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The eruption of the Hunga Tonga Hunga Ha'apai volcano on 15 January 2022 significantly impacted the lower and upper atmosphere globally. Using multi-instrument observations, we described disturbances from the sea surface to the ionosphere associated with atmospheric waves generated by the volcanic eruption. Perturbations were detected in atmospheric pressure, horizontal magnetic field, equatorial electrojet (EEJ), ionospheric plasma drifts, total electron content (TEC), mesospheric and lower thermospheric (MLT) neutral winds, and ionospheric virtual height measured at low magnetic latitudes in the western South American sector (mainly in Peru). The eastward Lamb wave propagation was observed at the Jicamarca Radio Observatory on the day of the eruption at 13:50 UT and on its way back from the antipodal point (westward) on the next day at 07:05 UT. Perturbations in the horizontal component of the magnetic field (indicative of EEJ variations) were detected between 12:00 and 22:00 UT. During the same period, GNSS-TEC measurements of traveling ionospheric disturbances (TIDs) coincided approximately with the arrival time of Lamb and tsunami waves. On the other hand, a large westward variation of MLT winds occurred near 18:00 UT over Peru. However, MLT perturbations due to possible westward waves from the antipode have not been identified. In addition, daytime vertical plasma drifts showed an unusual downward behavior between 12:00 and 16:00 UT, followed by an upward enhancement between 16:00 and 19:00 UT. Untypical daytime eastward zonal plasma drifts were observed when westward drifts were expected. Variations in the EEJ are highly correlated with perturbations in the vertical plasma drift exhibiting a counter-equatorial electrojet (CEEJ) between 12:00 and 16:00 UT. These observations of plasma drifts and EEJ are, so far, the only ground-based radar measurements of these parameters in the western South American region after the eruption. We attributed the ion drift and EEJ perturbations to large-scale thermospheric wind variations produced by the eruption, which altered the dynamo electric field in the Hall and Pedersen regions. These types of multiple and simultaneous observations can contribute to advancing our understanding of the ionospheric processes associated with natural hazard events and the interaction with lower atmospheric layers. [ABSTRACT FROM AUTHOR]

At present, the measurement of the near wave field of ships mostly relies on shipborne radar. The commonly used shipborne radar is incoherent and cannot obtain information on wave surface velocity. Therefore, the mathematical model of wave reconstruction is remarkably complex. As a new type of radar, coherent radar can obtain the radial velocity of the wave surface. Most wave surface reconstruction methods that use wave velocity are currently based on velocity potential. The difficulty of these methods lies in determining the initial value of the velocity integral. This paper proposes a wave surface reconstruction method based on an artificial boundary matrix. Numerical simulation data of regular and short-crest waves are used to verify the accuracy of this method. Simultaneously, the reconstruction stability under different wave velocity measurement errors is analyzed. The calculation results show that the proposed method can effectively realize the reconstruction of wave field. [ABSTRACT FROM AUTHOR]

We consider a dual function radar communications network and propose a waveform diversity-based approach to embed both target and scheduling data in the transmitter pulses. The scheduling data of radar signal bandwidth, carrier frequency, and waveform along with the information on target detection, range, and Doppler are shared among the network radar nodes without a need for designated communication links. Scheduling and target information are encoded to bits and transmitted over the radar coherent processing interval using up- and down-chirps. We derive exact expressions of probability of bit error and false alarm rate for sharing scheduling and target information data between two radar nodes. The effect of different parameters and data sharing on the radar performance is analyzed and validated by computer simulations. Using the cross-ambiguity function, we show the capability of the up- and down- chirp sequence transmission scheme in reducing range sidelobes compared to the conventional transmission of only the up- (or down-) chirps. To overcome the limited data rate associated with only using two chirp waveforms, we consider orthogonal chirp division multiplexing and derive its CAF. We demonstrate the suitability of OCDM for high data rate radar operations by examining the mainlobe width and sidelobe levels of the zero-Doppler and zero-delay cuts of the CAF. [ABSTRACT FROM AUTHOR]

