Echo Spectra Research Articles

A comprehensive computational model of animal biosonar signal processing.

Computational models of animal biosonar seek to identify critical aspects of echo processing responsible for the superior, real-time performance of echolocating bats and dolphins in target tracking and clutter rejection. The Spectrogram Correlation and Transformation (SCAT) model replicates aspects of biosonar imaging in both species by processing wideband biosonar sounds and echoes with auditory mechanisms identified from experiments with bats. The model acquires broadband biosonar broadcasts and echoes, represents them as time-frequency spectrograms using parallel bandpass filters, translates the filtered signals into ten parallel amplitude threshold levels, and then operates on the resulting time-of-occurrence values at each frequency to estimate overall echo range delay. It uses the structure of the echo spectrum by depicting it as a series of local frequency nulls arranged regularly along the frequency axis of the spectrograms after dechirping them relative to the broadcast. Computations take place entirely on the timing of threshold-crossing events for each echo relative to threshold-events for the broadcast. Threshold-crossing times take into account amplitude-latency trading, a physiological feature absent from conventional digital signal processing. Amplitude-latency trading transposes the profile of amplitudes across frequencies into a profile of time-registrations across frequencies. Target shape is extracted from the spacing of the object’s individual acoustic reflecting points, or glints, using the mutual interference pattern of peaks and nulls in the echo spectrum. These are merged with the overall range-delay estimate to produce a delay-based reconstruction of the object’s distance as well as its glints. Clutter echoes indiscriminately activate multiple parts in the null-detecting system, which then produces the equivalent glint-delay spacings in images, thus blurring the overall echo-delay estimates by adding spurious glint delays to the image. Blurring acts as an anticorrelation process that rejects clutter intrusion into perceptions.

Intrapulse Azimuth Frequency Scanning-Based 2-D Scanning SAR for HRWS Imaging

In this article, a novel 2-D scanning synthetic aperture radar mode for high-resolution and wide-swath imaging is introduced. Instead of the interpulse progressive azimuth beam scanning used in Terrain Observation by Progressive Scans (TOPS) mode, the intrapulse continuous azimuth frequency scanning is employed in this mode. The same azimuth resolution as the multichannel TOPS mode can be obtained through this mode in the case of single channel transmitting and receiving. And the wide-swath imaging in range is still achieved through the cyclically elevation beam scanning from burst to burst. Moreover, the antenna aperture can be fully exploited for transmitting in this mode. Therefore, it will outperform the multichannel TOPS mode especially in the case of active array antenna. The signal model of this mode is derived. Based on the analysis of the characteristics of echo spectrum, the design criteria for this mode are developed, followed by a design example. Finally, an imaging algorithm for this mode is presented. Theoretical analysis and numerical simulations verify the effectiveness of the proposed design criteria and imaging algorithm.

Echoes from phantom wormholes

We study the time evolution of the test scalar and electromagnetic fields perturbations in configurations of phantom wormholes surrounded by dark energy with state parameter $\omega< -1$. We observe obvious signals of echoes reflecting wormholes properties and disclose the physical reasons behind such phenomena. In particular, we find that the dark energy equation of state has a clear imprint in echoes in wave perturbations. When $\omega$ approaches the phantom divide $\omega=-1$ from below, the delay time of echoes becomes longer. The echo of gravitational wave is likely to be detected in the near future, the signature of the dark energy equation of state in the echo spectrum can serve as a local measurement of the dark energy.

Experimental study on echo spectrum modulation characteristics of plasma flow field simulated by wind tunnel

目标以极高速度在大气层内运动时，周围会因剧烈摩擦产生等离子体绕流场。等离子体绕流场运动速度分布不均匀，而且绕流场电子密度随时间动态变化，导致等离子体绕流场对入射其中的电磁波产生不均匀的频率调制，进而影响雷达的探测性能。为了复现等离子体绕流场在电磁波照射时产生的不均匀频谱调制现象，本文在中科院力学所JF-10风洞开展了等离子体绕流场回波频谱测量实验，通过信号源、环形器、天线和频谱仪组成的测量系统，以点频发射体制，获取了S、C波段的回波频谱数据，观察到了等离子体绕流场对目标回波频谱的调制现象，对测量现象的形成原因进行了讨论；基于测量数据，仿真分析了等离子体绕流场对目标一维距离像的散焦效应。

