Radar Line Of Sight Research Articles

Investigating Active Deformation in the Northern Longitudinal Valley and City of Hualien in Eastern Taiwan Using Persistent Scatterer and Small-Baseline SAR Interferometry

Rapid convergence between the Eurasian plate and the Philippine Sea plate in the northern Longitudinal Valley near the city of Hualien in eastern Taiwan has produced prominent fault scarps and large earthquakes (M≥7). This study attempts to more thoroughly understand the surface deformation activities near the faults and their relation to earthquakes near Hualien City using a stable target, multi-temporal radar interferometry method to extract the surface deformation information in this area. Our results indicate that the extracted deformation signals were comparable with those in our leveling data and have a very high spatial density (exceeding 70 point km^(-2)). Additionally, all faults in the northern Longitudinal Valley and Hualien City were not deformed monotonically. Rather, deformation time series are complex and frequent activities affected the general trends wherein, for example, 2005 earthquake swarm produced more than 10 mm of displacement near the Milun fault. Moreover, deformation rates in the study area generally ranged from around 10-20 mm yr^(-1) of relative displacement rates along the radar line of sight between the western part of Hualien City near the Central Range and the eastern part during the decade, while only the Mingyi fault displayed monotonic fault activities with change of displacement rates of approximately 5-10 mm yr^(-1) across the fault.

Scaling the 3-D Image of Spinning Space Debris via Bistatic Inverse Synthetic Aperture Radar

In 3-D inverse synthetic aperture radar (ISAR) imaging of spinning space debris, the image obtained via the available algorithm is modified by a scaling factor. Determined by the angle between the spinning vector and the radar line of sight, this factor cannot be estimated by a monostatic radar in a short imaging interval. This letter derives the bistatic ISAR (Bi-ISAR) geometry and signal model for 3-D imaging of space debris. Then, a 3-D image scaling algorithm is introduced based on the connections between the mono- and bistatic echoes of the same scatterer. Numeric simulations have proved the validity of the proposed algorithm.

Surface deformation associated with the 2008 Ms8.0 Wenchuan earthquake from ALOS L-band SAR interferometry

The Ms8.0 Wenchuan earthquake (in China) occurred on 12 May 2008 as a result of slip on the northeastern-striking Longmen Shan (LMS) faults beneath the rugged margin between the Qinghai-Tibet Plateau and Sichuan Basin. The catastrophic event caused significant surface ruptures and permanent ground displacement in a wide area. This paper concentrates on mapping surface deformation caused by the main shock with the interferometric synthetic aperture radar (InSAR) technology. The coseismic interferogram covering an area of over 83,000 km2 is computed with use of 46 SAR images that were collected along 6 adjacent ascending orbits by the L-band SAR sensor onboard the Japanese Advanced Land Observing Satellite (ALOS). The displacements measured at 16 GPS sites are used to check the accuracy of the InSAR deformation measurements. The radar coherence is computed and analyzed in relation to the topography and the normalized difference vegetation index (NDVI) estimated from the Landsat-7 imagery. The results show that the coseismic surface deformation can be mapped up to a centimeter-accuracy level even over the highly mountainous and heavily vegetated area with the L-band interferometer. It is also demonstrated that the L-band interferograms with time interval of months to years can still maintain acceptable radar coherence for deformation extraction over the area under the extreme conditions. The extracted InSAR deformation measurements show that the lands in the Sichuan Basin had moved 0.1–1.3 m toward the satellite along the radar line of sight (LOS) direction with an azimuth of 349.8° and an elevation angle of 51.3°, while the lands in the LMS area had moved 1.4 m at most away from the satellite.