This is the fourth paper in a series of articles devoted to modern methods of protection of coherent-pulse radars against the combined interference (i.e., an additive mixture of internal noise, masking noise jamming and clutter). It proposes the structures of an adaptive system of cooperative space-time signal processing (STSP) against the background of masking combined interference based on a two-dimensional adaptive lattice filter with matrix elementary lattice filters. These structures eliminate significant energy losses of adaptive systems of separate STSP caused by the non-classified nature of training samples of noise jamming and clutter in estimating the weight vectors of spatial and inter-period signal processing and the need to store the spatial weight vector during the time of inter-period compensation of clutter. The results of mathematical simulation of the developed parallel-sequential adaptive system of cooperative STSP with reduced number of complex multiplication operations as compared with direct inversion of interference correlation matrix are presented that confirm its high efficiency. [ABSTRACT FROM AUTHOR]

We propose and study a method of defense against intelligent simulating signal-like active interference on the basis of an assessment of the coherent properties of radar signals. The signalcoherence measure, which is defined as the entropy of the signal-energy distribution over the eigensubspaces of the signal correlation matrix, is proposed as a selection criterion. The main factors, which influence the coherence degree of radar signals, are analyzed. The computation cost during the implementation of the proposed method for estimating the quantitative measure of the signal coherence is estimated. The results of the full-scale experimental studies, which employ various types of aircraft (with a propeller engine and turbojets) and the special equipment, i.e., a repeater of simulating signal-like interference, are presented. The estimated degree of coherence of the signals from real airborne objects and the simulating signal-like interference, which has been obtained during the experimental studies, confirms the possibility of selecting the simulating signal-like interference by the proposed method. [ABSTRACT FROM AUTHOR]

With increased degrees of freedom of the transmitter, coherent waveform-agile radar system can change its transmission on-the-fly in response to target detection's requirement. This approach can provide better performance than a single waveform. In this paper, we consider unimodular sequence set design (USSD) problem based on summed range-Doppler ambiguity function (SRDAF) for coherent waveform-agile radar system. We present two algorithms for constructing unimodular sequence sets under the constant module constraint with desired minimized sidelobes on a predefined area of range-Doppler plane. The proposed algorithms are constructed based on singular value decomposition (SVD) and cyclic algorithm (CA), respectively. Numerical examples show the effectiveness of the proposed algorithms. [ABSTRACT FROM AUTHOR]

Optimal sounding sequences described by a pair of mutually symmetric Golomb rulers lasting from 12 to 18 pulses were synthesized within the generalization of the previously developed 13-pulse sounding sequence of SuperDARN radars. These sequences are pairs of identical sequences that are mutually symmetric in time, are separated by a lag, and do not contain an additional pulse used earlier to measure the power profile of the scattered signal. The optimal subsequences are sought by exhaustive search over the class of optimal and nearly optimal Golomb rulers. The optimality criterion is provision of the maximum information content of the received signal in the sense of a high spectral resolution with minimum accumulation time, as well as the requirement of its efficiency for measuring the power profile of the scattered signal. The work demonstrated the potential efficiency of one of the constructed signals (16-element one) for spectral measurements using the Ekaterinburg coherent decameter radar. [ABSTRACT FROM AUTHOR]

The challenge of residual stress as a result of wear deformation and thermal instability is the motivation behind this study. Nanosized ZrO2/ZrN particles were dispersed into zinc-rich electrolyte using dual anode electrolytic co-deposition (DAECD) route at constant time of deposition and applied current density. The hardness and wear behavior of the coatings were investigated using diamond base microhardness tester and MTR-300 abrasive test rig respectively. The structural properties of the coatings were systematically characterized by X-ray diffractometer (XRD), scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDS), atomic force microscopy, and metallurgical optical microscopy. The stability of the coatings was examined by isothermal furnace at 400 °C. From the results, it is observed that a good adhere coating with significant improvement in mechanical performance was attained. The hardness characteristic value increases from 34 HVN to 172.4 HVN for Zn-ZrO2-ZrN alloy. The plastic deformation was reduced from 0.2 g/min of the as-received sample to 0.0059 g/min for Zn-ZrO2-ZrN matrix. Hence, this establishes that the stability in the coated sample against the working substrate is attributed to the formation of coherent, uniform structural properties, and good strengthening effect from the multifaceted composite materials. [ABSTRACT FROM AUTHOR]

