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86-623: Previous SAR instruments, such as the radar on the Magellan mission to Venus , were large, massive, power-hungry, and expensive. Intended as a demonstration of cheap, lightweight SAR technology, the Mini-RF instrument was designed in response to these concerns. Because it was a technology demonstration, Mini-RF is sometimes not included in lists of LRO 's instruments. Radar is one of the few remote sensing tools capable of distinguishing water ice from other forms of water thought to be present of

172-408: A 2D sinusoid at a given frequency without distortion while minimizing the variance of the noise of the resulting image. The purpose is to compute the spectral estimate efficiently. Spectral estimate is given as where R is the covariance matrix, and a ω 1 , ω 2 ∗ {\displaystyle a_{\omega _{1},\omega _{2}}^{*}}

258-484: 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

344-622: A bibliometric study on the trends in publications related to landslides and InSAR. They found that the publication trends follow a power model, indicating that despite its inception in the last century, InSAR is a growing topical issue and has become established as a valuable tool for studying landslides. 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. InSAR can also be used to monitor

430-918: A given physical antenna. SAR is capable of high-resolution remote sensing , independent of flight altitude, and independent of weather, as SAR can select frequencies to avoid weather-caused signal attenuation. SAR has day and night imaging capability as illumination is provided by the SAR. SAR images have wide applications in remote sensing and mapping of surfaces of the Earth and other planets. Applications of SAR are numerous. Examples include topography, oceanography, glaciology, geology (for example, terrain discrimination and subsurface imaging). SAR can also be used in forestry to determine forest height, biomass, and deforestation. Volcano and earthquake monitoring use differential interferometry . SAR can also be applied for monitoring civil infrastructure stability such as bridges. SAR

516-630: A global scale and on a six-day repeat cycle. Airborne InSAR data acquisition systems are built by companies such as the American Intermap , the German AeroSensing , and the Brazilian OrbiSat . Terrestrial or ground-based SAR interferometry (TInSAR or GBInSAR) is a remote sensing technique for the displacement monitoring of slopes, rock scarps, volcanoes, landslides, buildings, infrastructures etc. This technique

602-540: 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. The processing chain used to produce interferograms varies according to

688-440: A meter down to several millimeters. As the SAR device on board the aircraft or spacecraft moves, the antenna location relative to the target changes with time. Signal processing of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions. This process forms the synthetic antenna aperture and allows the creation of higher-resolution images than would otherwise be possible with

774-660: 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. That specific implementation is patented. 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)

860-405: 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 known based on the time that it takes for the energy to make the round trip back to the satellite—but it is the extra fraction of a wavelength that is of particular interest and

946-551: A point ( ω x , ω y {\displaystyle \omega _{x},\omega _{y}} ) is given by: where ϕ ^ E V {\displaystyle {\hat {\phi }}_{EV}} is the amplitude of the image at a point ( ω x , ω y ) {\displaystyle \left(\omega _{x},\omega _{y}\right)} , v i _ {\displaystyle {\underline {v_{i}}}}

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1032-513: A point in the SAR image is in alignment to one of the signal subspace eigenvectors which is the peak in image estimate. Thus this method does not accurately represent the scattering intensity at each point, but show the particular points of the image. Backprojection Algorithm has two methods: Time-domain Backprojection and Frequency-domain Backprojection . The time-domain Backprojection has more advantages over frequency-domain and thus,

1118-467: 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

1204-409: 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

1290-771: 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. Thus, InSAR has become an indispensable tool to satisfactorily address many subsidence studies. Tomás et al. 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. Although InSAR technique can present some limitations when applied to landslides, it can also be used for monitoring landscape features such as landslides . Tomás et al. conducted

1376-555: Is a nonparametric covariance-based method, which uses an adaptive matched-filterbank approach and follows two main steps: The adaptive Capon bandpass filter is designed to minimize the power of the filter output, as well as pass the frequencies ( ω 1 , ω 2 {\displaystyle \omega _{1},\omega _{2}} ) without any attenuation, i.e., to satisfy, for each ( ω 1 , ω 2 {\displaystyle \omega _{1},\omega _{2}} ), where R