Water vapour distribution at urban scale using high-resolution numerical weather model and spaceborne SAR interferometric data

Abstract. The local distribution of water vapour in the urban area of Rome has been studied using both a high resolution mesoscale model (MM5) and Earth Remote Sensing-1 (ERS-1) satellite radar data. Interferometric Synthetic Aperture Radar (InSAR) techniques, after the removal of all other geometric effects, estimate excess path length variation between two different SAR acquisitions (Atmospheric Phase Screen: APS). APS are strictly related to the variations of the water vapour content along the radar line of sight. To the aim of assessing the MM5 ability to reproduce the gross features of the Integrated Water Vapour (IWV) spatial distribution, as a first step ECMWF IWV has been used as benchmark against which the high resolution MM5 model and InSAR APS maps have been compared. As a following step, the high resolution IWV MM5 maps have been compared with both InSAR and surface meteorological data. The results show that the high resolution IWV model maps compare well with the InSAR ones. Support to this finding is obtained by semivariogram analysis that clearly shows good agreement beside from a model bias.

The 1998Mw5.7 Zhangbei‐Shangyi (China) earthquake revisited: A buried thrust fault revealed with interferometric synthetic aperture radar

The 1998Mw5.7 Zhangbei‐Shangyi (China) earthquake is the largest to have occurred in northern China since the large 1976Ms7.8 Tangshan earthquake. Due to its proximity to Beijing, the capital of China, it has therefore gained a lot of attention. A great number of studies have been conducted using seismic and geodetic data, but few are able to identify conclusively the orientation of the primary fault plane for this earthquake. In this paper, two independent ERS synthetic aperture radar interferograms are used to determine precisely the location and magnitude of coseismic surface displacements (∼11 cm in the radar line of sight). Modeling the event as dislocation in an elastic half‐space suggests that the earthquake is associated with a buried shallow NNE‐SSW oriented thrust fault with a limited amount of lateral displacement, which is consistent with seismic intensity distribution and aftershock locations.

Supervised Classification and Estimation of Hydrometeors From C-Band Dual-Polarized Radars: A Bayesian Approach

In this paper, a Bayesian statistical approach for supervised classification and estimation of hydrometeors, using a C-band polarimetric radar, is presented and discussed. The Bayesian Radar Algorithm for Hydrometeor Classification at C-band (BRAHCC) is supervised by a backscattering microphysical model, aimed at representing ten different hydrometeor classes in water, ice, and mixed phase. The expected error budget is evaluated by means of contingency tables on the basis of C-band radar noisy and attenuated synthetic data. Its accuracy is better than that obtained from a previously developed fuzzy logic C-band classification algorithm. As a second step of the overall retrieval algorithm, a multivariate regression is adopted to derive water content statistical estimators, exploiting simulated polarimetric radar data for each hydrometeor class. The BRAHCC methodology is then applied to a convective hail event, observed by two C-band dual-polarized radars in a network configuration. The hydrometeor classification along the line of sight, connecting the two C-band radars, is performed using the BRAHCC applied to path-attenuation-corrected data. Qualitative results are consistent with those derived from the fuzzy logic algorithm. Hydrometeor water content temporal evolution is tracked along the radar line of sight. Hail vertical occurrence is derived and compared with an empirical hail detection index applied along the radar connection line during the whole event.

Measurements of the Doppler spectra of breaking waves

A number of experiments have been carried out in two large wave tanks with three different radar systems. The radar frequency, grazing angle, azimuth angle, water wavelength, wave steepness and the breaking wave strength were all varied systematically. The velocity of the peak Doppler power spectral density was found to depend on the phase velocity of the breaking wave in the radar line of sight, but was independent of the radar frequency. The spectral width depended on the phase velocity of the wave, but not on the grazing angle used. The peak Doppler power and radar cross-section of the breaking waves was found to scale with the radar wavelength (proplambda r 1.5 )

Coseismic Deformation of Chi-Chi Earthquake as Detected by Differential Synthetic Aperture Radar Interferometry and GPS Data