In this paper, we propose a new synthetic aperture radar (SAR) image detection algorithm based on the de-noising algorithm via the sparse representation and a new morphology edge detector. Firstly, we apply the Shearlet transform to the SAR image to get the sparse representation of it. Then, morphological edge detector with direction is applied to directional sub-band coefficients of the Shearlet which are recovered by the iterative de-noising process. Finally, the completed SAR image edge is obtained by merging each sub-band edge using Dempster-Shafer evidence theory. By completely using the directional sub-bands of the Shearlet transform, the proposed algorithm overcomes the disadvantages of transform detection algorithms which are very unrobust to noise and can also generate inaccurate edges. The experimental results demonstrate the effectiveness and superiority of our proposed algorithm in terms of the edge positioning accuracy, integrity, and the number of false edge points. [ABSTRACT FROM AUTHOR]

Micro-Doppler, induced by micro-motion of targets, is an important characteristic for target recognition once extracted via parameter estimation. However, micro-Doppler is usually too significant to result in aliasing in the terahertz band. According to this problem, a Doppler aliasing free micro-motion parameter estimation method based on the modulo Generalized Hough transform is proposed in this paper. Its basic idea is to search and match the parameters of aliasing micro-Doppler curves in the time-frequency distribution image. The high estimation precision and excellent noise immunity of this method are verified through simulations based on CST data and experiments based on a 0.33 THz radar system. [ABSTRACT FROM AUTHOR]

Abstract: We propose a method for measuring the harmonic vibration amplitude of a reflector using a coherent radar station. A mathematical model describing the dependence of the Doppler spectrum of the reflected radio signal on the ratio of the vibration amplitude to the radiation wavelength is constructed. The vibration amplitude is calculated from the amplitude ratio of the third to first or fourth to second harmonics. [ABSTRACT FROM AUTHOR]

Target detection for SAR images has many important applications; however there is a challenge that inherent speckle noise in SAR images may cause serious interference. Beamlet transform is a multi-scale image analysis method to extract line features in an image with strong anti-noise capacity. In this paper a method based on Beamlet transform is proposed for target detection for SAR images. It takes the advantage of Beamlet transform in feature extraction. Firstly Beamlet transform is applied on a SAR image to obtain Beamlet coefficients,which are then processed by a coefficient filtering algorithm to remove unreal Beamlet features caused by noise. The remained Beamlet features are fed to the BD-RDP (Beamlet-decorated recursive dyadic partition) algorithm for optimization and then clustered by NEC (Nearest Endpoint Clustering) algorithm to detect targets. The experimental results show that this method is able to detect target directly in a SAR image without pre-filtering. Further more, it still works well under the background of strong speckle noise. [ABSTRACT FROM AUTHOR]

The next generation of radar (radio detection and ranging) systems needs to be based on software-defined radio to adapt to variable environments, with higher carrier frequencies for smaller antennas and broadened bandwidth for increased resolution. Today's digital microwave components (synthesizers and analogue-to-digital converters) suffer from limited bandwidth with high noise at increasing frequencies, so that fully digital radar systems can work up to only a few gigahertz, and noisy analogue up- and downconversions are necessary for higher frequencies. In contrast, photonics provide high precision and ultrawide bandwidth, allowing both the flexible generation of extremely stable radio-frequency signals with arbitrary waveforms up to millimetre waves, and the detection of such signals and their precise direct digitization without downconversion. Until now, the photonics-based generation and detection of radio-frequency signals have been studied separately and have not been tested in a radar system. Here we present the development and the field trial results of a fully photonics-based coherent radar demonstrator carried out within the project PHODIR. The proposed architecture exploits a single pulsed laser for generating tunable radar signals and receiving their echoes, avoiding radio-frequency up- and downconversion and guaranteeing both the software-defined approach and high resolution. Its performance exceeds state-of-the-art electronics at carrier frequencies above two gigahertz, and the detection of non-cooperating aeroplanes confirms the effectiveness and expected precision of the system. [ABSTRACT FROM AUTHOR]