1462-550: Is a parameter-free sparse signal reconstruction based algorithm. It achieves super-resolution and is robust to highly correlated signals. The name emphasizes its basis on the asymptotically minimum variance (AMV) criterion. It is a powerful tool for the recovery of both the amplitude and frequency characteristics of multiple highly correlated sources in challenging environment (e.g., limited number of snapshots, low signal-to-noise ratio . Applications include synthetic-aperture radar imaging and various source localization. SAMV method

1548-742: Is also a special case of the FIR filtering approaches. It is seen that although the APES algorithm gives slightly wider spectral peaks than the Capon method, the former yields more accurate overall spectral estimates than the latter and the FFT method. FFT method is fast and simple but have larger sidelobes. Capon has high resolution but high computational complexity. EV also has high resolution and high computational complexity. APES has higher resolution, faster than capon and EV but high computational complexity. MUSIC method

1634-399: 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

1720-459: Is capable of achieving resolution higher than some established parametric methods, e.g., MUSIC , especially with highly correlated signals. Computational complexity of the SAMV method is higher due to its iterative procedure. This subspace decomposition method separates the eigenvectors of the autocovariance matrix into those corresponding to signals and to clutter. The amplitude of the image at

1806-405: 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

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1892-456: Is critical to operation of the EV method. The eigenvalue of the R matrix decides whether its corresponding eigenvector corresponds to the clutter or to the signal subspace. The MUSIC method is considered to be a poor performer in SAR applications. This method uses a constant instead of the clutter subspace. In this method, the denominator is equated to zero when a sinusoidal signal corresponding to

1978-407: Is essentially a realization of the mapping of the mathematical framework through generation of the variants and executing matrix operations. The performance of this implementation may vary from machine to machine, and the objective is to identify on which machine it performs best. The Capon spectral method, also called the minimum-variance method, is a multidimensional array-processing technique. It

2064-407: 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

2150-418: Is given by Interferometric synthetic-aperture radar Interferometric synthetic aperture radar , abbreviated InSAR (or deprecated 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

2236-407: Is measured to great accuracy. 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

2322-448: Is more preferred. The time-domain Backprojection forms images or spectrums by matching the data acquired from the radar and as per what it expects to receive. It can be considered as an ideal matched-filter for synthetic-aperture radar. There is no need of having a different motion compensation step due to its quality of handling non-ideal motion/sampling. It can also be used for various imaging geometries. In GEO-SAR, to focus specially on

2408-458: Is not advantageous to capture a waveform for each of both transmission directions for a given pair of antennas, because those waveforms will be identical. When multiple static antennas are used, the total number of unique echo waveforms that can be captured is where N is the number of unique antenna positions. The antenna stays in a fixed position. It may be orthogonal to the flight path, or it may be squinted slightly forward or backward. When

2494-671: Is not generally suitable for SAR imaging, as whitening the clutter eigenvalues destroys the spatial inhomogeneities associated with terrain clutter or other diffuse scattering in SAR imagery. But it offers higher frequency resolution in the resulting power spectral density (PSD) than the fast Fourier transform (FFT)-based methods. The backprojection algorithm is computationally expensive. It is specifically attractive for sensors that are wideband, wide-angle, and/or have long coherent apertures with substantial off-track motion. SAR requires that echo captures be taken at multiple antenna positions. The more captures taken (at different antenna locations)

2580-621: Is the coherency matrix and v i _ H {\displaystyle {\underline {v_{i}}}^{\mathsf {H}}} is the Hermitian of the coherency matrix, 1 λ i {\displaystyle {\frac {1}{\lambda _{i}}}} is the inverse of the eigenvalues of the clutter subspace, W ( ω x , ω y ) {\displaystyle W\left(\omega _{x},\omega _{y}\right)} are vectors defined as where ⊗ denotes