A rupture in the Chelungpu fault caused an Mw 7.6 earthquake on 21 September 1999 near Chi-Chi in central Taiwan. This earthquake was the most destructive experienced in Taiwan for the past century along this fault. In this study, we examined the earthquake-induced surface deformation pattern using differential synthetic aperture radar interferometry (D-InSAR) combined with global positioning system (GPS) data regarding the footwall of the Chelungpu fault. Six synthetic aperture radar (SAR) scenes, approximately 100×100 km each, recorded by the European Remote Sensing Satellite 2 (ERS-2), spanning the rupture area, were selected for study. The data were used to generate a high-resolution, wide-area map of displacements in flat or semi-flat areas. Interferograms show radar line contours indicating line-of-sight (LOS) changes corresponding to surface displacements caused by earthquake ruptures. These results were compared to synthetic interferograms generated from GPS data. Displacements shown by GPS data were interpolated onto wide-area maps and transformed to coincide with the radar LOS direction. The resulting coseismic displacement contour map showed a lobed pattern consistent with the precise GPS based displacement field. Highly accurate vertical displacement was determined using D-InSAR data using the coordinate transform method, while GPS data was effective in showing the horizontal component. Thus, this study confirmed the effectiveness of the D-InSAR method for determining the coseismic deformation caused by the Chi-Chi earthquake at the footwall of the Chelungpu fault.

Unexpected spectral characteristics of VHF radar signals from 150-km region over Jicamarca

The physical mechanisms that cause daytime 150‐km echoes at equatorial latitudes are still puzzling. However, there is a general consensus that the radar signals coming from this altitude region should present a narrow spectra (<20 m s−1). Moreover 150‐km echoes are believed to come from field‐aligned irregularities, and to be very aspect sensitive, i.e., they come from regions where the radar line of sight is perpendicular to the magnetic field (B). In this letter, we present evidence of a surprising result: radar signals around 150 km altitudes are also obtained when the radar points few degrees off‐perpendicular to B, and more importantly, their spectra are very wide (>1000 m s−1). On the other hand, the time and altitudinal characteristics of these off‐perpendicular observations are the same as those presented by the so‐called 150‐km echoes. Moreover, the off‐perpendicular echo strengths are several orders of magnitude weaker than perpendicular echoes. Our preliminary interpretation of these results suggests that the spectra of these signals are controlled by ion (electron) dynamics when they are observed off‐perpendicular (perpendicular) to B.

Application of SAR interferometry to a large thrust deformation: the 1999Mw= 7.6 Chichi earthquake in central Taiwan

SUMMARY Application of the interferometric method to four ERS2-SAR images acquired before and after the 1999 Chichi earthquake has allowed determination of the coseismic surface displacement in the footwall area of the Chelungpu fault. The interferometric results revealed a relative shortening in the round trip distance between the radar antenna and the ground of the footwall side of Chelungpu fault, during the earthquake. This shortening progressively increased from the west to the east and reaches the maximum amount of approximately 26 cm near the central segment of the Chelungpu fault. Our interferometric results have been precisely examined using a dense GPS network in the investigated area. We mapped the GPS coseismic measurements into the radar line of sight and implemented a forward simulation of SAR interferogram from this synthesized result to control our unwrapping performance. In this study, these two observations are compared with a 3-D dislocation model of the fault. Finally, a deformation analysis based on our interferometric result has indicated that a segment with irregular deformation behaviour can be distinguished in the footwall area of the Chelungpu fault. This segment may result from either the influence of inherited basement faults or the presence of a structural terrace that provide local opportunities for superficial deformation.

Localized deformation at Miyakejima volcano based on JERS‐1 radar interferometry: 1992–1998

Stacked radar interferograms at Miyakejima volcano (Japan) between 1992 and 1998 showed two localized significantly deforming areas with a magnitude of 4∼6 mm/yr in the radar line of sight. One area is close to the 1983 eruption vent, and, using a simple closed analytical formulation, the deformation is interpreted as due to a thermoelastic contraction of a formerly intruded magma. The other one detected in the previous caldera is explained by a depressurization source, whose depth (500 m) coincides with that of a low resistivity zone interpreted as a hydrothermal reservoir. Also, our computed CO2 flux is consistent with in situ measurement data. The reservoir volume is significantly less than the collapsed volume in 2000, and its depth is much shallower than the void detected prior to the collapse. Hence, the depressurization would not directly induce the caldera collapse in 2000.