Present-day ideas of the formation of inhomogeneities in midlatitude lower-ionosphere plasma are considered. Experimental and theoretical results are described that testify to the existence of anisotropic structures developing at lower-ionosphere altitudes and bring about strong electric fields, narrow echo peaks, and quasi-periodic echoes (detected by coherent radar). The origin of inhomogeneities and structures observed in plasma and neutral components of the lower ionosphere is analyzed. [ABSTRACT FROM AUTHOR]

The transfer of high-quality time-frequency signals between remote locations underpins many applications, including precision navigation and timing, clock-based geodesy, long-baseline interferometry, coherent radar arrays, tests of general relativity and fundamental constants, and future redefinition of the second. However, present microwave-based time-frequency transfer is inadequate for state-of-the-art optical clocks and oscillators that have femtosecond-level timing jitter and accuracies below 1 × 10−17. Commensurate optically based transfer methods are therefore needed. Here we demonstrate optical time-frequency transfer over free space via two-way exchange between coherent frequency combs, each phase-locked to the local optical oscillator. We achieve 1 fs timing deviation, residual instability below 1 × 10−18 at 1,000 s and systematic offsets below 4 × 10−19, despite frequent signal fading due to atmospheric turbulence or obstructions across the 2 km link. This free-space transfer can enable terrestrial links to support clock-based geodesy. Combined with satellite-based optical communications, it provides a path towards global-scale geodesy, high-accuracy time-frequency distribution and satellite-based relativity experiments. [ABSTRACT FROM AUTHOR]

Interferometric synthetic aperture radar phase data include not only signals due to crustal movements, but also those associated with microwave propagation delay through the atmosphere. In particular, the effect of water vapor can generate apparent signals in the order of a few centimeters or more, and prevent us from detecting such geophysical signals as those due to secular crustal deformation. To examine if and to what extent numerical weather model (NWM) outputs are helpful to reduce the tropospheric delay signals at spatial scales of 5-50 km wavelengths, we compared three approaches of tropospheric signal reduction, using 54 interferograms in central Hokkaido, Japan. The first approach is the conventional topography-correlated delay correction that is based on the regional digital elevation model (DEM). The second approach is based on the Japan Meteorological Agency's operational meso-scale analysis model (MSM) data, where we compute tropospheric delays and subtract them from the interferogram. However, the MSM data are available at predefined epochs and their spatial resolution is about 10 km; therefore, we need to interpolate both temporally and spatially to match with interferograms. Expecting to obtain a more physically plausible reduction of the tropospheric effects, we ran a 1-km mesh high-resolution numerical weather model WRF (Weather Research and Forecasting model) by ourselves, using the MSM data as the initial and boundary conditions. The third approach is similar to the second approach, except that we make use of the WRF-based tropospheric data. Results show that if the topography-correlated phases are significant, both the conventional DEM-based approach and the MSM-based approach reveal comparable performances. However, when the topography-correlated phases are insignificant, none of the approaches can efficiently reduce the tropospheric phases. Although it could reduce the tropospheric signals in a local area, in none of the case studies did the WRF model produce the 'best' performance. Whereas the global atmospheric model outputs are shown to be effective in reducing long-wavelength tropospheric signals, we consider that further improvements are needed for the initial and boundary condition data for high-resolution NWM, so that the NWM-based approach will become more reliable even in the case of a non-stratified troposphere. [ABSTRACT FROM AUTHOR]

We analyze the operation of an oceanographic cross-track interferometric synthetic aperture radar and present estimates of the signal-to-noise ratio in the obtained interferograms. It is shown that after filtering additive phase noise, the interferograms allow one to restore the profile of a sufficiently large-scale part of water-surface roughness. The criterion of large-scale roughness is determined mainly by the ratio R/V of the oblique distance R to the velocity V of a carrier of the interferometric synthetic aperture radar, as well as by the character and propagation direction of the surface roughness. [ABSTRACT FROM AUTHOR]