FDBP-InSAR: An Efficient Algorithm for InSAR Imaging via Frequency Domain Back Projection

High-quality focusing with accurate phase-preserving is a significant and challenging step in interferometric synthetic aperture radar (InSAR) imaging. Compared with conventional frequency-based imaging algorithms, the time-domain back-projection algorithm (TDBPA) can greatly ensure the accuracy of imaging and phase-preserving by point-to-point coherent integration but suffers from huge computational complexity. In this paper, we propose an efficient InSAR imaging method, called a frequency-domain back-projection algorithm (FDBPA), to achieve high-resolution focusing and accurate phase-preserving of InSAR imaging. More specifically, FDBPA is utilized to replace the traditional point-to-point coherent integration of TDBPA with frequency-domain transform. It divides the echo spectrum into uniform grids and transforms the range compression data into the range frequency domain. Phase compensation and non-uniform Fourier transform of the underlying scene are implemented to achieve image focusing in the wavenumber domain. Then, the interferometric phase of the target scene can be preserved by accurate phase compensation of the target’s distance. FDBPA avoids the repetitive calculation of index values and point-to-point coherent integration which reduces the time complexity compared with TDBPA. The characteristics of focusing and phase-preserving of our method are analyzed via simulations and experiments. The results demonstrate the efficiency and high-quality imaging of the FDBPA method. It can improve the imaging efficiency by more than three times, while keeping similar imaging accuracy compared with TDBPA.

Fiber Bragg Grating Sensor Based on Refractive Index Segment Code of Mobile Modulation

Fiber Bragg Grating (FBG) is achieved by refractive index modulation. Temperature and strain can be measured by FBG. The echo spectral distribution is determined by the refractive index of the fiber and the grating pitch. Therefore, there are few types of conventional refractive index modulation methods, and there are few types of echo spectrum. In this paper, an index code modulation based on IoT method was proposed. The FBG is divided into a plurality of interval segments in the method. The spectral shape of the echo is controlled by encoding and modulating each interval. When the segment size satisfies some requirements, the positive and negative modes of the sub-FBG are calculated by the coupled-mode theory, and then the mode functions of each segment are solved by the matrix transmission algorithm. The equivalent function of the reflectance distribution field is obtained. According to the relevant parameters of FBG, the mathematical model of the FBG modulation spectral distribution is established by the refractive index modulation method. Through the simulation analysis of the fractional refractive index modulation, the results show that the echo spectral peak, half-width, etc. have a larger tunable range. In the experiment, a femtosecond laser was used to accurately encode and modulate the refractive index, and two different types of segmented FBGs were fabricated according to design requirements. The experimental results show that the design of different segmented structures has a significant impact on the spectral distribution of the echo. Spectral shape control can be achieved by refractive index modulation method.

Classification of biosonar target echoes based on coarse and fine spectral features in the bottlenose dolphin (Tursiops truncatus)

Previous bottlenose dolphin studies suggest that the coarse envelope of an echo spectrum ("macrostructure") has hierarchical dominance over finer-scale spectral features ("microstructure") during synthetic echo discrimination tasks. In this study, two dolphins listened to and discriminated between underwater sound stimuli consisting of pairs of clicks with different micro- and macrostructures. After conditioning dolphins to reliably discriminate between two "anchor" stimuli with different micro- and macrostructures, probe stimuli, which contained a macrostructure identical to one of the anchor stimuli and the microstructure of the alternate anchor, were infrequently presented. Dolphins responded to probes in a manner consistent with macrostructure primacy.

Echo signal simulation system

In order to meet the demand of realistic simulation for complex target echo signal in water, the indirect method of frequency domain is employed to simulate the target echo signal in this paper. The plate element method is first used to forecast complex target echo characteristics in typical water. After generating frequency response function data, the data is imported into the echo signal simulation system. The system adopts FPGA real-time signal processing system based on PXIe bus. It receives hydrophone output signal by A/D sampling, extracts the incident acoustic signal and makes frequency domain transformation. After multiplying the transformed signal and frequency response function, scattering echo spectrum is obtained. And then the time domain transformation is made to get the time series of the echo signal. The series is loaded into the transmitting transducer by the power amplifier and the echo signal is simulated. The system has the high fidelity of the model, stable and reliable hardware system, and software system for easy operation.

