Interferometry Synthetic Aperture Research Articles

A Comprehensive Comparison of Far-Field and Near-Field Imaging Radiometry in Synthetic Aperture Interferometry

A Comprehensive Comparison of Far-Field and Near-Field Imaging Radiometry in Synthetic Aperture Interferometry

Transmission mode synthetic aperture interferometry using a compact coherent imager and switched fiber-optic illumination

This paper discusses a modular implementation of synthetic aperture interferometry (SAI) using inexpensive telecommunications technologies that can be expanded for use in optical instruments with arbitrarily large fields of view. Exploiting multiple image sensors and multiple illumination sources, transmission and reflection SAI geometries, capable of emulating the phase contrast and interference modes of conventional interference microscopes, are considered. The point-spread and transfer characteristics of these geometries are illustrated for the case of a transmission mode instrument. It is shown that although imaging is strictly field dependent (i.e. shift-variant), the response of modular instruments is periodic and with careful choice of illumination and sensor positioning can be made quasi shift-invariant. We demonstrate SAI in a transmission configuration comprising of a single coherent imager that holographically records the field scattered from a test target sequentially illuminated by 16 different plane waves. This set-up is equivalent to a single module of a larger SAI system and since brightfield regions in the individual reconstructed images overlap, provides an ideal platform to examine the effect of tilt and phase offset errors in the illumination. By applying a first order correction and assuming phase continuity over the overlapping regions, we demonstrate a significant improvement in the SAI reconstruction.

An Algebraic Comparison of Synthetic Aperture Interferometry and Digital Beam Forming in Imaging Radiometry

Digital beam forming (DBF) and synthetic aperture interferometry (SAI) are signal processing techniques that mix the signals collected by an antenna array to obtain high-resolution images with the aid of a computer. This note aims at comparing these two approaches from an algebraic perspective with the illustrations of simulations conducted at microwaves frequencies within the frame of the Soil Moisture and Ocean Salinity (SMOS) mission. Although the two techniques are using the same signals and sharing the same goal, there are several differences that deserve attention. From the algebraic point of view, it is the case for the singular values distributions of the respective modeling matrices which are both rank-deficient but do not have the same sensitivity to the diversity of the array’s elementary antennas radiation patterns. As a consequence of this difference, the level and the angular signature of the reconstruction floor error are significantly lower with the DBF paradigm than with the SAI one.

Monitoring the two-dimensional deformation of the old landslide in Woda Village with radar interferometry technology

Recently, large-scale sudden landslide disasters have occurred more frequently in the western parts of China. These disasters have characteristics of high position and concealment, and it is challenging to solve disaster prevention using the naked eye and traditional geological means. Using GPS surface monitoring instruments to monitor landslide deformation is currently common, but it is limited because of high costs, harsh natural environment, and human interference. With the rise of interferometric synthetic aperture radar (InSAR), the quality of radar satellite data has improved, providing a new way for long-term monitoring and analysis of landslide historical deformation dynamics. This study used Sentinrl-1A data provided by ESA (March 20, 2017– September 28, 2019) to conduct two-dimensional deformation monitoring of an old landslide in Woda Village, Yanbi Township, Jiangda County, Changdu, Tibet, and compared and analyzed two methods for rechecking with onsite engineering geological survey. The results show that detecting the overall and local deformation of the old landslide using InSAR technology is highly consistent with the field survey phenomenon. The deformation of the front and rear edges of the landslide is small, and the deformation of the middle part is large (>-55mm/a). Combined with rainfall data analysis, the landslide deformation and rainfall have a good response law, showing that rainfall affects the resurrection area in the front of the landslide. The overall performance is a partially resurrected traction sliding, and the main accumulation area in front of the old landslide is shown as a strong push from the middle to the front edge. InSAR technology demonstrates the potential of synthetic aperture interferometry in applying landslide monitoring to deformation monitoring results of the landslide and provides new ideas and references for similar old landslide inducement analysis and monitoring and early warning work.

Combined Geodetic and Seismological Study of the December 2020 Mw = 4.6 Thiva (Central Greece) Shallow Earthquake

