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'''''Note - This
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One fringe of phase difference is generated by a ground motion of half the wavelength, since this corresponds to a whole wavelength increase in the two-way travel distance. Phase shifts are only resolvable relative to other points in the interferogram. Absolute deformation can be inferred by assuming one area in the interferogram (for example a point away from expected deformation sources) experienced no deformation, or by using a ground control (GPS or similar) to establish the absolute movement of a point.
=== Processing === ▼
The processing chain used to produce interferograms varies according to the software used and the precise application, but will usually include some combination of the following steps. ▼
Two SAR images are required to produce an interferogram; these may be obtained pre-processed, or produced from raw data by the user prior to InSAR processing. The two images must first be [[Image registration|co-registered]], using a [[correlation]] procedure to find the offset and difference in geometry between the two amplitude images. One SAR image is then [[Resampling|re-sampled]] to match the geometry of the other, meaning each [[pixel]] represents the same ground area in both images. The interferogram is then formed by [[cross product|cross-multiplication]] of each pixel in the two images, and the interferometric phase due to the [[reference ellipsoid]] is removed, a process referred to as flattening. For deformation applications a DEM can be used in conjunction with the baseline data to simulate the contribution of the topography to the interferometric phase, this can then be removed from the interferogram. ▼
Once the basic interferogram has been produced, it is commonly filtered using an adaptive power-spectrum filter to amplify the phase signal. For most quantitative applications the consecutive fringes present in the interferogram will then have to ''unwrapped'', which involves interpolating over the 0 to 2π phase jumps to produce a continuous deformation field. At some point, before or after unwrapping, incoherent reas of the image may be masked out. The final processing stage involves [[geocoding]] the image, which involves resampling the interferogram from the acquisition geometry (related to direction of satellite path) into the desired ]]geographic projection]]. ▼
=== Difficulties with InSAR ===
A variety of factors govern the choice of images which can be used for interferometry. The simplest is data availability - radar instruments used for interferometry commonly don't operate continuously, acquiring data only when programmed to do so. For future requirements it may be possible to request acquisition of data, but for many areas of the world archived data may be sparse. Data availability is further constrained by baseline criteria. Availability of a suitable DEM may also be a factor for two-pass InSAR; commonly 90m [[SRTM]] data may be available for many areas, but at high lattitudes or in areas of [[SRTM#No-data areas|poor coverage]] alternative datasets must be found.
A fundamental requirement of the removal of the ground signal is that the sum of phase contributions from the individual targets within the pixel remains constant between the two images and is completely removed. However there are several factors that can cause this criterion to fail. Firstly the two images must be accurately co-registered to a sub-pixel level to ensure that the same targets are contributing to that pixel. There is also a geometric constraint on the maximum length of the baseline - the difference in viewing angles must not cause phase to change over a pixel by more than a wavelength. The effects of topography also influence the condition, and baselines need to be shorter if terrain gradients are high. Where registration is poor or the maximum baseline is exceeded the pixel phase will become incoherent - the phase becomes essentially random from pixel to pixel rather than varying smoothly, and the area appears noisy. This is also true for anything else that changes the contributions to the phase within each pixel, for example changes to the ground targets in each pixel caused by vegetation growth, landslides, agriculture or snow cover.
Another source of error present in most interferograms is caused by the propagation of the waves through the atmosphere. If the wave traveled through a vacuum it should theoretically be possible (subject to sufficient accuracy of timing) to use the two-way travel-time of the wave in combination with the phase to calculate the exact distance to the ground. However the velocity of the wave through the atmosphere is lower than the [[speed of light]] in a [[vacuum]], and is a function of air temperature, pressure and the partial pressure of water vapour {{cn}}. It is this unknown phase delay that prevents the integer number of wavelengths being calculated. If the atmosphere was horizontally [[homogeneous]] over the length scale of an interferogram and vertically over that of the topography then the effect would simply be a constant phase difference between the two images which, since phase difference is measured relative to other points in the interferogram, would not contribute to the signal. However the atmosphere is laterally [[heterogeneous]] on length scales both larger and smaller than typical deformation signals. This spurious signal can appear completely unrelated to the surface features of the image, however in other cases the atmospheric phase delay is caused by vertical inhomogeneity at low altitudes and this may result in fringes appearing to correspond with the topography.
== Data Sources ==▼
== Producing interferograms ==
▲The processing chain used to produce interferograms varies according to the software used and the precise application, but will usually include some combination of the following steps.
▲Two SAR images are required to produce an interferogram; these may be obtained pre-processed, or produced from raw data by the user prior to InSAR processing. The two images must first be [[Image registration|co-registered]], using a [[correlation]] procedure to find the offset and difference in geometry between the two amplitude images. One SAR image is then [[Resampling|re-sampled]] to match the geometry of the other, meaning each [[pixel]] represents the same ground area in both images. The interferogram is then formed by [[cross product|cross-multiplication]] of each pixel in the two images, and the interferometric phase due to the [[reference ellipsoid]] is removed, a process referred to as flattening. For deformation applications a DEM can be used in conjunction with the baseline data to simulate the contribution of the topography to the interferometric phase, this can then be removed from the interferogram.
