NationMaster - Encyclopedia: Interferometric synthetic aperture radar

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. ^[1] ^[2] ^[3] 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 landlsides, and also in structural engineering, in particular monitoring of subsidence and structural stability. Image File history File links Current_event_marker. ... To meet Wikipedias quality standards, this article may require cleanup. ... This long range radar antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll. ... It has been suggested that geodetic system be merged into this article or section. ... Synthetic aperture radar image of Death Valley colored using polarimetry In the broadest sense, remote sensing is the measurement or acquisition of information of an object or phenomenon, by a recording device that is not in physical or intimate contact with the object. ... The surface of Venus, as imaged by the Magellan probe using SAR Synthetic aperture radar (SAR) is a form of radar in which sophisticated post-processing of radar data is used to produce a very narrow effective beam. ... 3D rendering of a DEM of Tithonium Chasma on Mars A digital elevation model (DEM) is a representation of the topography of the Earth or another surface in digital format, that is, by coordinates and numerical descriptions of altitude. ...

Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... European Remote-Sensing satellite (ERS) was the European Space Agencys first Earth-observing satellite. ... Ä°zmit (also known as Kocaeli; previously known as Ismid or Isnikmid) is a city in the northwestern part of Anatolia, Turkey. ...

Technique

Synthetic aperture radar

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 back from the target, as opposed to passive sensing where the reflections detected come 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. Image File history File links Size of this preview: 600 Ã— 600 pixels Full resolution (1000 Ã— 1000 pixel, file size: 333 KB, MIME type: image/jpeg) Original Caption Released with Image: Data acquired on April 13, 1994 and on October 4, 1994 from the X-band Synthetic Aperture Radar on board... Image File history File links Size of this preview: 600 Ã— 600 pixels Full resolution (1000 Ã— 1000 pixel, file size: 333 KB, MIME type: image/jpeg) Original Caption Released with Image: Data acquired on April 13, 1994 and on October 4, 1994 from the X-band Synthetic Aperture Radar on board... KÄ«lauea is an active volcano in the Hawaiian Islands, one of five shield volcanoes that together form the Island of Hawaii. ... The surface of Venus, as imaged by the Magellan probe using SAR Synthetic aperture radar (SAR) is a form of radar in which sophisticated post-processing of radar data is used to produce a very narrow effective beam. ... The surface of Venus, as imaged by the Magellan probe using SAR Synthetic aperture radar (SAR) is a form of radar in which sophisticated post-processing of radar data is used to produce a very narrow effective beam. ... This long range radar antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll. ... A Yagi-Uda beam antenna Short Wave Curtain Antenna (Moosbrunn, Austria) A building rooftop supporting numerous dish and sectored mobile telecommunications antennas (Doncaster, Victoria, Australia) An antenna or aerial is a transducer designed to transmit or receive radio waves which are a class of electromagnetic waves. ... It has been suggested that this article or section be merged with light. ... Microwaves are electromagnetic waves with wavelengths longer than those of terahertz (THz) frequencies, but relatively short for radio waves. ...

Phase

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. Image File history File links Phase_shift. ... Image File history File links Phase_shift. ... Amplitude is a nonnegative scalar measure of a waves magnitude of oscillation, that is, magnitude of the maximum disturbance in the medium during one wave cycle. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The wavelength is the distance between repeating units of a wave pattern. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

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. In mathematics and physics, the radian is a unit of angle measure. ...

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. Look up reflection in Wiktionary, the free dictionary. ... A dielectric, or electrical insulator, is a substance that is highly resistant to electric current. ...

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). Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... KÄ«lauea is an active volcano in the Hawaiian Islands, one of five shield volcanoes that together form the Island of Hawaii. ... It has been suggested that Geomorphometry be merged into this article or section. ... Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is a technique to create the illusion of depth in a photograph, movie, or other two-dimensional image, by presenting a slightly different image to each eye. ... 3D rendering of a DEM of Tithonium Chasma on Mars A digital elevation model (DEM) is a digital representation of ground surface topography or terrain. ...

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. 3D rendering of a DEM of Tithonium Chasma on Mars A digital elevation model (DEM) is a digital representation of ground surface topography or terrain. ...