On 2 December 2020, a moderate and shallow Mw = 4.6 earthquake occurred in Boeotia (Central Greece) near the city of Thiva. Despite its magnitude, the co-seismic ground deformation field was detectable and measurable by Sentinel-1, ascending and descending, synthetic aperture interferometry radar (InSAR) acquisitions. The closest available GNSS station to the epicenter, located 11 km west, measured no deformation, as expected. We proceeded to the inversion of the deformation source. Moreover, we reassessed seismological data to identify the activated zone, associated with the mainshock and the aftershock sequence. Additionally, we used the rupture plane information from InSAR to better determine the focal mechanism and the centroid location of the mainshock. We observed that the mainshock occurred at a shallower depth and the rupture then expanded downdip, as revealed by the aftershock distribution. Our geodetic inversion modelling indicated the activation of a normal fault with a small left-lateral component, length of 2.0 km, width of 1.7 km, average slip of 0.2 m, a low dip angle of 33°, and a SW dip-direction. The inferred fault top was buried at a depth of ~0.5 km, rooted at a depth of ~1.4 km, with its geodetic centroid buried at 1.0 km. It was aligned with the Kallithea fault. In addition, the dip-up projection of the modeled fault to the surface was located very close (~0.4 km SW) to the mapped (by existing geological observations) trace of the Kallithea fault. The ruptured area was settled in a transition zone. We suggest the installation of at least one GNSS and seismological station near Kallithea; as the activated zone (inferred by the aftershock sequence and InSAR results) could yield events with M ≥ 5.0, according to empirical laws relating to rupture zone dimensions and earthquake magnitude.

Coherent imager module with a large field of view for synthetic aperture interferometry applications.

Optical areal profilometry of large precision-engineered surfaces require high-resolution measurements over large fields of view. Synthetic Aperture Interferometry (SAI) offers an alternative to the conventional approach of stitching small fields of view (FOV) obtained with Coherent Scanning Interferometry (CSI) using high-NA objectives. In SAI, low-resolution digital holograms are recorded for different illumination and observation directions and they are added coherently to produce a high-resolution reconstruction over a large FOV. This paper describes the design, fabrication and characterization of a large FOV, compact and low-cost coherent imager (CI) as a building block of a coherent sensor array for a SAI system. The CI consists of a CMOS photodetector array with 1.12 µm pixel pitch, a square entrance pupil and a highly divergent reference beam that emerges from a pinhole milled with a focused ion beam on the cylindrical cladding at the tip of an optical fibre. In order to accurately reconstruct the digital holograms, the wavefront of the reference beam is estimated by localizing the reference source relative to the photodetector array. This is done using an optimization approach that simultaneously reconstructs plane waves that reach the aperture from 121 different illumination directions and guarantees a phase root-mean-squared (RMS) error of less than a fifth of the wavelength across the CI entrance pupil at a boundary of the FOV. The CI performance is demonstrated with a holographic reconstruction of a 0.110 m wide object placed at a distance of 0.085 m, i.e. a FOV = ±0.57 rad, the highest reported to date with a holographic camera.

An approach for accurately retrieving the vertical deformation component from two-track InSAR measurements

ABSTRACTAs is well known, both conventional differential synthetic aperture radar interferometry (D-InSAR) and multi-temporal synthetic aperture radar interferometry (MT-InSAR) have a common limitation that they only can measure the deformation component along the radar line of sight (LOS) direction. However, in the majority of disaster investigations, there is more interest in the vertical deformation component than that of the horizontal direction, for example, in measuring ground subsidence in urban areas, or ground subsidence due to underground mining. To estimate the vertical deformation component accurately, it is in theory necessary to create at least three independent equations, and solve the vertical, the North–South, and the East–West deformation components by exploiting at least three-track InSAR LOS measurements or combining at least two-track InSAR LOS measurements with azimuth measurements. However, these methods are greatly limited and sometimes not even practical because there is typically little chance of obtaining a three-track SAR data set covering the same area in the same time span, and the accuracy of the azimuth measurements is far lower than that of InSAR LOS measurements. In this article, we found that it is possible to solve for the vertical deformation component from two-track InSAR LOS measurements in some circumstances. Then, an approach for accurately retrieving the vertical deformation component from two-track InSAR LOS measurements is proposed, and the analytical expression is presented. The approach is illustrated through an investigation of the ground subsidence in an area of Beijing, China. Unlike previous methods, this approach can accurately retrieve the vertical deformation component from two-track InSAR LOS measurements, and provide more reliable results for improving the interpretation of ground subsidence phenomena.

Mapping Three-Dimensional Co-Seismic Deformations Fields by Combining Multiple-Aperture Interferometry and Differential Interferometric Synthetic Aperture Radar Techniques

DInSAR technique based displacement measurement is sensitive to only radar line-of-sight, which renders it hard to determine 3D co-seismic deformations. To infer these deformations, a method based on the robust least squares method by combining the multiple-aperture interferometry (MAI) and DInSAR is applied to one ascending and one descending interferometric pair for the 2003 Bam, Iran, earthquake. The results reveal that co-seismic deformations not has the level strike-slip in north-south direction, but also has a large vertical component and a horizontal component in east-west direction. In areas with high interferometry coherence (>0.6), the standard deviation of MAI results is less than 0.075 m, and the 3D deformations are consistent with results of combining DInSAR and Offset Tracking technology in up-down and east-west directions with the maximum difference of 0.031 m. But the differences in south-north direction are relatively large on decimeter level. Therefore, the proposed method can be used to model reliable and accurate 3D surface displacement fields and benefit for analyzing the causes and effects of the earthquake.