Diurnal variation of atmospheric metal Na layer and nighttime top extension detected by a Na lidar with narrowband spectral filters at Beijing, China

Diurnal observations of metal Na layer can be implemented by making use of narrowband Faraday filters. On one hand, the narrowband spectral filters help to effectively suppress the background noise; on the other hand, the narrow bandwidth leads to different transmissions of Na layer resonance fluorescence signal and stratosphere Rayleigh scattering signal. The influence of the narrow spectral transmission on Na density retrieval cannot be canceled out by Rayleigh normalization and would result in a systematic error. In this paper, the Na density retrieval method when narrowband Faraday filters are used in the lidar receiver is discussed based on the transmission spectrum of Faraday filters and echo spectra of Rayleigh scattering and Na fluorescence. The results of Na layer diurnal variations that have been obtained from Na lidar observations at Beijing, China are presented. Considerable diurnal variation in Na density on the layer top and bottom are clearly revealed on a logarithmic scale. Intriguingly, larger height-scale nighttime extensions on the Na layer topside have been frequently observed from early summer to autumn. These observational results can provide valuable evidence for studying solar effect on the metal layer variation, atmospheric dynamical and chemical processes in the mesosphere and lower thermosphere region.

A Case Study of the Daytime Intense Radar Backscatter and Strong Ionospheric Scintillation Related to the Low‐Latitude E‐Region Irregularities

AbstractThe special daytime echoes, which present the V‐shaped echo pattern between 90–180 km ranges in the range‐time‐intensity (RTI) plot, were occasionally recorded by the Hainan coherent scatter phased array radar (HCOPAR) located at the low latitude of China. Four cases with distinct echo traces are presented to display the common characteristics of the V‐shaped echoes. They usually occur between 10:00–15:00 LT in the daytime with the enhanced top frequency and blanketing frequency of Es layer (ftEs and fbEs). Their left/right wings present negative/positive Doppler velocity. A case on 16 August 2015 was investigated by the joint observation of the HCOPAR, the Hainan Digisonde, and a scintillation receiver. The echo spectra show that the V‐shaped echoes were backscattered from the type 2 irregularities. The formation of the V‐shaped echo pattern was due to the leakage of the antenna sidelobe for the very intense scatter from Es layer. The irregularities were found traveling westward with the velocity of 41.12 ± 2.08 m/s. While the occurrence of the intense irregularities, there were periodic peaks appearing on the ftEs and fbEs curves, and the maximum ftEs‐fbEs reached 10.31 MHz. At the same time, the strong ionospheric scintillation event was recorded by the scintillation receiver. The wind shear is considered to produce the sharp density gradient in the Es plasma and then induce the gradient drift instability. The large‐scale gravity waves have the possibility to induce the E‐region wind shear.

A study of marine slicks damping effect on Doppler spectra of backscattered echoes from rough sea surfaces

ABSTRACT Doppler spectrum of backscattered sea echoes from slick-covered sea surfaces is investigated in this paper, with emphasis on exploring the influence of marine sea slicks damping effect on Doppler spectral signatures including Doppler shift and spectral bandwidth. Model of local balance of wave energy inputs and outputs in wavenumber space is utilized for describing marine slicks damping effect, and the ‘choppy wave’ model is adopted for describing non-linear hydrodynamic interactions between ocean waves. The second-order small-slope approximation model combined with Monte Carlo technique is utilized for evaluating Doppler spectrum of backscattered field from clean or contaminated sea surfaces. Numerical simulations show that Doppler spectral bandwidths in slicks are reduced for almost all of the incidence angles ranging from to , which is qualitatively consistent with wave-tank measurements. The Doppler shifts in slicks, however, can increase or decrease, depending strongly on incidence angle, which is also in qualitative agreement with measurements. The Doppler spectrum of radar sea echoes is promising in the detection marine slicks by virtue of radar Doppler signatures.