Early exploitation of satellite-based InSAR included use of [[Seasat]] data in the 1980s, but the potential of the technique was expanded in the 1990s, with the launch of [[ERS-1]] (1991), [[JERS-1]] (1992), [[RADARSAT-1]] and [[ERS-2]] (1995). These platforms provided the stable, well-known orbits and short baselines necessary for InSAR. More recently, the 11-day NASA STS-99 mission in February of 2000 used a SAR antenna mounted on the [[space shuttle]] to gather data for the [[Shuttle Radar Topography Mission]]. In 2002 [[ESA]] launched the ASAR instrument, designed as a sucessor to ERS, aboard [[Envisat]]. While the majority of InSAR to date has utilised the C-band sensors, recent missions such as the [[Advanced Land Observation Satellite|ALOS PALSAR]] and [[TerraSAR-X]] are expanding the available data in the L- and C-band. ▼
▲Once the basic interferogram has been produced, it is commonly filtered using an adaptive power-spectrum filter to amplify the phase signal. For most quantitative applications the consecutive fringes present in the interferogram will then have to ''unwrapped'', which involves interpolating over the 0 to 2π phase jumps to produce a continuous deformation field. At some point, before or after unwrapping, incoherent reas of the image may be masked out. The final processing stage involves [[geocoding]] the image, which involves resampling the interferogram from the acquisition geometry (related to direction of satellite path) into the desired ]]geographic projection]].
A variety of InSAR processing packages are commonly used, several are available free for academic use.
*[[ROI_PAC]] - produced by [[NASA]]'s [[Jet Propulsion Laboratory]] and [[CalTech]]. UNIX based, can be freely downloaded from [http://www.openchannelfoundation.com/projects/ROI_PAC/index.html The Open Channel Foundation].
*DORIS - processing suite from [[Delft University of Technology]], code is C++ based, making it multi-platform portable. Can be downloaded for academic uses from the [http://enterprise.lr.tudelft.nl/doris/ DORIS] homepage.
*Gamma - Commercial software suite [http://www.gamma-rs.ch/software.php].
*Pulsar - Commercial software suite, UNIX based [http://www.phoenixsystems.co.uk/].
▲=== Data Sources ===
▲Early exploitation of satellite-based InSAR included use of [[Seasat]] data in the 1980s, but the potential of the technique was expanded in the 1990s, with the launch of [[ERS-1]] (1991), [[JERS-1]] (1992), [[RADARSAT-1]] and [[ERS-2]] (1995). These platforms provided the stable, well-known orbits and short baselines necessary for InSAR. More recently, the 11-day NASA STS-99 mission in February of 2000 used a SAR antenna mounted on the [[space shuttle]] to gather data for the [[Shuttle Radar Topography Mission]]. In 2002 [[ESA]] launched the ASAR instrument, designed as a sucessor to ERS, aboard [[Envisat]]. While the majority of InSAR to date has utilised the C-band sensors, recent missions such as the [[Advanced Land Observation Satellite|ALOS PALSAR]] and [[TerraSAR-X]] are expanding the available data in the L- and
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== Applications ==
=== Tectonic ===
InSAR can be used to measure [[tectonic]] deformation, for example ground movements due to [[earthquakes]]. It was first used for the 1992 [[Landers]] earthquake <ref>Massonnet, D., Rossi, M., Carmona, C., Adragna, F., Peltzer, G., Feigl, K. & Rabaute, T. 1993 The displacement field of the Landers earthquake mapped by radar interferometry. Nature 364, 138{142}</ref>, but has since been utilised extensively for a wide variety of earthquakes all over the world. In particular the 1999 [[1999 İzmit earthquake|Izmit]] and 2003 [[Bam, Iran#2003 earthquake|Bam]] earthquakes were extensively studied. <ref>[http://www.esa.int/esaEO/SEMLD1W4QWD_index_0.html] Envisat's rainbow vision detects ground moving at pace fingernails grow. August 6th 2004, accessed March 20th 2007 </ref> <ref>[http://earth.esa.int/ers/article_archive/izmit_1999.html] The Izmit Earthquake of 17 August 1999 in Turkey. Accessed March 20th 2007 </ref> InSAR can also be used to monitor creep and strain accumulation on [[Geologic_fault|faults]].