A study of an interesting meteorological episode over the Owens Valley, California, USA during the Terrain-Induced Rotor EXperiment was conducted using a recently adapted statistical interpolation method to retrieve wind-velocity vectors from Doppler lidar data. This vector retrieval method has been adapted from radar data assimilation techniques. Results show that the method allows better preservation of local variations in the flow field than other techniques. In addition, a high resolution Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) run is used to understand the large-scale flow within the valley and compared with lidar retrievals. Observations from 1030 UTC to 1230 UTC (0230 local time to 0430 local time) on March 27, 2006 are presented. Lidar observations show complex and uncharacteristic flows such as sudden bursts of westerly cross-valley wind mixing with the dominant up-valley wind. Model results from COAMPS and other in-situ instrumentation are used to corroborate and complement these observations. The optimal interpolation technique for Doppler lidar data vector retrieval appears well suited for scenarios with complex spatial variations in the flow field. [ABSTRACT FROM AUTHOR]

Features of interaction of microwave electromagnetic radiation with one-dimensional photonic crystals made on the basis of microstrip transmission lines with a variable substrate permittivity and variable width of the upper strip conductor are investigated. A method for determining the parameters of solid and liquid dielectrics from the measured frequency dependences of the coefficient of transmission of electromagnetic radiation interacting with the microstrip photonic structure, in which the samples under investigation act as irregularities, is theoretically substantiated and experimentally implemented. The results of measuring the complex permittivity of a number of liquid dielectrics are presented. [ABSTRACT FROM AUTHOR]

The extraction of groundwater can generate land subsidence by causing the compaction of susceptible aquifer systems, typically unconsolidated alluvial or basin-fill aquifer systems comprising aquifers and aquitards. Various ground-based and remotely sensed methods are used to measure and map subsidence. Many areas of subsidence caused by groundwater pumping have been identified and monitored, and corrective measures to slow or halt subsidence have been devised. Two principal means are used to mitigate subsidence caused by groundwater withdrawal-reduction of groundwater withdrawal, and artificial recharge. Analysis and simulation of aquifer-system compaction follow from the basic relations between head, stress, compressibility, and groundwater flow and are addressed primarily using two approaches-one based on conventional groundwater flow theory and one based on linear poroelasticity theory. Research and development to improve the assessment and analysis of aquifer-system compaction, the accompanying subsidence and potential ground ruptures are needed in the topic areas of the hydromechanical behavior of aquitards, the role of horizontal deformation, the application of differential synthetic aperture radar interferometry, and the regional-scale simulation of coupled groundwater flow and aquifer-system deformation to support resource management and hazard mitigation measures. [ABSTRACT FROM AUTHOR]

Synthetic aperture radar (SAR) interferometry (InSAR) is a geodetic tool widely applied in the studies of earth-surface deformation. This technique has the benefits of high spatial resolution and centimetre-scale accuracy. Differential SAR interferometry (DInSAR) is used to measure ground deformation with repeat-pass SAR images. This study applied DInSAR and persistent scatterers InSAR (PSInSAR) for detecting land subsidence in the Pingtung Plain, southern Taiwan, between 1995 and 2000. In recent years, serious land subsidence occurred along coastal regions of Taiwan as a consequence of over-pumping of underground water. Results of this study revealed that the critical subsidence region is located on the coast near the estuary of Linpien River. It is also found that subsidence was significantly higher during the dry season than the wet season. The maximum annual subsidence rate of the dry season is up to −11.51 cm/year in critical subsidence region and the vertical land movement rate is much slower during the wet season. The average subsidence rates in wet and dry seasons are −0.31 and −3.37 cm/year, respectively. As a result, the subsidence rate in dry seasons is about 3 cm larger than in wet seasons. [ABSTRACT FROM AUTHOR]