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. It also means that vertical motion and components of horizontal motion parallel to the plane of the line of sight cannot be separately resolved. ERS can refer to: The card game Egyptian Ratscrew Either of the two European Remote-Sensing Satellites The Electoral Reform Society Ellipsoidal Reflector Spotlight ERS http://en. ... Fig. ...

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. Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. ...

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 latitudes or in areas of poor coverage alternative datasets must be found. The Shuttle Radar Topography Mission (SRTM) is a research effort that obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth to date. ... The Shuttle Radar Topography Mission (SRTM) is a research effort that obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth to date. ...

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. In computer vision, sets of data acquired by sampling the same scene or object at different times, or from different perspectives, will be in different coordinate systems. ...

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.^[4] 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. A line showing the speed of light on a scale model of Earth and the Moon The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness. It is the speed of all electromagnetic radiation... Look up Vacuum in Wiktionary, the free dictionary. ... 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. ... Look up Homogeneous in Wiktionary, the free dictionary. ... Look up Heterogeneous in Wiktionary, the free dictionary. ...

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, 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 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. In computer vision, sets of data acquired by sampling the same scene or object at different times, or from different perspectives, will be in different coordinate systems. ... Positive linear correlations between 1000 pairs of numbers. ... Resampling is a term used in statistics to describe a variety of methods for computing summary statistics using subsets of available data (jackknife), drawing randomly with replacement from a set of data points (bootstrapping), or switching labels on data points when performing significance tests (permutation test). ... This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. ... For the crossed product in algebra and functional analysis, see crossed product. ... In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the true figure of the Earth or other planetary body. ...

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 areas 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. Television signal splitter consisting of a hi-pass and a low-pass filter. ... Geocoding is the process of assigning geographic identifiers (e. ... Equidistant cylindrical projection of the Globe Equidistant cylindrical projection of a composite satellite image (NASA) The Plate Carrée or Equidistant Cylindrical Projection or Geographic Projection, is a very simple map projection that has been in use since the earliest days of spherical cartography. ...

Software

A variety of InSAR processing packages are commonly used, several are available free for academic use.

ROI_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. ... The National Aeronautics and Space Administration (NASA) is an agency of the United States federal government, responsible for the nations public space program. ... The NASA Jet Propulsion Laboratory (JPL), in Pasadena and La CaÃ±ada Flintridge, near Los Angeles, California, USA, builds and operates unmanned spacecraft for the National Aeronautics and Space Administration (NASA). ... California Institute of Technology The California Institute of Technology (commonly known as Caltech) is a private, coeducational university located in Pasadena, California, in the United States. ... Founded in 1842, the Delft University of Technology, in Delft, the Netherlands, is one of the oldest, largest, and most comprehensive technical universities in the Netherlands, with over 13,000 students and 2,100 scientists (including 200 professors). ... The Centre National dÃ‰tudes Spatiales is the French government space agency (administratively, a public establishment of industrial and commercial character). Its headquarters are located in central Paris. ...

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-defined 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 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 and TerraSAR-X are expanding the available data in the L- and X-band. Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Seasat (Courtesy NASA/JPL-Caltech) Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earths oceans and had onboard the first spaceborne synthetic aperture radar (SAR). ... Earth Resources Satellite-1. ... The Japanese Earth Resources Satellite (1) was launched in 1992 by the Japanese space agency (NASDA). ... RADARSAT-1 is Canadas first commercial Earth observation satellite. ... European Remote-Sensing satellite (ERS) was the European Space Agencys first Earth-observing satellite. ... NASAs Space Shuttle, officially called Space Transportation System (STS), is the United States governments current manned launch vehicle. ... The Shuttle Radar Topography Mission (SRTM) is a research effort that obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth to date. ... This article is about the European Space Agency. ... The Envisat (Environmental Satellite) satellite is an Earth-observing satellite built by the European Space Agency. ... Advanced Land Observation Satellite (ALOS), also called Daichi, is a 4-ton Japanese satellite. ... TerraSAR-X is an Earth observation satellite, built and operated by the German Aerospace Center and EADS Astrium. ...