2666-392: Is the covariance matrix , h ω 1 , ω 2 ∗ {\displaystyle h_{\omega _{1},\omega _{2}}^{*}} is the complex conjugate transpose of the impulse response of the FIR filter, a ω 1 , ω 2 {\displaystyle a_{\omega _{1},\omega _{2}}}

Mini-RF - Misplaced Pages Continue

2752-439: Is the 2D Fourier vector, defined as a ω 1 , ω 2 ≜ a ω 1 ⊗ a ω 2 {\displaystyle a_{\omega _{1},\omega _{2}}\triangleq a_{\omega _{1}}\otimes a_{\omega _{2}}} , ⊗ {\displaystyle \otimes } denotes Kronecker product. Therefore, it passes

2838-457: Is the 2D complex-conjugate transpose of the Fourier vector. The computation of this equation over all frequencies is time-consuming. It is seen that the forward–backward Capon estimator yields better estimation than the forward-only classical capon approach. The main reason behind this is that while the forward–backward Capon uses both the forward and backward data vectors to obtain the estimate of

2924-415: Is therefore independent of natural illumination and images can be taken at night. Radar uses electromagnetic radiation at microwave frequencies; the atmospheric absorption at typical radar wavelengths is very low, meaning observations are not prevented by cloud cover. SAR makes use of the amplitude and the absolute phase of the return signal data. In contrast, interferometry uses differential phase of

3010-513: Is used in majority of the spectral estimation algorithms, and there are many fast algorithms for computing the multidimensional discrete Fourier transform. Computational Kronecker-core array algebra is a popular algorithm used as new variant of FFT algorithms for the processing in multidimensional synthetic-aperture radar (SAR) systems. This algorithm uses a study of theoretical properties of input/output data indexing sets and groups of permutations. A branch of finite multi-dimensional linear algebra

3096-411: Is used to identify similarities and differences among various FFT algorithm variants and to create new variants. Each multidimensional DFT computation is expressed in matrix form. The multidimensional DFT matrix, in turn, is disintegrated into a set of factors, called functional primitives, which are individually identified with an underlying software/hardware computational design. The FFT implementation

3182-402: Is used to produce a very narrow effective beam. It can be used to form images of relatively immobile targets; moving targets can be blurred or displaced in the formed images. SAR is a form of active remote sensing – the antenna transmits radiation that is reflected from the image area, as opposed to passive sensing, where the reflection is detected from ambient illumination. SAR image acquisition

3268-408: Is useful in environment monitoring such as oil spills, flooding, urban growth, military surveillance: including strategic policy and tactical assessment. SAR can be implemented as inverse SAR by observing a moving target over a substantial time with a stationary antenna. A synthetic-aperture radar is an imaging radar mounted on a moving platform. SAR is a Doppler technique. It is based on

3354-460: The Kronecker product of the two vectors. MUSIC detects frequencies in a signal by performing an eigen decomposition on the covariance matrix of a data vector of the samples obtained from the samples of the received signal. When all of the eigenvectors are included in the clutter subspace (model order = 0) the EV method becomes identical to the Capon method. Thus the determination of model order

3440-409: 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

3526-443: 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

Mini-RF - Misplaced Pages Continue

3612-664: 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 (SRTM). In 2002 ESA launched

3698-682: The ASAR instrument, designed as a successor to ERS, aboard Envisat . While the majority of InSAR to date has utilized 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. 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

3784-424: The Capon method, but more accurate spectral estimates for amplitude in SAR. In the Capon method, although the spectral peaks are narrower than the APES, the sidelobes are higher than that for the APES. As a result, the estimate for the amplitude is expected to be less accurate for the Capon method than for the APES method. The APES method requires about 1.5 times more computation than the Capon method. SAMV method

