Interferometric synthetic aperture radar
Encyclopedia
Interferometric synthetic aperture radar, also abbreviated InSAR or IfSAR, is a radar
technique used in geodesy
and remote sensing
. This geodetic method uses two or more synthetic aperture radar
(SAR) images to generate maps of surface deformation or digital elevation, using differences in the phase of the waves returning to the satellite
, or aircraft. The technique can potentially measure centimetre-scale changes in deformation over timespans of days to
years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and also in structural engineering, in particular monitoring of subsidence and structural stability.
Synthetic aperture radar
(SAR) is a form of radar
in which sophisticated processing of radar data is used to produce a very narrow effective beam. It can only be used by moving instruments over relatively immobile targets. It is a form of active remote sensing - the antenna
transmits radiation which is then reflected from the target, as opposed to passive sensing, where the reflection is detected from ambient illumination. The image acquisition is therefore independent of the natural illumination and images can be taken at night. Radar uses electromagnetic radiation
with microwave
frequencies; the atmospheric absorption at typical radar wavelengths is very low, meaning observations are not prevented by cloud cover.
Most SAR applications make use of the amplitude
of the return signal, and ignore the phase
data. However interferometry uses the phase of the reflected radiation. Since the outgoing wave is produced by the satellite, the phase is known, and can be compared to the phase of the return signal. The phase of the return wave depends on the distance to the ground, since the path length to the ground and back will consist of a number of whole wavelengths plus some fraction of a wavelength. This is observable as a phase difference or phase shift in the returning wave. The total distance to the satellite (i.e. the number of whole wavelengths) is not known, but the extra fraction of a wavelength can be measured extremely accurately.
In practice, the phase is also affected by several other factors, which together make the raw phase return in any one SAR image 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, due to the cyclic nature of phase, is recorded as repeating fringes which each represent a full 2π cycle.
, 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 different 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 topography
, meaning an extra phase difference is introduced by a stereoscopic effect. The longer the baseline, the smaller the topographic height needed to produce a fringe of phase change - known as the altitude of ambiguity. This effect can be exploited to calculate the topographic height, and used to produce a digital elevation model
(DEM).
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 with InSAR). 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 (GPS or similar) to establish the absolute movement of a point.
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 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 depends on air temperature, pressure and the partial pressure
of water vapour. 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.
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 co-registered
, using a correlation
procedure to find the offset and difference in geometry between the two amplitude images. One SAR image is then 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-multiplication
of each pixel in the two images, and the interferometric phase due to the curvature of the Earth
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 be 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 areas of the image may be masked out. The final processing stage involves geocoding
the image, which resamples the interferogram from the acquisition geometry (related to direction of satellite path) into the desired geographic projection.
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-defined orbits and short baselines necessary for InSAR. More recently, the 11-day NASA STS-99 mission in February 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 successor to ERS, aboard Envisat
. While the majority of InSAR to date has utilised the C-band sensors, recent missions such as the ALOS PALSAR
, TerraSAR-X
and COSMO SKYMED
are expanding the available data in the L- and X-band.
, but has since been utilised extensively for a wide variety of earthquakes all over the world. In particular the 1999 Izmit
and 2003 Bam earthquakes were extensively studied. InSAR can also be used to monitor creep and strain accumulation on faults.
distribution at depth, gravitational spreading of volcanic edifices, and volcano-tectonic deformation signals. Early work on volcanic InSAR included studies on Mount Etna
, and Kilauea
, 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. Recently InSAR has also been used to study rifting processes in Ethiopia.
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.
Interferograms can be used to produce 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 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.
Politecnico di Milano patented the technology in 1999 and created the spin-off company Tele-Rilevamento Europa - TRE in 2000 to commercialize the technology and perform ongoing research.
Some research centres and other companies were inspired to develop 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 created by ESA to define the second generation of radar interferometry techniques.
Commonly such techniques are most useful in urban areas with lots of permanent structures, for example the PSI studies of European cities undertaken by the Terrafirma project. The Terrafirma project (led by Fugro NPA
) 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 5 years this service has supplied information relating to urban subsidence and uplift, slope stability and landslides, seismic and volcanic deformation, coastlines and flood plains.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
technique used in geodesy
Geodesy
Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...
and remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
. This geodetic method uses two or more synthetic aperture radar
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
(SAR) images to generate maps of surface deformation or digital elevation, using differences in the phase of the waves returning to the satellite
, or aircraft. The technique can potentially measure centimetre-scale changes in deformation over timespans of days to
years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and also in structural engineering, in particular monitoring of subsidence and structural stability.