VHF coherent radar signals from the E region ionosphere and the relationship to electron drift velocity and ion acoustic velocity

The Scandinavian Twin Auroral Radar Experiment (STARE) coherent radar system measures the Doppler shifts caused by ∼1 m plasma waves in the high‐latitude E region ionosphere. These Doppler velocities are here related to the electron drift velocity and ion acoustic velocity derived from measurements with the incoherent radar system European Incoherent Scatter (EISCAT). The Doppler velocity is limited in magnitude to near the ion acoustic velocity in the plasma. For large flow angles θ, i.e., the angle between the radar line of sight and the electron drift velocity, the Doppler shifts are equal to the component of the electron drift velocity on the line of sight. For θ ∼ 40° the Doppler velocity is equal to the ion acoustic velocity at 105‐km altitude, and for decreasing flow angle the Doppler velocity increases. For 0° < θ < 60° the variation with flow angle can be described as cosαθ, where the α decreases from 0.8 to 0.2 with an increase in drift speed from ∼400 to 1600 ms−1. The ratio of the line‐of‐sight velocity for θ ∼ 0° to the ion acoustic velocity decreases from 1.2 at low velocities to 1.05 at large velocities. The systematic variations of the Doppler shifts with drift speed and flow angle make it possible, in principle, to recover the electron velocity from the coherent radar measurements. The observations are used to illustrate how well the recovery is possible in practice.

The Mw 7.1, Hector Mine, California earthquake: surface rupture, surface displacement field, and fault slip solution from ERS SAR data

Abstract The Mw7.1, Hector Mine, California earthquake of October 16, 1999 produced a 50 km-long surface rupture, ∼20 km east of the surface rupture of the 1992 Landers earthquake. ERS2 radar data acquired before and after the event provide geodetic measurements of two components of the co-seismic surface displacement field. (1) Interferometric processing of the data provides the range (radar line of sight) component of the displacement field with a precision of a few millimeters. (2) The sub-pixel correlation of the before and after amplitude images provides a second component of the surface displacement field, parallel to the satellite track, with a precision of ∼20 cm. The azimuth offset map reveals a ∼50 km long, NNW surface rupture formed of three sections: a N40°W southern section following the Bullion fault, a N10°W central section cutting through the Bullion Mountains, and a northern N40°W section extending up to the Lavic Lake. A north–south-trending, localized gradient in the azimuth offset field suggests that the central section of the fault extends to the north at depth over a distance of ∼10 km. The slip distribution curve constructed using the azimuth offsets has a bell shape, with a maximum of 6.5 m at the north end of the central fault section. The fault slip model derived from the inversion of the azimuth and from range displacement data indicates that 3–5.8 m of right-lateral slip occurred between the depths of 0 and 8 km along the northern and the central sections of the fault. The solution bears a down-dip component of slip of ∼2 m, west side down, along the southern part of the northern fault section. The down-dip component of slip is consistent with compression on a northeast dipping fault.

Uplift of Tainan Tableland (SW Taiwan) revealed by SAR Interferometry

Interferometric processing of five SAR‐ERS images reveals uplift of the Tainan Tableland (SW of Taiwan) during the period 1996–1998. The maximum measured ground motion for these two years is 2.8 cm along the radar line of sight towards the satellite, indicating for the displacement vector a vertical component of 3.2 cm, and a horizontal component of 1.6 cm towards the WSW considering additional information from GPS data. The reconstructed displacement field is consistent with the geological interpretation of the Tainan Tableland as an actively growing anticline connected to the Taiwan fold‐and‐thrust belt. This implies that the deformation front is located farther west than usually assumed in the Tainan area. The large Tainan city is thus located in an active deformation zone. Seismic hazard assessment is however difficult because the mechanisms and kinematics are not known in detail.