DInSAR technique for three-dimensional coastal spit simulation from radarsat-1 fine mode data

This work presents a new approach to 3D spit simulation using differential synthetic aperture interferometry (DInSAR). In doing so, conventional DInSAR procedures are implemented to three repeat passes of RADARSAT-1 SAR fine mode data (F1). Further, a new application of using fuzzy B-spline algorithm is implemented with phase unwrapping technique. The study shows that the performance of DInSAR method using fuzzy B-spline is better than the DInSAR technique, which is validated by the coefficient of determination r 2 of 0.96, probability p of 0.002, and accuracy (RMSE) of ± 0.034 m, with 90% confidence intervals. In conclusion, integration of fuzzy B-spline with phase unwrapping produces an accurate 3D coastal geomorphology reconstruction.

Three-Dimensional Coastal Geomorphology Deformation Modelling Using Differential Synthetic Aperture Interferometry

This work presents a new approach for three-dimensional (3D) coastal deformation simulation using differential synthetic aperture interferometry (DInSAR). In doing so, conventional InSAR procedures are implemented to three repeat passes of RADARSAT-1 SAR fine mode data (F1). Further, the DInSAR method is implemented with the phase unwrapping technique. Consequently, DInSAR is used to eliminate the phase decorrelation impact from the interferograms. The study shows the accurate performance of DInSAR with a root mean square error of 0:02 ±0:21 m and 90% confidence intervals. In conclusion, the DInSAR technique produces an accurate 3D coastal geomorphology reconstruction

Research on Flat Earth Removal Techniques of INSAR

As a new microwave remote sensing technique, synthetic aperture interferometry has been developed rapidly in last thirty years. It is based on synthetic aperture. The imaging algorithm of the synthetic aperture radar interferometry (INSAR) is introduced. Two approaches of flat earth removal are derived and compared. The influence of the flat earth removal processing to the interformetry phase statistics distribution and phase derivative is analyzed. The results indicate that the flat earth removal technology makes the phase unwrapping easier and improves the precision of it.

Synthetic aperture interferometry: error analysis

Synthetic aperture interferometry (SAI) is a novel way of testing aspherics and has a potential for in-process measurement of aspherics [Appl. Opt. 42, 701 (2003)]. A method to measure steep aspherics using the SAI technique has been previously reported [Appl. Opt. 47, 1705 (2008)]. Here we investigate the computation of surface form using the SAI technique in different configurations and discuss the computational errors. A two-pass measurement strategy is proposed to reduce the computational errors, and a detailed investigation is carried out to determine the effect of alignment errors on the measurement process.

Measurement of steep aspheric surfaces using an anamorphic probe

Synthetic aperture interferometry has been previously proposed as a possible in-process method to measure aspheric form (R. Tomlinson, Appl. Opt.42, 701, 2003.). Preliminary demonstration utilized a scanning probe consisting of a pair of bare single mode fibers to perform source and receive functions. It was found that this probe did not have sufficient numerical aperture (NA) to measure steep surfaces and that simply increasing the NA decreases the light gathering efficiency substantially. In this paper, we introduce supplementary optics to increase the NA, and the light gathering efficiency has been increased by adopting an anamorphic design. A spherical test optic of known form is measured to demonstrate the capability of the new probe design.

Crustal deformation associated with the Noto Hanto Earthquake in 2007 in Japan

Abstract The Global Positioning System (GPS) network in Japan detected coseismic deformation from the 2007 Noto Hanto earthquake (M w = 6.9). The result indicates a 21-cm southwestward displacement and a 7-cm upheaval at the GPS site near the epicenter. Synthetic Aperture Interferometry (InSAR) shows an approximately 50-cm movement toward the “Daichi” satellite, in ascending orbit, near the epicenter. The estimated fault slip distribution based on GPS and InSAR results shows a large slip area ranging up to 2 m near and northeast of the hypocenter. The slip area beneath the Sea of Japan southwest of the hypocenter also shows large slippage of around 2 m. Aftershocks are distributed in the asperity area and its vicinity, demonstrating that the asperity hypothesis clearly does not hold completely for this earthquake. Computed vertical displacements using the model are consistent with the geomorphological evidence of long-term uplift.

Synthetic aperture interferometry: in-process measurement of aspheric optics.

A scanning probe consisting of a source and receive fiber pair is used to measure the phase difference between wave fronts scattered from the front and rear surfaces of an aspheric optic. This system can be thought of as a classical interferometer with an aperture synthesized from the data collected along the path of the probe. If the form of either surface is known, the other can be deduced. In contrast with classical interferometers, the method does not need test or null plates and has the potential to be integrated into the manufacturing process.