Synthetic aperture radar
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
(SAR) is a form of radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
in which sophisticated processing of radar data is used to produce a very narrow effective beam. It can only be used by moving instruments over relatively immobile targets. It is a form of active remote sensing - the antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
transmits radiation which is then reflected from the target, as opposed to passive sensing, where the reflection is detected from ambient illumination. The image acquisition is therefore independent of the natural illumination and images can be taken at night. Radar uses electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
with microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
frequencies; the atmospheric absorption at typical radar wavelengths is very low, meaning observations are not prevented by cloud cover.
Phase
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of the return signal, and ignore the phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
data. However interferometry uses the phase of the reflected radiation. Since the outgoing wave is produced by the satellite, the phase is known, and can be compared to the phase of the return signal. The phase of the return wave depends on the distance to the ground, since the path length to the ground and back will consist of a number of whole wavelengths plus some fraction of a wavelength. This is observable as a phase difference or phase shift in the returning wave. The total distance to the satellite (i.e. the number of whole wavelengths) is not known, but the extra fraction of a wavelength can be measured extremely accurately.
In practice, the phase is also affected by several other factors, which together make the raw phase return in any one SAR image 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, due to the cyclic nature of phase, is recorded as repeating fringes which 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 reflectionReflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
, 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
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing 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 different 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.
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
, meaning an extra phase difference is introduced by a stereoscopic effect. The longer the baseline, the smaller the topographic height needed to produce a fringe of phase change - known as the altitude of ambiguity. This effect can be exploited to calculate the topographic height, and used to produce a digital elevation model
Digital elevation model
A digital elevation model is a digital model or 3-D representation of a terrain's surface — commonly for a planet , moon, or asteroid — created from terrain elevation data....
(DEM).
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
Digital elevation model
A digital elevation model is a digital model or 3-D representation of a terrain's surface — commonly for a planet , moon, or asteroid — created from terrain elevation data....
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 with InSAR). 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
European Remote-Sensing Satellite
European remote sensing satellite was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782–785 km.-Instruments:...
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 (GPS or similar) to establish the absolute movement of a point.
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 SRTMShuttle Radar Topography Mission
The Shuttle Radar Topography Mission is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009...
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
Image registration
Image registration is the process of transforming different sets of data into one coordinate system. Data may be multiple photographs, data from different sensors, from different times, or from different viewpoints. It is used in computer vision, medical imaging, military automatic target...
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 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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, and depends on air temperature, pressure and the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of water vapour. 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.
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 co-registered
Image registration
Image registration is the process of transforming different sets of data into one coordinate system. Data may be multiple photographs, data from different sensors, from different times, or from different viewpoints. It is used in computer vision, medical imaging, military automatic target...
, using a correlation
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
procedure to find the offset and difference in geometry between the two amplitude images. One SAR image is then re-sampled
Resampling
Resampling may refer to:* Resampling , several related audio processes* Resampling , resampling methods in statistics* Resampling , scaling of bitmap images* Sample rate conversion-See also:* Downsampling* Upsampling...
to match the geometry of the other, meaning each pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
represents the same ground area in both images. The interferogram is then formed by cross-multiplication
Cross product
In mathematics, the cross product, vector product, or Gibbs vector product is a binary operation on two vectors in three-dimensional space. It results in a vector which is perpendicular to both of the vectors being multiplied and normal to the plane containing them...
of each pixel in the two images, and the interferometric phase due to the curvature of the Earth
Reference ellipsoid
In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body....
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
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
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 be 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 areas of the image may be masked out. The final processing stage involves geocoding
Geocoding
Geocoding is the process of finding associated geographic coordinates from other geographic data, such as street addresses, or zip codes...
the image, which resamples the interferogram from the acquisition geometry (related to direction of satellite path) into the desired geographic projection.
Terrestrial SAR Interferometry (TInSAR)
Terrestrial SAR Interferometry (TInSAR) is a remote sensing technique for the displacement monitoring of slopes, rock scarps, volcanoes, landslides, buildings, infrastructures etc. The TInSAR 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 satellite moving around an orbit. SAR technique allow 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). The antenna emits and receive microwave impulses, and by the measurement of the phase difference between two images it is possible to compute the displacement of all the pixel of the SAR image. The accuracy in the displacement measurement is millimetric or submillimetric depending on the specific local and atmospheric conditions.Software
A variety of InSAR processing packages are commonly used, several are available free or free for academic use.- GMTSAR: An InSAR processing system based on Generic Mapping Tools - open source GNU General Public License - http://topex.ucsd.edu/gmtsar
- IMAGINE SAR Interferometry- commercial processing package embedded in ERDAS IMAGINEERDAS IMAGINEERDAS IMAGINE is a remote sensing application with raster graphics editor capabilities designed by ERDAS for geospatial applications. The latest version is 2010, version 10.1. ERDAS IMAGINE is aimed primarily at geospatial raster data processing and allows the user to prepare, display and enhance...
remote sensing software suite, code is C++ based. http://gi.leica-geosystems.com/default.aspx homepage. - ROI PACROI PACROI_PAC is a software package created by the Jet Propulsion Laboratory division of NASA and CalTech for processing SAR images to create InSAR images, named interferograms. ROI_PAC stands for Repeat Orbit Interferometry PACkage...