=== Volcanic ===
InSAR can be used in a variety of [[volcanic]] settings, including deformation associated with [[eruptions]], inter-eruption strain caused by changes in [[magma]] distribution at depth, [[gravitational]] spreading of volcanic edifices, and volcano-tectonic deformation signals.<ref>Wadge, G. (2003) A strategy for the observation of volcanism on Earth from space. Phil. Trans. Royal Soc.Lond. A, 361, 145-156.</ref> Early work on volcanic InSAR included studies on [[Mount Etna]] <ref>D. Massonnet, P. Briole, and A. Amaud, “Deflation of Mount Etna monitored by spaceborne radar interferometry”, Nature, vol. 375, pp. 567-570, 1995</ref>, and [[Kilauea]]<ref>Rosen, P. A., S. Hensley, H. A. Zebker, F. H. Webb, and E. J. Fielding (1996), Surface deformation and coherence measurements of Kilauea Volcano, Hawaii, from SIR C radar interferometry, J. Geophys. Res., 101(E10), 23,109–23,126.</ref>, with many more volcanoes being studied as the field developed. The technique is now widely used for academic research into volcanic deformation, although its use as an operational monitoring technique for volcano observatories has been limited by issues such as orbital repeat times, lack of archived data, coherence and atmospheric errors.<ref>Stevens, N.F. and Wadge, G, 2004. Towards operational repeat-pass SAR interferometry at active volcanoes. Natural Hazards, 33, 47-76</ref> Recently InSAR has also been used to study [[rifting]] processes in Ethiopia. <ref> Wright, T.J.; Ebinger, C.; Biggs, J.; Ayele, A.; Yirgu, G.; Keir, D.; Stork, A. (2006) Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode, Nature, 442, pp.291-294. doi:10.1038/nature04978 [http://earth.leeds.ac.uk/~eartjw/papers/Wright_Afar_Nature2006.pdf] (accessed 20th March 2007). </ref>
=== Subsidence ===
Ground [[subsidence]] from a variety of causes has been successfully measured using InSAR, in particular subsidence caused by oil or water extraction from underground reservoirs, subsurface [[mining]] and collapse of old mines. It can also be used for monitoring the stability of built structures and landscape features such as [[landslides]].<ref>[http://www.eomd.esa.int/booklets/booklet183.asp] Ground motion, European Space Agency, accessed March 21st 2007. </ref>
=== DEM generation ===
Interferograms can be used to produce [[digital elevation map|digital elevation maps]] (DEMs) using the [[stereoscopic]] effect caused by slight differences in observation position between the two images. When using two images produced by the same sensor with a separation in time, it must be assumed other phase contributions (for example from deformation or atmospheric effects) are minimal. In 1995 the two [[European Remote-Sensing Satellite|ERS]] satellites flew in tandem with a one-day separation for this purpose. A second approach is to use two antennas mounted some distance apart on the same platform, and acquire the images at the same time, which ensures no atmospheric or deformation signals are present. This approach was followed by NASA's [[SRTM]] mission aboard the [[space shuttle]] in 2000. InSAR-derived DEMs can be used for later two-pass deformation studies, or for use in other geophysical applications.
=== Persistent Scatterer InSAR ===
Persistent or Permanent Scatterer techniques are a relatively recent development from conventional InSAR, and rely on studying pixels which remain coherent over a sequence of interferograms. There are a variety of different algorithms and techniques available. Commonly the technique is most useful in urban areas with lots of permanent structures, for example the PS InSAR studies of European cities undertaken by the Terrafirma project. <ref>[http://www.esa.int/esaEO/SEMO6B8ZMRE_index_0.html] Ground movement risks identified by Terrafirma. Dated 8 September 2006, accessed March 21st 2007 </ref>
== References ==
<references/>
==See also==
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== Further Reading ==
*Radar Interferometry: Data Interpretation and Error Analysis, Ramon F. Hanssen. Kluwer Academic, 2001. ISBN-10: 0792369459, ISBN-13: 9780792369455
*Massonnet, D., and K. L. Feigl (1998), Radar interferometry and its application to changes in the earth’s surface, Rev. Geophys., 36(4), 441–500.
*[http://www.earth.ox.ac.uk/%7Etimw/papers/Wright_Visions_PTRS2002.pdf Remote monitoring of the earthquake cycle using satellite radar interferometry.]Tim J. Wright, Phil. Trans. R. Soc. Lond. A, 360, 2873-2888, 2002.
*[http://www.geo.cornell.edu/eas/PeoplePlaces/Faculty/matt/vol59no7p68_69.pdf InSAR, a tool for measuring Earth's surface deformation] Matthew E. Pritchard
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== To Do List ==
*Add lots of references
*Put more in
*Add more detail to the PS section
*Search for other relevant categories
*More about topo applications
*Look at ordering of Technique sections
*Standardise formatting of references, and check against guidelines
*Find out how to link ISBNs and DOIs properly
*Check formatting against the style guidelines
*Spellcheck
*Standardise linebreaks/blank lines
*Find more introductory-level items for Further Reading
*Find some pictures
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