Inverse Radon transform-based large micromotion target detection algorithm in synthetic aperture radar

When the micromotion (MM) target has a large MM amplitude, the bandwidth of the synthetic aperture radar (SAR) azimuth echo will be greater than the pulse repetition frequency (PRF) of SAR. According to Nyquist sampling theorem, spectrum aliasing will occur at this time, and conventional MM target detection and estimation algorithms may lose effect. To solve the problem of large MM target detection with echo bandwidth greater than the PRF, an inverse Radon transform (IRT)-based large MM target detection algorithm is proposed. The study found that the time-frequency (TF) spectrum of large MM target azimuth echo is usually a folded sinusoidal curve. Then we spliced enough identical TF spectrums to restore an intact sinusoidal curve. The intact sinusoidal curve can be mapped into a peak in the parameter space using IRT. Hence, the algorithm can detect large MM target and estimate its parameters accurately. Data processing results prove the effectiveness of the algorithm. At the same time, the performance analysis proves that the operation speed and antinoise performance of the algorithm are better than that of the Hough transform algorithm.

Signal Modeling and Analysis for Elevation Frequency Scanning HRWS SAR

Elevation frequency scanning synthetic aperture radar (EF-SCAN SAR) can isolate range ambiguities in the range-frequency domain. Therefore, range ambiguity suppression can be accomplished through range-frequency bandpass filtering, which can outperform spatial filtering-based ambiguity suppression. In this article, the analytic space–time signal model of an intrapulse frequency scanning array is derived. Based on the analysis of the characteristics of this signal model, waveform design criteria for EF-SCAN SAR are developed to effectively separate the range ambiguities in the range frequency domain. The ground echo model of EF-SCAN SAR is also developed based on this space–time signal model. The impact of intrapulse frequency scanning on the performance of high-resolution and wide-swath (HRWS) imaging is analyzed based on the derivation and analysis of the echo spectrum. Finally, an imaging algorithm for EF-SCAN SAR is developed based on this analysis. Numerical simulations verify the effectiveness of the proposed waveform design criteria and imaging algorithm.

Small‐Scale Void‐Size Determination in Reinforced Concrete Using GPR

The detection and evaluation of void in concrete are imperative issues in health monitoring of civil engineering. However, the void is difficult to be detected at its early stage of formation on account of its small scale and concealment. Although, in view of the remarkable performance such as precision and continuity, ground penetrating radar (GPR) is widely employed in some special fields of nondestructive testing, the strong reflection of rebar which could submerge the echo of void is now a stumbling block for the application of GPR in reinforced concrete. In order to develop the tactic of small‐scale void‐size determination with considerable noise originated from rebar, a two‐stage regression method is proposed in this study. First, a superposition model of target echoes was established to analyze the relationship between the space‐time distribution and spectrum of the rebar echo and the void echo, and a curvelet transform was employed to reduce the strong reflection of the rebar. Then, the echo amplitude of small‐scale void was investigated, and a regression method was put forward to determine the void size of small scale. Finally, forward simulation and experiment were carried out to verify the feasibility of the proposed method. Results demonstrated that the spectral distribution of the rebar echo is significantly higher than that of the void echo. The curvelet‐based low‐pass filter can effectively ensure the discrimination of void echo. The regression method performs a high‐accuracy void‐size determination with more than 90% in small scale.

MELISSA: System description and spectral features of pre‐ and post‐midnight F‐region echoes