3870-570: The Mini-RF transmitter had suffered a critical failure. The receiver continues working, allowing occasional bistatic radar measurements, where the radar signal is transmitted from the Earth, reflected off the Moon, and received by the Mini-RF. The original principal investigator of Mini-RF, Stewart Nozette , was arrested for espionage. Nozette was replaced by Ben Bussey, then of APL, the Applied Physics Laboratory where Mini-RF

3956-668: The Moon , such as hydrated minerals and water adsorbed onto the lunar surface. Although the LCROSS mission, which deliberately crashed a probe into the lunar surface to look for water, detected water in Cabeus Crater, Mini-RF did not detect the presence of thick deposits of water ice at the LCROSS impact site, however, the presence of less than 10 cm sized ice fragments could not be ruled out. In January, 2011, after completion of Mini-RF's primary mission objectives, NASA announced that

4042-423: The SAR image stack are a sampled version of the Fourier transform of reflectivity in elevation direction, but the Fourier transform is irregular. Thus the spectral estimation techniques are used to improve the resolution and reduce speckle compared to the results of conventional Fourier transform SAR imaging techniques. FFT (Fast Fourier Transform i.e., periodogram or matched filter ) is one such method, which

4128-437: The absolute movement of a point. 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

4214-511: The antenna aperture travels along the flight path, a signal is transmitted at a rate equal to the pulse repetition frequency (PRF). The lower boundary of the PRF is determined by the Doppler bandwidth of the radar. The backscatter of each of these signals is commutatively added on a pixel-by-pixel basis to attain the fine azimuth resolution desired in radar imagery. The spotlight synthetic aperture

4300-490: The antenna — each object will have its own doppler shift. A precise frequency analysis of the radar reflections will thus allow the construction of a detailed image. In order to realise this concept, electromagnetic waves are transmitted sequentially, the echoes are collected and the system electronics digitizes and stores the data for subsequent processing. As transmission and reception occur at different times, they map to different small positions. The well ordered combination of

4386-411: The antenna's field of view. The 3D processing is done in two stages. The azimuth and range direction are focused for the generation of 2D (azimuth-range) high-resolution images, after which a digital elevation model (DEM) is used to measure the phase differences between complex images, which is determined from different look angles to recover the height information. This height information, along with

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4472-522: 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

4558-617: The azimuth-range coordinates provided by 2-D SAR focusing, gives the third dimension, which is the elevation. The first step requires only standard processing algorithms, for the second step, additional pre-processing such as image co-registration and phase calibration is used. In addition, multiple baselines can be used to extend 3D imaging to the time dimension . 4D and multi-D SAR imaging allows imaging of complex scenarios, such as urban areas, and has improved performance with respect to classical interferometric techniques such as persistent scatterer interferometry (PSI). SAR algorithms model

4644-420: 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

4730-413: The covariance matrix, the forward-only Capon uses only the forward data vectors to estimate the covariance matrix. The APES (amplitude and phase estimation) method is also a matched-filter-bank method, which assumes that the phase history data is a sum of 2D sinusoids in noise. APES spectral estimator has 2-step filtering interpretation: Empirically, the APES method results in wider spectral peaks than

4816-465: The fact that "radar reflections from discrete objects in a passing radar beam field each [have] a minute Doppler, or speed, shift relative to the antenna". Carl Wiley, working at Goodyear, Arizona, (which later became Goodyear Aerospace, and eventually Lockheed Martin Corporation) in 1951, suggested the principle that — because each object in the radar beam has a slightly different speed relative to

4902-455: 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 been used to study rifting processes in Ethiopia. Ground subsidence from

4988-450: 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

5074-411: 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

5160-440: 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

5246-458: The large synthetic antenna aperture (the size of the antenna). Typically, the larger the aperture, the higher the image resolution will be, regardless of whether the aperture is physical (a large antenna) or synthetic (a moving antenna) – this allows SAR to create high-resolution images with comparatively small physical antennas. For a fixed antenna size and orientation, objects which are further away remain illuminated longer – therefore SAR has