Despite the intense attention paid to internal wave (IW) investigation, the most experimentally studied and theoretically described are internal gravitational waves in shelf zones of oceans and tidal seas appearing during the interaction of tidal currents with the margin of the shelf. Information on surface manifestations of internal waves in enclosed seas, such as the Black and Caspian seas, is almost absent. In this paper, the results of study of the peculiarities of generation and propagation of nontidal internal waves are presented; the study is carried out on the basis of combined analysis in data of marine surface radiolocation and data of optical and infrared satellite-borne sensors. The experimental basis of investigation is radar images of the marine surface derived with the help of Synthetic Aperture Radars, onboard the Envisat and ERS-2 satellites. Additionally, the data of the following sensors belonging to the optical and infrared ranges were used for the purpose of radar image interpretation: MODIS (onboard the Aqua/Terra satellite), MERIS (Envisat), and AVHRR (NOAA). Surface manifestations of IWs in the northeastern part of the Black Sea and in the Caspian Sea have been found in radar imagery for the first time, their pattern of spatial and temporal variation has been reconstructed. The possible factors leading to generation of the observed nontidal IWs are determined and suppositions about the corresponding generation mechanisms are made. In particular, the IW manifestations recorded in the northeastern part of the Black Sea are localized in the vicinity of the boundaries of eddies or hydrological fronts; this fact evidences for the frontal mechanism of generation, at which IWs are radiated by a nonstationary (moving and/or inertially oscillating) front. The most probable main sources of generation of IWs detected in the Caspian Sea are longitudinal one-knot seiches, whose knot point is located near the Apsheron sill. [ABSTRACT FROM AUTHOR]

Large-eddy simulation has become an important tool for the study of the atmospheric boundary layer. However, since large-eddy simulation does not simulate small scales, which do interact to some degree with large scales, and does not explicitly resolve the viscous sublayer, it is reasonable to ask if these limitations affect significantly the ability of large-eddy simulation to simulate large-scale coherent structures. This issue is investigated here through the analysis of simulated coherent structures with the proper orthogonal decomposition technique. We compare large-eddy simulation of the atmospheric boundary layer with direct numerical simulation of channel flow. Despite the differences of the two flow types it is expected that the atmospheric boundary layer should exhibit similar structures as those in the channel flow, since these large-scale coherent structures arise from the same primary instability generated by the interaction of the mean flow with the wall surface in both flows. It is shown here that several important similarities are present in the two simulations: (i) coherent structures in the spanwise-vertical plane consist of a strong ejection between a pair of counter-rotating vortices; (ii) each vortex in the pair is inclined from the wall in the spanwise direction with a tilt angle of approximately 45°; (iii) the vortex pair curves up in the streamwise direction. Overall, this comparison adds further confidence in the ability of large-eddy simulation to produce large-scale structures even when wall models are used. Truncated reconstruction of instantaneous turbulent fields is carried out, testing the ability of the proper orthogonal decomposition technique to approximate the original turbulent field with only a few of the most important eigenmodes. It is observed that the proper orthogonal decomposition reconstructs the turbulent kinetic energy more efficiently than the vorticity. [ABSTRACT FROM AUTHOR]

Sunspot activity is usually described by either sunspot numbers or sunspot areas. The smoothed monthly mean sunspot numbers (SNs) and the smoothed monthly mean areas (SAs) in the time interval from November 1874 to September 2007 are used to analyze their phase synchronization. Both the linear method (fast Fourier transform) and some nonlinear approaches (continuous wavelet transform, cross-wavelet transform, wavelet coherence, cross-recurrence plot, and line of synchronization) are utilized to show the phase relation between the two series. There is a high level of phase synchronization between SNs and SAs, but the phase synchronization is detected only in their low-frequency components, corresponding to time scales of about 7 to 12 years. Their high-frequency components show a noisy behavior with strong phase mixing. Coherent phase variables should exist only for a frequency band with periodicities around the dominating 11-year cycle for SNs and SAs. There are some small phase differences between them. SNs lag SAs during most of the considered time interval, and they are in general more asynchronous around the minimum and maximum times of a cycle than at the ascending and descending phases. [ABSTRACT FROM AUTHOR]