AbstractMost of the low‐latitude ionospheric radar observations in South America come from the Jicamarca Radio Observatory, located in the western longitude sector (∼75°W). The deployment of the 30 MHz FAPESP‐Clemson‐INPE (FCI) coherent backscatter radar in the magnetic equatorial site of São Luis, Brazil, in 2001 allowed observations to be made in the eastern sector (∼45°W). However, despite being operational for several years (2001–2012), FCI only made observations during daytime and pre‐midnight hours, with a few exceptions. Here, we describe an upgraded system that replaced the FCI radar and present results of full‐night F‐region observations. This radar is referred to as Measurements of Equatorial and Low‐latitude Ionospheric irregularities over São Luís, South America (MELISSA), and made observations between March 2014 and December 2018. We present results of our analyses of pre‐ and post‐midnight F‐region echoes with focus on the spectral features of post‐midnight echoes and how they compare to spectra of echoes observed in the post‐sunset sector. The radar observations indicate that post‐midnight F‐region irregularities were generated locally and were not a result of “fossil” structures generated much earlier in time (in other longitude sectors) and that drifted into the radar field‐of‐view. This also includes cases where the echoes are weak and that would be associated with decaying equatorial spread F (ESF) structures. Collocated digisonde observations show modest but noticeable F‐region apparent uplifts prior to post‐midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizing F‐region conditions leading to ESF development.

Numerical and Experimental Study on Backscattering Doppler Characteristics From 2-D Nonlinear Sealike Surface at Low Grazing Angle

This article studies both experimentally and numerically the backscattering radar cross section (RCS) and Doppler spectrum characteristics from 2-D linear and nonlinear time-varying sea-like surface at UHF band under low grazing incidence. The small slope approximation and choppy wave model are applied to solve rough surface scattering and generate nonlinear sea waves, respectively. A coherent Doppler radar operating at 340 MHz was deployed at the tip of Huangqi peninsula in the southeast coast of China to measure the echo spectrum from the actual sea surfaces. At the same time, an ocean buoy was placed in the radar coverage to monitor the sea states. The RCS and Doppler spectrum are analyzed and compared comprehensively between radar measurements and numerical predictions. After compensating the influence of wind direction, the responses of radar echo power and numerical predicted RCS to wind speed are basically consistent and are more sensitive in low sea states. The correlation of Doppler spectra between radar measurements and numerical simulations exceed 0.96 during the whole experiment. The intensity of the high-order peaks increases rapidly with the wind speed, while the intensity of the Bragg peak decreases slightly. Doppler spectrum will be shifted by an amount proportional to radial surface current.

Cognitive hierarchy of acoustic power spectrum features for simulated biosonar target echoes in the bottlenose dolphin (Tursiops truncatus)

Previous work with bottlenose dolphins [Dubrovsky et al., “Mechanisms of signal discrimination and identification in the auditory system of Tursiops truncatus,” in Marine Mammal Sensory Systems, edited by Thomas et al. (Plenum, New York, 1992), pp. 235–240] suggested that the perception of coarse envelopes of echo power spectra (“macrostructure”) is hierarchically dominant to finer-scale spectral features (“microstructure”). In the present study, two dolphins passively listened to and discriminated between two standard click doublets having different micro- and macrostructure. The dolphins were provided food reinforcement for remaining on an underwater station after the “negative” stimulus (two clicks with 100-μs separation) and for touching a paddle after the “positive” stimulus (two clicks with 150-μs separation). The dolphins were then presented with probe stimuli that were hybrids of the two standard stimuli. Each probe had a macrostructure identical to the positive or negative stimulus but the microstructure of the alternate standard. Preliminary results show the dolphins responding to probes in a manner consistent with macrostructure supremacy. This is in line with previous work suggesting that the macrostructure is hierarchically dominant over microstructure. [Work supported by the Office of Naval Research.]Previous work with bottlenose dolphins [Dubrovsky et al., “Mechanisms of signal discrimination and identification in the auditory system of Tursiops truncatus,” in Marine Mammal Sensory Systems, edited by Thomas et al. (Plenum, New York, 1992), pp. 235–240] suggested that the perception of coarse envelopes of echo power spectra (“macrostructure”) is hierarchically dominant to finer-scale spectral features (“microstructure”). In the present study, two dolphins passively listened to and discriminated between two standard click doublets having different micro- and macrostructure. The dolphins were provided food reinforcement for remaining on an underwater station after the “negative” stimulus (two clicks with 100-μs separation) and for touching a paddle after the “positive” stimulus (two clicks with 150-μs separation). The dolphins were then presented with probe stimuli that were hybrids of the two standard stimuli. Each probe had a macrostructure identical to the positive or negative stimulus but the micros...