Three-dimensional ISAR imaging of maneuvering targets using three receivers

The conventional ISAR image is a two-dimensional (2-D) projection of a three-dimensional (3-D) object surface. The image (projection) plane is related to the motion of a target with respect to the line of radar sight (LOS). In general, the image plane and the image scale in the cross range direction can not be determined by the traditional ISAR system with one receiver unless the target motion knowledge is known. In this paper, we propose a new ISAR system with three receivers. Using the three-receiver ISAR system, 3-D images of maneuvering targets can be generated, where the knowledge of the target motion is not required.

Surface displacement of the March 26, 1997 Kagoshima‐Ken‐Hokuseibu Earthquake in Japan from synthetic aperture radar interferometry

A JERS 1 differential L‐band synthetic aperture radar (SAR) interferogram of the March 26,1997 Kagoshima‐ken‐hokuseibu earthquake (Mw=6.1) in southwestern Japan shows about 9 cm peak‐to‐peak coseismic surface displacement in the radar line‐of‐sight (LOS) direction. A permanent GPS array detected 1 to 2 cm horizontal displacements from this earthquake. By inverting the SAR and GPS data together, we estimated a fault mechanism without any seismological data. A theoretical radar LOS displacement pattern from a single fault model of the earthquake motion matches the SAR and GPS observations closely. The model assumes left lateral slip of 0.46 cm with rake angle of 19° on a rectangular fault plane of dimensions 11 km (width) by 12 km (length). We demonstrate that L‐band SAR interferometry can describe several cm surface displacement in detail and construct a fault model. However, despite the acquisitions being during the cold season, there are apparent water vapor signatures in the interferogram with equivalent path delays of up to 1.5 cm.

Constraints on Spin State and Hapke Parameters of Asteroid 4769 Castalia Using Lightcurves and a Radar-Derived Shape Model

A 167-parameter, 3-D shape model of the Earth-crossing asteroid Castalia, obtained from inversion of delay-Doppler images (Hudson and Ostro, 1994,Science263,940–943) constrained the object's pole to lie on a cone of half angle 55 ± 10° centered on the radar line of sight (right ascension 0.3 hr, declination 25.4°) at the time of observations (Aug. 22, 1989) but could not constrain the pole's azimuthal orientation or the sense of rotation. Here we fit lightcurves obtained at Table Mountain Observatory on Aug. 23–25 with lightcurves calculated from Castalia's shape and Hapke's photometric model (Hapke 1981,J. Geophys. Res.86,3039–3054; 1984,Icarus59,41–59; 1986,Icarus67,264–280). The fits strongly constrain Castalia's spin state to one of two possibilities. One has a north pole within ∼13° of λ = 253°, β = 56° and a sidereal spin periodP= 4.089 ± 0.001 hr. The other has a south pole within ∼13° of λ = 242°, β = 7° and a sidereal spin periodP= 4.094 ± 0.001 hr. The north-pole solution has global-average Hapke photometric parametersw= 0.38 ± 0.07,g= −0.11 ± 0.09, θ = 46° ± 10° while the south-pole solution hasw= 0.24 ± 0.07,g= −0.30 ± 0.09, θ = 25° ± 10°. Due to the large solar phase angles (60°–90°) the fits are not sensitive to the opposition surge parametershandB0. At a more subtle level, the least-bifurcated shape (“lower-bound model”) allowed by the radar data (Hudson and Ostro, 1994,Science263,940–943) consistently gives statistically better fits than the more bifurcated possibilities. However, the differences are small enough that the possibility that they are due to unmodeled photometric effects cannot be discounted. The fits are improved by allowing the macroscopic roughness parameter θ to be a function of surface position. Statistically, this process improves the north-pole solution more than the south-pole solution suggesting that the radar observations viewed a northern latitude, thereby indicating a resolution of the “mirror ambiguity” inherent in the single-date radar data set.

