Echo Spectra Research Articles

Bottlenose dolphins direct sonar clicks off-axis of targets to maximize Fisher Information about target bearing

Aiming the sonar beam directly at a potential target maximizes the detection information available to an echolocating animal. In contrast, estimating the angular location of a target relies on changes in the received echo spectrum due to the frequency dependence of the transmit beam pattern. The Fisher Information quantifies the available information on angular location in terms of the sensitivity of the received echo spectrum as a function of bearing. We calculated the Fisher Information for a dolphin's echolocation signal and determined the maximum angular location information occurs when the sonar beam is such that the target falls slightly off-axis. To compare the predicted beam aim to the actual beam aim of an echolocating dolphin, we recorded the echolocation signals of a bottlenose dolphin with a 16-element hydrophone array while the animal performed a target detection task. The dolphin consistently pointed its sonar beam 7 degrees away from the target, which is the beam aim that maximizes the Fisher Information for the dolphin signal. [Work supported by NSF and ONR.]

An Improved M-PRT Technique for Spectral Analysis of Weather Radar Observations

The exploitation of Doppler radars for weather observations is strongly constrained by the well-known range–velocity dilemma. To overcome the range and velocity ambiguities, dual and triple staggered pulse-repetition time (PRT) techniques are commonly used in Doppler radar systems. Today, a triple-PRT (3-PRT) scheme is operational in France. These techniques imply nonuniform sampling of the weather signal, inducing multiple replicas in the Doppler spectrum. The situation is particularly complicated for short-wavelength radars, where larger extension factors of the unambiguous Nyquist interval are needed. To overcome these difficulties, a novel technique called OptM-PRT is proposed. It mainly consists in optimizing the transmission scheme based on multiple pulse repetition time, so that the corresponding autocorrelation function is well filled. The Doppler spectrum is therefore reconstructed with much less ambiguities, from the computation of the autocorrelation function of radar signal and its Fourier transform. Considering both 3-PRT and Opt9-PRT schemes, the magnitude and Doppler velocity of radar returns in rain are simulated for different spectral widths, with and without elimination of the spectral lines of ground clutter. When the ground clutter is filtered out, the 3-PRT is found to better reproduce the Doppler velocity, whereas the Opt9-PRT better restitutes the magnitude of the signal. In the presence of noise, the Opt9-PRT scheme produces the best result for both the magnitude and velocity. The 3- and Opt9-PRT techniques have been applied to the C-band Doppler radar operating in Bourges, France. The experimental results show that Opt9-PRT efficiently reconstructs the Doppler spectrum of rain echoes.

On the origin of oscillations in two-dimensional spectra of excitonically-coupled molecular systems

We investigate an artificial molecular dimer made of two dipole coupled cyanine dye monomers in which a strong coherent coupling between electronic and vibrational degrees of freedom arises. Clear signatures of this coupling are reflected in an oscillatory time evolution of the off-diagonal vibronic cross peaks in the two-dimensional optical photon echo spectrum. We find a strong coherence component damped by fast electronic dephasing ( fs) accompanied by a much weaker component which decays on the longer time scales (ps) associated to vibrational dephasing. We find that vibronic coupling does not cause longer dephasing times of the dominant photo echo component but additional weak but long-lived components emerge.

Target strength spectra of tracked individual fish in schools

Broadband sonar echoes received from individual fish in schools contain information that might be useful for species discrimination. The echo characteristics are closely related to the incident angle of the sound to fish body, corresponding to the apparent tilt angle of a fish from a viewpoint of a transducer. Therefore, it is necessary to isolate individual fish echoes and estimate the incident angles. In this study, a broadband split-beam echo sounder system was used to extract features from free-swimming fish schools. Echoes from individual fish in dense schools were measured and their position was estimated using the system with a downward-oriented acoustic beam. Individual fish were tracked in schools of Japanese jack mackerel Trachurus japonicus, chub mackerel Scomber japonicus, and red sea bream Pagrus major with the system. The target strength (TS) spectra of echoes ranging from 80 to 120 kHz were calculated and sorted on the basis of the incident angle, which was estimated by the tracking result. Using this approach, TS spectra were obtained from entire schools of free-swimming fish, and the obtained TS spectra were found to be dependent on fish species and the incident angles of the individual fish involved.

Effect of zinc substitution on molecular dynamics of protoporphyrin-IX

We report the effects of zinc metal substitution on the molecular dynamics of protoporphyrin-IX in dichloromethane solvent by spectrally resolving the femtosecond photon echo spectrum. We have found that the coherence and population dynamics change due to the presence of Zn metal in the protoporphyrin-IX system. Zinc metal reduces the conformational disorder in the molecular structure of protoporphyrin-IX and in turn increases the planarity of the molecule resulting in better delocalization of π-electrons in the system. This effect decreases the inhomogeneous broadening of the photon echo spectra of Zn-substituted protoporphyrin-IX.