Application of High-Frequency Electron Paramagnetic Resonance/Electron Spin Echo for the Identification of the Impurity Composition and Electronic Structure of Ceramics Based Garnets

Electron paramagnetic resonance (EPR) spectra of Ce3+, Yb3+, Cr3+, and Gd3+ impurity ions in yttrium aluminum garnet Y3Al5O12 (YAG) ceramics were detected and identified at frequency of 94 GHz. The advantage of measuring the EPR spectra in the high-frequency range compared to the standard EPR technique is shown, which makes it possible to separate the EPR spectra characterized by different anisotropic g-factors and also to isolate the EPR signals due to the splitting of the fine structure for centers with high-spin states. In ceramics with a high content of magnetic Gd3+ ions, EPR and electron spin echo (ESE) spectra of multi-ionic gadolinium complexes were observed, and EPR spectra of complexes with the maximum number of exchange-coupled gadolinium ions are seen at low temperatures. The temperature dependences of the EPR spectra indicates ferromagnetic ordering of exchange-coupled complexes of gadolinium.

High-dimensional feature extraction of sea clutter and target signal for intelligent maritime monitoring network

As one of the source sensors of maritime intelligent traffic network, radar plays an important role in maritime monitoring and early warning. The number of features extracted by the traditional maritime radar target detection method in feature domain is small, and the sea clutter and target echo features are often not linearly separable, so the radar detection performance is seriously affected by sea clutter. To solve this problem, this paper combines the time–frequency domain processing method with the residual neural network, and uses the time–frequency transform method to improve the signal-to-clutter ratio (SCR) and the degree of difference between sea clutter and target echo. On this basis, the high-dimensional time–frequency spectrum features are extracted by using the residual neural network to improve the utilization rate of radar echo information, form a high-dimensional feature space of sea clutter and target echo that are non-linearly separable, and realize the binary classification of sea clutter and target echo. It is verified by the measured data of X-band radar that the proposed target detection method can extract the deep features of the time–frequency spectrum of radar echo, has a high classification accuracy even in the case of low SCR, and has the potential to detect weak targets in the background of strong sea clutter. In addition, the influence of different time–frequency transform methods and polarization modes on target detection performance is further analyzed. The comparative study shows that different time–frequency transform methods and polarization modes have little influence on the classification accuracy of the proposed method. In comparison, under the conditions of fractional Fourier transform and cross-polarization, the proposed method has higher classification accuracy.

Echolocating bats perceive natural-size targets as a unitary class using micro-spectral ripples in echoes.

Echolocating big brown bats emit frequency-modulated (FM) sounds covering ultrasonic frequencies in two harmonic sweeps (FM1 from 50-60 kHz sweeping down to 20-25 kHz, FM2 from 100-110 kHz sweeping down to 45 kHz). Using a complex interplay of acoustic cues, the bats perceive object distance from echo delay and object shape from echo spectra. Typical natural targets-flying insects-return discrete reflections, called glints, from prominent body parts (e.g., head, wings). Insect sizes are mostly 0.5 cm to about 3.5 cm, corresponding to reflected time separations of 30 to 210 μs. When closely spaced reflections overlap, they interfere to create a characteristic echo spectrum containing repetitive peaks and nulls. Time spacing of these glints (Δt) is transposed into spectral ripples at frequency intervals (Δf = 1/Δt) from about 30 kHz down to 5 kHz. The bat's perception of rippled echoes as a distinct class was tested in two-alternative forced-choice discrimination experiments with a standard stimulus of 100 μs glint separation (10 kHz ripples), in the middle of natural ripple separations (30 kHz to 5 kHz). Stimuli were electronically generated virtual targets with simulated 2-glint sizes from smaller than 0.5 cm to larger than 3.5 cm. Bats perceived insect-sized virtual objects (glint delays of 36-300 μs) as similar to the 100-μs standard, while smaller or larger virtual objects were perceived as not similar. Insect-sized echoes contained a pattern of microscale spectral ripples that stood apart from the coarser, macroscale spectral features of smaller objects or the temporally separate reflections of larger objects. (PsycINFO Database Record (c) 2019 APA, all rights reserved).