Surface deformation and coherence measurements of Kilauea Volcano, Hawaii, from SIR‐C radar interferometry

The shuttle imaging radar C/X synthetic aperture radar (SIR‐C/X‐SAR) radar on board the space shuttle Endeavor imaged Kilauea Volcano, Hawaii, in April and October 1994 for the purpose of measuring active surface deformation by the methods of repeat‐pass differential radar interferometry. Observations at 24 cm (L band) and 5.6 cm (C band) wavelengths were reduced to interferograms showing apparent surface deformation over the 6‐month interval and over a succession of 1‐day intervals in October. A statistically significant local phase signature in the 6‐month interferogram is coincident with the Pu'u O'o lava vent. Interpreted as deformation, the signal implies centimeter‐scale deflation in an area several kilometers wide surrounding the vent. Peak deflation is roughly 14 cm if the deformation is purely vertical, centered southward of the Pu'u O'o caldera. Delays in the radar signal phase induced by atmospheric refractivity anomalies introduce spurious apparent deformation signatures, at the level of 12 cm peak‐to‐peak in the radar line‐of‐sight direction. Though the phase observations are suggestive of the wide‐area deformation measured by Global Positioning System (GPS) methods, the atmospheric effects are large enough to limit the interpretation of the result. It is difficult to characterize centimeter‐scale deformations spatially distributed over tens of kilometers using differential interferometry without supporting simultaneous, spatially distributed measurements of refractivity along the radar line of sight. Studies of the interferometric correlation of images acquired at different times show that L band is far superior to C band in the vegetated areas, even when the observations are separated by only 1 day. These results imply longer wavelength instruments are more appropriate for studying surfaces by repeat‐pass observations.

Kalman filters for a missile with radar and/or imaging sensor

The scenario covers a missile in a head-on attack against a combat aircraft, performing evasive maneuvers in the vertical plane. The improvement of the estimation of guidance signals is investigated when the radar as prime source for target information is supported and finally replaced by an imaging sensor. Kalman filters estimate the guidance signals for an augmented proportional navigation, the line-of-sight (LOS) rate and the target's normal acceleration. Filtering of these signals from the radar measurement is heavily affected by glint. An imaging sensor in the frequency band of visible light works on a much lower noise level. Flight under pure radar control is treated as a basis for comparison. In the case of uncoupled filters an imaging sensor measures the target's projection, which allows the estimation of projection rate. Via kinematic and flight mechanical relations the target acceleration can be determined. A second filter estimates the LOS rate from the radar measurement. Coupling of the filters by feeding the estimated target acceleration of the optical system into the radar's LOS rate filter improves its signal. A significant increase of performance is achieved when the imaging sensor replaces the radar also as source for LOS information.

Magnetic aspect angle effects in radar aurora at 48.5 MHz, corrected for refraction

It is now widely recognized that at lower VHF frequencies, refraction by electron density structures in the auroral E region can greatly affect the character of radio auroral backscatter, making the quantitative study of magnetic aspect angle effects difficult or impossible. In the present study of data taken with the 48.5‐MHz Bistatic Auroral Radar System (BARS) in central Canada, these difficulties have been minimized by using only data obtained from spatially and temporally uniform events. After correcting for effects of refraction, it has been found that true large aspect angle backscatter occurs for magnetic aspect angles exceeding 6°. Doppler velocities from the smaller aspect angle Red Lake radar behaved as if proportional to the line‐of‐sight component of the convection drift velocity; the resulting flow directions were found to be in good agreement with estimates obtained from magnetometers located in the BARS field of view. The constant of proportionality between the radar velocity and the drift velocity component appears to decrease by a factor of approximately 2 as the aspect angle increases from 3° to 6°. At 3° aspect angle, the backscatter power decreased with increasing aspect angle by about 13 dB/deg (the aspect sensitivity), similar to what has been found in other studies at UHF. In common with those other studies this aspect sensitivity is found to decrease with increasing aspect angle, but more rapidly, falling to less than 4 dB/deg at aspect angles of 5°. In contrast to previous studies at small aspect angle, there is no evidence of a dependence of backscattered power on the flow angle, the angle between the radar line of sight and the direction of E × B drift.