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5332-452: The more reliable the target characterization. Multiple captures can be obtained by moving a single antenna to different locations, by placing multiple stationary antennas at different locations, or combinations thereof. The advantage of a single moving antenna is that it can be easily placed in any number of positions to provide any number of monostatic waveforms. For example, an antenna mounted on an airplane takes many captures per second as

5418-417: The motion of the radar antenna over a target region to provide finer spatial resolution than conventional stationary beam-scanning radars. SAR is typically mounted on a moving platform, such as an aircraft or spacecraft, and has its origins in an advanced form of side looking airborne radar (SLAR). The distance the SAR device travels over a target during the period when the target scene is illuminated creates

5504-446: 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

5590-558: The plane travels. The principal advantages of multiple static antennas are that a moving target can be characterized (assuming the capture electronics are fast enough), that no vehicle or motion machinery is necessary, and that antenna positions need not be derived from other, sometimes unreliable, information. (One problem with SAR aboard an airplane is knowing precise antenna positions as the plane travels). For multiple static antennas, all combinations of monostatic and multistatic radar waveform captures are possible. Note, however, that it

5676-436: The property of creating larger synthetic apertures for more distant objects, which results in a consistent spatial resolution over a range of viewing distances. To create a SAR image, successive pulses of radio waves are transmitted to "illuminate" a target scene, and the echo of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single beam-forming antenna, with wavelengths of

5762-416: 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

5848-429: The raw data was recorded on film and the postprocessing by matched filter was implemented optically using lenses of conical, cylindrical and spherical shape. The Range-Doppler algorithm is an example of a more recent approach. Synthetic-aperture radar determines the 3D reflectivity from measured SAR data. It is basically a spectrum estimation, because for a specific cell of an image, the complex-value SAR measurements of

5934-410: The received signals builds a virtual aperture that is much longer than the physical antenna width. That is the source of the term "synthetic aperture," giving it the property of an imaging radar. The range direction is perpendicular to the flight track and perpendicular to the azimuth direction, which is also known as the along-track direction because it is in line with the position of the object within

6020-410: The reflected radiation, either from multiple passes along the same trajectory and/or from multiple displaced phase centers (antennas) on a single pass. 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

6106-433: The relative moving track, the backprojection algorithm works very well. It uses the concept of Azimuth Processing in the time domain. For the satellite-ground geometry, GEO-SAR plays a significant role. The procedure of this concept is elaborated as follows. Capon and APES can yield more accurate spectral estimates with much lower sidelobes and more narrow spectral peaks than the fast Fourier transform (FFT) method, which

6192-418: 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. The most important factor affecting the phase is the interaction with

6278-460: The satellite or aircraft. The technique can potentially measure millimetre-scale changes in deformation over spans of days to years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and 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

6364-580: 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. InSAR can be used to measure tectonic deformation, for example ground movements due to earthquakes . It

6450-456: The scene as a set of point targets that do not interact with each other (the Born approximation ). While the details of various SAR algorithms differ, SAR processing in each case is the application of a matched filter to the raw data, for each pixel in the output image, where the matched filter coefficients are the response from a single isolated point target. In the early days of SAR processing,

6536-404: 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

6622-544: The stability of built structures. 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, dike stability, forensic engineering and many other uses. Interferograms can be used to produce digital elevation maps (DEMs) using the stereoscopic effect caused by slight differences in observation position between

6708-474: 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. 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

6794-408: 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

6880-401: 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

6966-399: 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

7052-401: 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 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

7138-440: 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

7224-438: Was assembled from components developed by a consortium of industry team members. Bussey accepted a position at NASA Headquarters and was replaced by the current principal investigator, Wes Patterson, also of APL. Synthetic aperture radar Synthetic-aperture radar ( SAR ) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses

7310-704: Was first used for the 1992 Landers earthquake , 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 . 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. Early work on volcanic InSAR included studies on Mount Etna , and Kilauea , with many more volcanoes being studied as

7396-518: 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. The Terrafirma project provides

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