We propose the adaptive S-method-based technique for imaging of SAR and ISAR targets. This approach can be applied in the 1D and 2D modes. It is a postprocessing technique, since the first stage is the standard radar imaging with the 2D Fourier transform. In addition, selection of the adaptive parameter in this technique is efficient and can be performed based on simple rules in real time. The proposed technique produces highly concentrated (focused) radar image without interferences commonly associated with time-frequency representations and without defocusing target images that are already focused in the 2D Fourier domain. [ABSTRACT FROM AUTHOR]

When reconstructing the ISAR image of a maneuvering target rotating approximately with the equal changing acceleration, the received signals can be considered as the multicomponent cubic phase signals. If the model of rotating with constant acceleration is used here, the errors of the reconstructed images cannot be neglected, and a lot of pseudoscatterers are produced. Hence, the ISAR imaging of rotating target with equal changing acceleration based on the cubic phase (CP) function is presented in this paper. The CP function is applied to estimate the parameters of the multicomponent cubic phase signals, and the clear instantaneous ISAR images are obtained. The results of simulated data demonstrate the effectiveness of the method proposed. [ABSTRACT FROM AUTHOR]

This paper presents a new approach for the squint-mode spotlight SAR imaging. Like the frequency scaling algorithm, this method starts with the received signal dechirped in range. According to the geometry for the squint mode, the reference range of the dechirping function is defined as the range between the scene center and the synthetic aperture center. In our work, the residual video phase is compensated firstly to facilitate the following processing. Then the range-cell migration with a high-order range-azimuth coupling form is processed by a nonlinear frequency scaling operation, which is different from the original frequency scaling one. Due to these improvements, the algorithm can be used to process high squint SAR data with a wide swath and a high resolution. In addition, some simulation results are given at the end of this paper to demonstrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

To determine whether spaceborne Synthetic Aperture Radar (SAR), specifically fine beam RADARSAT-1 C-band, could be used to provide quantitative data on white mangrove ( Laguncularia racemosa (Gaertner)) forests, backscatter coefficients (σ°) were examined in relation to structural parameter data collected from plots located in a mangrove forest of the Mexican Pacific. Significant coefficients of determination were recorded between the backscatter coefficients and the logarithms of both Leaf Area Index (LAI) and mean stem height at two incident angles for both the dry and wet seasons. The highest coefficients of determination for LAI ( r 2 = 0.60) and mean stem height ( r 2 = 0.72) were observed using the shallower (~40°) and steeper (~47°) incident angles, respectively. No significant relationships were recorded between the backscatter coefficients and either stem density, basal area or mean DBH. Given the results of this investigation, it is recommended that for cloud covered regions, fine beam RADARSAT-1 data could be used by resource managers when they require a quick method for surveying structural damage to mangroves resulting from both natural and anthropogenic causes. [ABSTRACT FROM AUTHOR]

We propose a model of coherent-echo signals during the monostatic sounding of the ionosphere. The model is based on the previously obtained radar equation for separate samples of the scattered signal. The dielectric-permittivity perturbation described by a discrete set of spatial harmonics modulated in space and time is used as the scattering irregularities. The model was tested using Irkutsk incoherent-scatter radar data obtained during the coherent-echo observations in July 15 and 16, 2000. The test shows that the model is suitable for describing the observed characteristics of separate sample spectra of the coherent-echo signals. [ABSTRACT FROM AUTHOR]