- produced by NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Jet Propulsion LaboratoryJet Propulsion LaboratoryJet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
and Caltech. UNIX based, can be freely downloaded from The Open Channel Foundation. - DORIS - processing suite from Delft University of TechnologyDelft University of TechnologyDelft University of Technology , also known as TU Delft, is the largest and oldest Dutch public technical university, located in Delft, Netherlands...
, code is C++ based, making it multi-platform portable. Distributed as open source with conditions from the DORIS homepage. - Gamma Software - Commercial software suite consisting of different modules covering SAR data processing, SAR Interferometry, differential SAR Interferometry, and Interferometric Point Target Analysis, runs on Solaris, Linux, Mac OS X, Windows, large discount for Research Institutes http://www.gamma-rs.ch/software/.
- SARscape - Commercial software suite consisting of different modules covering SAR data processing, SAR and ScanSAR focusing and Interferometry, differential SAR Interferometry, Persistent Scatterers and SBAS, Polarimetry and Polarimetric Interferometry, running as an extension of ENVI under Windows and Linux http://www.sarmap.ch/.
- Pulsar - Commercial software suite, UNIX based http://www.phoenixsystems.co.uk/.
- DIAPASON - Originally developed by the French Space Agency CNESCNESThe is the French government space agency . Established under President Charles de Gaulle in 1961, its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and Research...
, and maintained by Altamira Information, Commercial software suite - UNIX & Windows based http://www.altamira-information.com/. - RAT (Radar Tools) - SAR polarimetry (PolSAR), interferometry (InSAR), polarimetric interferometry (PolInSAR) and more, free software suite http://radartools.berlios.de/
- Orfeo ToolBoxOrfeo toolboxOrfeo Toolbox is a library for remote sensing image processing. The project had been initiated by the French space agency in 2006 and is under heavy developments and the participation from the open source community is currently growing. The goal is to provide potential users of satellite images...
(OTB) - UNIX & Windows based, free software suite http://www.orfeo-toolbox.org/
Data Sources
Seasat
SEASAT was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board the first spaceborne synthetic aperture radar . The mission was designed to demonstrate the feasibility of global satellite monitoring of oceanographic phenomena and to help determine...
data in the 1980s, but the potential of the technique was expanded in the 1990s, with the launch of ERS-1 (1991), JERS-1
JERS-1
Japanese Earth Resources Satellite 1 was a satellite launched in 1992 by the Japan Aerospace Exploration Agency...
(1992), RADARSAT-1
RADARSAT-1
Radarsat-1 is Canada's first commercial Earth observation satellite.-Mission:It was launched at 14h22 UTC on November 4, 1995 from Vandenberg AFB in California, into a sun-synchronous orbit above the Earth with an altitude of 798 kilometers and inclination of 98.6 degrees...
and ERS-2 (1995). These platforms provided the stable, well-defined orbits and short baselines necessary for InSAR. More recently, the 11-day NASA STS-99 mission in February 2000 used a SAR antenna mounted on the space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
to gather data for the Shuttle Radar Topography Mission
Shuttle Radar Topography Mission
The Shuttle Radar Topography Mission is an international research effort that obtained digital elevation models on a near-global scale from 56° S to 60° N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009...
. In 2002 ESA launched the ASAR instrument, designed as a successor to ERS, aboard Envisat
Envisat
Envisat is an Earth-observing satellite. It was launched on 1 March 2002 aboard an Ariane 5 from the Guyana Space Centre in Kourou, French Guyana into a Sun synchronous polar orbit at an altitude of...
. While the majority of InSAR to date has utilised the C-band sensors, recent missions such as the ALOS PALSAR
Advanced Land Observation Satellite
Advanced Land Observation Satellite , also called Daichi, is a 4-ton Japanese satellite. It was launched from Tanegashima island, Japan on 24 January 2006 by a H-IIA rocket. The launch had been delayed three times by weather and sensor problems...
