet approach.
    IEEE Transactions on Geoscience and Remote Sensing, 37(2):705-715, March 1999.
  1. Alessandro Ferretti, Claudio Prati, and Fabio Rocca.
    Nonlinear subsidence rate estimation using permanent scatterers in differential SAR interferometry.
    IEEE Transactions on Geoscience and Remote Sensing, 38(5):2202-2212, September 2000.
  1. Alessandro Ferretti, Claudio Prati, and Fabio Rocca.
    Permanent scatterers in SAR interferometry.
    IEEE Transactions on Geoscience and Remote Sensing, 39(1):8-20, January 2001.
  1. Ramon F Hanssen.
    Radar Interferometry: Data Interpretation and Error Analysis.
    Kluwer Academic Publishers, Dordrecht, 2001.
  1. Ramon F Hanssen, Arnout J Feijt, and Roland Klees.
    Comparison of precipitable water vapor observations by spaceborne radar interferometry and Meteosat 6.7-
    mm radiometry.
    Journal of Atmospheric and Oceanic Technology, 18(5):756-764, May 2001.
  1. Ramon F Hanssen, Tammy M Weckwerth, Howard A Zebker, and Roland Klees.
    High-resolution water vapor mapping from interferometric radar measurements.
    Science, 283:1295-1297, February-26 1999.
  1. Jörn Hoffmann, Howard A Zebker, Devin L Galloway, and Falk Amelung.
    Seasonal subsidence and rebound in Las Vegas Valley, Nevada, observed by synthetic aperture radar interferometry.
    Water Resources Research, 37(6):1551-1566, June 2001.
  1. Rolando L Jordan, Edward R Caro, Yunjin Kim, Michael Kobrick, Yuhsyen Shen, Frederick V Stuhr, and Marian U Werner.
    Shuttle radar topography mapper (SRTM).
    In Giorgio Franceschetti, Christopher J Oliver, James C Shiue, and Shahram Tajbakhsh, editors, Microwave Sensing and Synthetic Aperture Radar, pages 412-422.
    SPIE, Bellingham, 1996.
  1. Johan Jacob Mohr and Sřren Nřrvan Madsen.
    Geometric calibration of ERS satellite SAR images.
    IEEE Transactions on Geoscience and Remote Sensing, 39(4):842-850, 2001.
  1. David T Sandwell and Evelyn J Price.
    Phase gradient approach to stacking interferograms.
    Journal of Geophysical Research, 103(B12):30183-30204, December 1998.
  1. David T Sandwell and Lydie Sichoix.
    Topographic phase recovery from stacked ERS interferometry and a low resolution digital elevation model.
    Journal of Geophysical Research, 105(B12):28211-28222, 2000.
  1. David T Sandwell, Lydie Sichoix, Duncan Agnew, Yehuda Bock, and Jean-Bernard Minster.
    Near real-time radar interferometry of the Mw 7.1 Hector Mine earthquake.
    Geophysical Research Letters, 27(19):3101-3104, 2000.
  1. Marian Werner.
    Shuttle radar topography mission (SRTM). the X-Band SAR interferometer.
    In 28th EuMC workshop proceedings. EuMW Amsterdam, 9 October 1998, 1998.
  1. L S Wray, A J Wilkinson, and M R Inggs.
    Synthetic aperture radar image simulator for interferometry.
    In ISRSE, Cape Town, 27-31 March 2000, 2000.
  1. Howard A Zebker, Falk Amelung, and Sjonni Jonsson.
    Remote sensing of volcano surface and internal processes using radar interferometry.
    In Peter J Mouginis-Mark, Joy A Crisp, and Jonathan H Fink, editors, Remote sensing of active volcanism, Geophysical Monographs 116, pages 179-205. American Geophysical Union, Washington, DC, 2000.


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