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,^[5] 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. ^[7] ^[8] InSAR can also be used to monitor creep and strain accumulation on faults. ... Global earthquake epicenters, 1963–1998. ... Lander College for Men is a division of Touro College, based in Kew Gardens Hills, Queens, New York, was created as an antidote to Yeshiva University. ... The 1999 Ä°zmit earthquake had a magnitude of 7. ... Bam in 2002. ... Geologic faults, fault lines or simply faults are planar rock fractures, which show evidence of relative movement. ...

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.^[9] Early work on volcanic InSAR included studies on Mount Etna,^[6] and Kilauea,^[10] 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.^[11] Recently InSAR has also been used to study rifting processes in Ethiopia.^[12] This article is about volcanoes in geology. ... Eruption can refer to: Volcanic eruption The eruption of teeth through the gum Eruption (band) This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... Magma is molten rock located beneath the surface of the Earth (or any other terrestrial planet), and which often collects in a magma chamber. ... This article covers the physics of gravitation. ... Mount Etna (also known as Mongibeddu in Sicilian and Mongibello in Italian, a combination of Latin mont- and Arabic jebel, both meaning mountain) is an active volcano on the east coast of Sicily, close to Messina and Catania. ... KÄ«lauea is an active volcano in the Hawaiian Islands, one of five shield volcanoes that together form the Island of Hawaii. ... In geology, a rift is a place where the Earths lithosphere is expanding. ...

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.^[13] Image File history File links Size of this preview: 456 Ã— 599 pixels Full resolution (504 Ã— 662 pixel, file size: 88 KB, MIME type: image/jpeg) Original Caption Released with Image: This figure shows a comparison of interferograms from four different years mapping the rapid ground subsidence over the Lost Hills... Image File history File links Size of this preview: 456 Ã— 599 pixels Full resolution (504 Ã— 662 pixel, file size: 88 KB, MIME type: image/jpeg) Original Caption Released with Image: This figure shows a comparison of interferograms from four different years mapping the rapid ground subsidence over the Lost Hills... A road destroyed by subsidence and shear. ... This article is about mineral extraction. ... This entry refers to the geological term landslide. ...

DEM generation

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. Image File history File links Size of this preview: 800 Ã— 358 pixels Full resolution (2220 Ã— 994 pixel, file size: 778 KB, MIME type: image/jpeg) URL: http://visibleearth. ... Image File history File links Size of this preview: 800 Ã— 358 pixels Full resolution (2220 Ã— 994 pixel, file size: 778 KB, MIME type: image/jpeg) URL: http://visibleearth. ... 3D rendering of a DEM of Tithonium Chasma on Mars A digital elevation model (DEM) is a representation of the topography of the Earth or another surface in digital format, that is, by coordinates and numerical descriptions of altitude. ... Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is a technique to create the illusion of depth in a photograph, movie, or other two-dimensional image, by presenting a slightly different image to each eye. ... European Remote-Sensing satellite (ERS) was the European Space Agencys first Earth-observing satellite. ... The Shuttle Radar Topography Mission (SRTM) is a research effort that obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth to date. ... NASAs Space Shuttle, officially called Space Transportation System (STS), is the United States governments current manned launch vehicle. ...

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. ^[14]

References

2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the CE era. ... March 22 is the 81st day of the year in the Gregorian calendar (82nd in leap years). ... 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the CE era. ... March 22 is the 81st day of the year in the Gregorian calendar (82nd in leap years). ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the CE era. ... March 21 is the 80th day of the year in the Gregorian calendar (81st in leap years). ... 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the CE era. ... March 21 is the 80th day of the year in the Gregorian calendar (81st in leap years). ...

Contents

Technique

Synthetic aperture radar

Phase

Factors affecting phase

Difficulties with InSAR

Producing interferograms

Software

Data Sources

Applications

Tectonic

Volcanic

Subsidence

DEM generation

Persistent Scatterer InSAR

References

See also

Further Reading

Contents

Technique GA_googleFillSlot("encyclopedia_square");

Synthetic aperture radar

Phase

Factors affecting phase

Difficulties with InSAR

Producing interferograms

Software

Data Sources

Applications

Tectonic

Volcanic

Subsidence

DEM generation

Persistent Scatterer InSAR

References

See also

Further Reading

Technique