, TerraSAR-X
TerraSAR-X
TerraSAR-X, a German Earth observation satellite, is a joint venture being carried out under a public-private-partnership between the German Aerospace Center DLR and EADS Astrium GmbH; the exclusive commercial exploitation rights are held by the geo-information service provider Infoterra GmbH....
and COSMO SKYMED
COSMO-SkyMed
COSMO-SkyMed is an Earth observation satellite system funded by the Italian Ministry of Research and Ministry of Defence and...
are expanding the available data in the L- and X-band.
Tectonic
InSAR can be used to measure tectonic deformation, for example ground movements due to earthquakes. It was first used for the 1992 Landers earthquake1992 Landers earthquake
The 1992 Landers earthquake was a magnitude 7.3 earthquake that occurred on June 28, 1992 with an epicenter near the town of Landers, California...
, but has since been utilised extensively for a wide variety of earthquakes all over the world. In particular the 1999 Izmit
1999 Izmit earthquake
The 1999 İzmit earthquake was a 7.6 magnitude earthquake that struck northwestern Turkey on August 17, 1999, at about 3:02am local time. The event lasted for 37 seconds, killing around 17,000 people and leaving approximately half a million people homeless...
and 2003 Bam earthquakes were extensively studied. InSAR can also be used to monitor creep and strain accumulation on faults.
Volcanic
InSAR can be used in a variety of volcanic settings, including deformation associated with eruptions, inter-eruption strain caused by changes in magmaMagma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
distribution at depth, gravitational spreading of volcanic edifices, and volcano-tectonic deformation signals. Early work on volcanic InSAR included studies on Mount Etna
Mount Etna
Mount Etna is an active stratovolcano on the east coast of Sicily, close to Messina and Catania. It is the tallest active volcano in Europe, currently standing high, though this varies with summit eruptions; the mountain is 21 m higher than it was in 1981.. It is the highest mountain in...
, and Kilauea
Kilauea
Kīlauea is a volcano in the Hawaiian Islands, and one of five shield volcanoes that together form the island of Hawaii. Kīlauea means "spewing" or "much spreading" in the Hawaiian language, referring to its frequent outpouring of lava. The Puu Ōō cone has been continuously erupting in the eastern...
, 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. Recently InSAR has also been used to study rifting processes in Ethiopia.
Subsidence
Subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
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
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
and collapse of old mines. It can also be used for monitoring the stability of built structures, and landscape features such as 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.DEM generation
European Remote-Sensing Satellite
European remote sensing satellite was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782–785 km.-Instruments:...
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
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
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. In 1999, researchers at Politecnico di Milano, Italy, developed a new multi-image approach in which one searches the stack of images for objects on the ground providing consistent and stable radar reflections back to the satellite. These objects could be the size of a pixel or, more commonly, sub-pixel sized, and are present in every image in the stack.Politecnico di Milano patented the technology in 1999 and created the spin-off company Tele-Rilevamento Europa - TRE in 2000 to commercialize the technology and perform ongoing research.
Some research centres and other companies were inspired to develop 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 created by ESA to define the second generation of radar interferometry techniques.
Commonly such techniques are most useful in urban areas with lots of permanent structures, for example the PSI studies of European cities undertaken by the Terrafirma project. The Terrafirma project (led by Fugro NPA
Fugro NPA
Fugro NPA is the longest-established satellite mapping specialist in Europe, with expertise in geoscience applications of earth observation and remote sensing...
) 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 5 years this service has supplied information relating to urban subsidence and uplift, slope stability and landslides, seismic and volcanic deformation, coastlines and flood plains.
See also
- Terrestrial SAR Interferometry (TInSAR)
- InterferometryInterferometryInterferometry refers to a family of techniques in which electromagnetic waves are superimposed 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,...
- ROI PACROI PACROI_PAC is a software package created by the Jet Propulsion Laboratory division of NASA and CalTech for processing SAR images to create InSAR images, named interferograms. ROI_PAC stands for Repeat Orbit Interferometry PACkage...
- Remote sensingRemote sensingRemote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
- RadarRadarRadar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
- Synthetic aperture radarSynthetic aperture radarSynthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
Further reading
- Terrestrial SAR Interferometry(TInSAR)
- IFSAR Technology
- InSAR, a tool for measuring Earth's surface deformation Matthew E. Pritchard
- Radar interferometry tutorial
- USGS InSAR factsheet
- InSAR Principles, ESA publication, TM19, February 2007.
- B. Kampes, Radar Interferometry – Persistent Scatterer Technique, Kluwer Academic Publishers, Dordrecht,The Netherlands, 2006. ISBN 978-1402045769