Coherent radiation in the range from soft X-rays up to hard X-rays, produced by the low-emittance electron beam of MAMI, can be used for various applications. Novel types of interferometers have been developed for the measurement of the complex index of refraction of thin self-supporting foils. For the vacuum ultraviolet and soft X-ray region the interferometer consists of two collinear undulators, and a grating spectrometer. A foil placed between the undulators causes a phase shift and an attenuation of the oscillation amplitude. The complex index of refraction has been measured at the L2,3-absorption edges of nickel. A novel method is described for the measurement of the X-ray magnetic circular birefringence. For the hard X-ray region the interferometer consists of two foils at which the 855MeV electron beam produces transition radiation. Distinct interference oscillations have been observed as a function of both, the photon emission angle and the distance between the foils. The refractive index decrement δ(ω) of a 2μm thick nickel sample foil has been measured at X-ray energies around the K absorption edge at 8333eV and at 9930eV with an accuracy of better than 1.5 %. The line width of parametric X radiation (PXR) was measured in backward geometry with a Si single-crystal monochromator. Upper limits of the line width of 42meV, 50meV, and 44meV, have been determined for the (333), (444) and (555) reflections at photon energies of 5932eV, 7909eV, and 9887eV, respectively. Small angle scattering of the electrons in the crystal leads to a stochastic frequency modulation of the exponentially damped wave train which results in the line broadening. To elucidate the quest if the production of PXR is a kinematical or a dynamical process the radiation from silicon single-crystal targets, emitted close to the electron direction, has been studied. The observed interference structures and the narrow-band radiation in forward direction shows that PXR is produced in a dynamical process. [ABSTRACT FROM AUTHOR]

In the light of the concept of spherical wave source, the theoretical model of nearfield acoustic holography (NAH) based on the spherical wave superposition method (SWSM), including reconstruction of expansion coefficients, prediction of acoustic field, error sensitivity analysis, regularization method and a searching method with dual measurement surfaces for determining the optimal number of expansion terms, is established. Subsequently, the spherical wave source boundary point method (SWSBPM) and its application in the NAH are introduced briefly. Considering the similarity of the SWSM and the SWSBPM for realizing the NAH, they are compared. The similarities and differences of the two methods are illuminated by a rigorous mathematical justification and two experiments on a single source and two coherent sources in the semi-free acoustic field. And, the superiority of the NAH based on the SWSBPM is demonstrated. [ABSTRACT FROM AUTHOR]

Diffraction phenomena accompanying the propagation of narrow directed beams of coherent radiation in a medium with anisotropy of absorption and gain are theoretically analyzed. A quasi-optical equation is obtained, which differs from the traditional equation by the complexity of the diffraction coefficient, and this leads to effective diffusion of radiation in anisotropically absorbing media. Because of this, the intensity of minima observed in the far-field region in the case of diffraction of coherent radiation by a slit does not vanish, but has a finite value. The solution for Gaussian beams is constructed. It is shown that the beam width increases as the square root of the path length in an anisotropic medium, whereas the angular divergence varies in inverse proportion to the square root of the path length. [ABSTRACT FROM AUTHOR]

Distributed radars have the potential to combine coherently for achieving a high signal-to-noise ratio (SNR) while maintaining a moderate antenna size. The key to coherently combining multiple radars is obtaining accurate coherent parameters (CPs), which are used to adjust the transmitting/receiving time and phase of each radar. One approach for CP estimation is to transmit orthogonal waveforms. However, ideally, orthogonal waveforms occupying the same frequency band may not be found in practice. Cross-correlation energy leakage exists between non-orthogonal waveforms, which seriously impairs the accurate acquisition of CPs. To solve this problem, we propose a clean signal reconstruction approach for CP estimation. This approach reconstructs clean echoes by gradually stripping out the cross-correlation energy leakage with a reconstruction-elimination-reconstruction framework. And CPs are obtained from these reconstructed clean echoes. Since the majority of cross-correction energy leakages are eliminated, enhanced CP estimation performance can be achieved. Verified simulations are designed for a dual radar scenario. Results show that the proposed approach significantly improves the performance of CP estimation while reducing the SNR requirement for coherently combining multiple radars. [ABSTRACT FROM AUTHOR]

The article presents the Asia-Pacific Economic Cooperation (APEC) Cooperation for Earthquake Simulation (ACES) related research and activities. It offers unique and innovated opportunity to scientist at all levels to gain new understanding of earthquake nucleation process, precursory phenomena and space-time earthquake patterns. It also tries to expand into studies of new satellite measurements that focuses on Interferometric Synthetic Aperture Radar (InSAR) data.