Interferometric synthetic-aperture radar
Interferometric synthetic aperture radar, abbreviated InSAR (or deprecated IfSAR), is a
Technique
Synthetic aperture radar
Phase
SAR makes use of the
In practice, the phase of the return signal is affected by several factors, which together can make the absolute phase return in any SAR data collection essentially arbitrary, with no correlation from pixel to pixel. To get any useful information from the phase, some of these effects must be isolated and removed. Interferometry uses two images of the same area taken from the same position (or, for topographic applications, slightly different positions) and finds the difference in phase between them, producing an image known as an interferogram. This is measured in radians of phase difference and, because of the cyclic nature of phase, is recorded as repeating fringes that each represent a full 2π cycle.
Factors affecting phase
The most important factor affecting the phase is the interaction with the ground surface. The phase of the wave may change on reflection, depending on the properties of the material. The reflected signal back from any one pixel is the summed contribution to the phase from many smaller 'targets' in that ground area, each with different dielectric properties and distances from the satellite, meaning the returned signal is arbitrary and completely uncorrelated with that from adjacent pixels. Importantly though, it is consistent – provided nothing on the ground changes the contributions from each target should sum identically each time, and hence be removed from the interferogram.
Once the ground effects have been removed, the major signal present in the interferogram is a contribution from orbital effects. For interferometry to work, the satellites must be as close as possible to the same spatial position when the images are acquired. This means that images from two satellite platforms with different orbits cannot be compared, and for a given satellite data from the same orbital track must be used. In practice the perpendicular distance between them, known as the baseline, is often known to within a few centimetres but can only be controlled on a scale of tens to hundreds of metres. This slight difference causes a regular difference in phase that changes smoothly across the interferogram and can be modelled and removed.
The slight difference in satellite position also alters the distortion caused by
If the height of the topography is already known, the topographic phase contribution can be calculated and removed. This has traditionally been done in two ways. In the two-pass method, elevation data from an externally derived DEM is used in conjunction with the orbital information to calculate the phase contribution. In the three-pass method two images acquired a short time apart are used to create an interferogram, which is assumed to have no deformation signal and therefore represent the topographic contribution. This interferogram is then subtracted from a third image with a longer time separation to give the residual phase due to deformation.
Once the ground, orbital and topographic contributions have been removed the interferogram contains the deformation signal, along with any remaining noise (see Difficulties below). The signal measured in the interferogram represents the change in phase caused by an increase or decrease in distance from the ground pixel to the satellite, therefore only the component of the ground motion parallel to the satellite line of sight vector will cause a phase difference to be observed. For sensors like ERS with a small incidence angle this measures vertical motion well, but is insensitive to horizontal motion perpendicular to the line of sight (approximately north-south). It also means that vertical motion and components of horizontal motion parallel to the plane of the line of sight (approximately east-west) cannot be separately resolved.
One fringe of phase difference is generated by a ground motion of half the radar 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 (
Difficulties
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 90 m SRTM data may be available for many areas, but at high latitudes or in areas of 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 ground 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 the width of one 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 co-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 travelled through 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
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. In 1999, researchers at
Some research centres and companies, were inspired to develop variations of their own algorithms which would also overcome InSAR's limitations. In scientific literature, these techniques are collectively referred to as persistent scatterer interferometry or PSI techniques. The term persistent scatterer interferometry (PSI) was proposed by European Space Agency (ESA) to define the second generation of radar interferometry techniques. This term is nowadays commonly accepted by scientific and the end user community.
Commonly such techniques are most useful in urban areas with many permanent structures, for example the PSI studies of European geohazard sites undertaken by the Terrafirma project.[5] The Terrafirma project provides a ground motion hazard information service, distributed throughout Europe via national geological surveys and institutions. The objective of this service is to help save lives, improve safety, and reduce economic loss through the use of state-of-the-art PSI information. Over the last 9 years this service has supplied information relating to urban subsidence and uplift, slope stability and landslides, seismic and volcanic deformation, coastlines and flood plains.
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
Once the basic interferogram has been produced, it is commonly
Hardware
Spaceborne
Early exploitation of satellite-based InSAR included use of
Sentinel-1A and Sentinel-1B, both C-band sensors, were launched by the ESA in 2014 and 2016, respectively. Together, they provide InSAR coverage on a global scale and on a 6-day repeat cycle.
Airborne
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Airborne InSAR data acquisition systems are built by companies such as the American
Terrestrial or ground-based
Terrestrial or ground-based SAR interferometry (TInSAR or GBInSAR) is a remote sensing technique for the displacement monitoring of slopes,[7] rock scarps, volcanoes, landslides, buildings, infrastructures etc. This technique is based on the same operational principles of the satellite SAR interferometry, but the synthetic aperture of the radar (SAR) is obtained by an antenna moving on a rail instead of a satellite moving around an orbit. SAR technique allows 2D radar image of the investigated scenario to be achieved, with a high range resolution (along the instrumental line of sight) and cross-range resolution (along the scan direction). Two antennas respectively emit and receive microwave signals and, by calculating the phase difference between two measurements taken in two different times, it is possible to compute the displacement of all the pixels of the SAR image. The accuracy in the displacement measurement is of the same order of magnitude as the EM wavelength and depends also on the specific local and atmospheric conditions.
Applications
Tectonic
InSAR can be used to measure
Volcanic
InSAR can be used in a variety of
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,[17] subsurface mining and collapse of old mines.[18] Thus, InSAR has become an indispensable tool to satisfactorily address many subsidence studies. Tomás et al.[19] performed a cost analysis that allowed to identify the strongest points of InSAR techniques compared with other conventional techniques: (1) higher data acquisition frequency and spatial coverage; and (2) lower annual cost per measurement point and per square kilometre.
Landslides
Although InSAR technique can present some limitations when applied to landslides,
Ice flow
Glacial motion and deformation have been successfully measured using satellite interferometry. The technique allows remote, high-resolution measurement of changes in glacial structure, ice flow, and shifts in ice dynamics, all of which agree closely with ground observations.[24]
Infrastructure and building monitoring
InSAR can also be used to monitor the stability of built structures.[25] Very high resolution SAR data (such as derived from the TerraSAR-X StripMap mode or COSMO-Skymed HIMAGE mode) are especially suitable for this task. InSAR is used for monitoring highway and railway settlements,[26][27] dike stability,[28] forensic engineering[29] and many other uses.
DEM generation
Interferograms can be used to produce
See also
References
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- ISBN 9780792369455
- ^ "Terrafirma.eu.com: A pan-European ground hazard information service". Retrieved 22 January 2013.
- ^ "Revista Pesquisa Fapesp".
- ^ Longstaff, I.D. (2011). "Comparing real beam and synthetic aperture techniques for Slope Stability Radar" (PDF). Whitepaper, University of Queensland, Australia.[permanent dead link]
- S2CID 4355142
- ^ "Envisat's rainbow vision detects ground moving at pace fingernails grow". European Space Agency. August 6, 2004. Retrieved 2007-03-22.
- ^ "The Izmit Earthquake of 17 August 1999 in Turkey". European Space Agency. Archived from the original on 2007-03-10. Retrieved 2007-03-22.
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- ^ "Ground motion". European Space Agency. Archived from the original on 2008-05-21. Retrieved 2007-03-21.
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- .
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- .
Further reading
- B. Kampes, Radar Interferometry – Persistent Scatterer Technique, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2006. ISBN 978-1-4020-4576-9
External links
- InSAR, a tool for measuring Earth's surface deformation Matthew E. Pritchard
- USGS InSAR factsheet
- InSAR Principles, ESA publication, TM19, February 2007.