Frequency-Domain Backprojection Algorithm for Synthetic Aperture Radar Imaging

A new frequency-domain backprojection method for processing synthetic aperture radar (SAR) data is presented. It forms SAR images by backprojecting the spectrum of the radar echo into the image wavenumber domain and is capable of fully correcting wavefront curvature and handling a general aperture geometry. From each reconstructed spectral pixel, information about the entire scene can be obtained, allowing the formation of interim lower resolution images during the processing. In addition, the method allows for an arbitrary sampling of the image spectrum. It forms SAR images by backprojecting the spectrum of the radar echo into the image wavenumber domain. Processing of simulated and experimental ultrawideband/widebeam SAR raw data demonstrates the efficacy of this new method.

Striation processing of continuously active sonar data

Previous work by the authors has shown that striations associated with the waveguide invariant can provide a new physics-based tracking constraint that improves tracking performance in pulsed active sonar data. A necessary pre-processing step is to isolate the target return from the direct arrival and other target echoes. Striation-based beamforming can be used as an optional pre-processing step to preserve the multipath structure in the beamformer output of a horizontal line array. Recent interest in continuously active sonar (CAS) processing has motivated research to determine how striations might be similarly exploited for tracking when the sonar is operated with a high duty cycle. In CAS signals, the direct arrival is almost always overlapping in time with the target echo return and the standard time-gating technique cannot isolate the echo return. In this paper, a time-dependent filter is introduced to isolate and extract the CAS echo spectrum. The conditions in which striation-based beamforming should be applied to preserve the striation structure in the beam output are also explored. The physics-based signal processing algorithms discussed here are expected to provide valuable information about the target’s multipath structure, which can be exploited in an extended Kalman filter for enhanced tracker performance.

Enhanced-Resolution Single-Shot 2DFT Spectroscopy by Spatial Spectral Interferometry.

We demonstrate use of spatial interference for the complete electric field reconstruction of two-dimensional (2D) coherent spectroscopic signals generated through four-wave mixing (4WM) in a single laser shot. Until now, the amplitude and phase characterization of 4WM signals has relied primarily on Fourier transform spectral interferometry (FTSI), which limits the measurement's sensitivity and resolution. We show that spatial spectral interferometry (SSI) is a generalized approach to 4WM signal detection that eliminates these inherent limitations of FTSI without introducing additional experimental complexity. SSI is used to measure the 2D photon echo spectra of two systems with dramatically different line widths, the coupled D line transitions in rubidium vapor and the energy-transfer dynamics in the light-harvesting protein LH2.

Numerical simulations of bistatic sea echo spectrum in high frequency band

Numerical simulations of bistatic sea echo spectrum are helpful to the comprehension of interaction mechanism between radio waves of bistatic HF radar and ocean waves. Moreover, the simulated sea echo spectrum in line with the fact is of great significance for testing the validity and applicability of inversion algorithm. In this paper, numerical simulations of bistatic sea echo spectrum in high frequency band are conducted and the effects of wind speed, wind direction and bistatic angle are discussed. When the bistatic angle is fixed, the trends of bistatic sea echo spectrum under the influence of wind speed and wind direction are the same as those of mono-static sea echo spectrum. However, the integrable singularities resulting from electromagnetic coupling coefficient and the corresponding secondary peaks of bistatic sea echo spectrum vary with the bistatic angle. And the intensity and frequency distribution range of second-order spectrum decrease with the increasing bistatic angle.

Improving coherent field imaging quality by suppressing the influence of transmitting aperture spacing error

Coherent field imaging is based on the assumption of equal transmitting apertures spacing and equal spectrum of laser, and high-resolution image is reconstructed by iteratively computing the frequency spectrum. However, the inevitable transmitting aperture spacing error of laser is a key factor to affect the coherent field imaging quality in the application. Aiming at the problem of degrading imaging quality caused by the transmitting aperture spacing error, we discuss the mechanism of influence of aperture spacing error on imaging quality and propose a novel method of improving imaging quality from the perspective of suppressing the influence of transmitting aperture spacing error. Firstly, the mechanism of the influence of aperture spacing error on imaging quality and laser echo spectrum is analyzed in detail. Then we derive a frequency spectrum error propagation model. Based on the model, the iterative effect of frequency spectrum error is investigated and the trend of variation in imaging quality with frequency spectrum error is given. We propose a theoretical equation, in which the transmitting aperture spacing error has no influence on frequency spectrum nor imaging quality. To solve the above equation, an optimized method of linear programming is proposed and the optimized matrix of aperture spacing error is obtained. In practice, the influence of aperture spacing error on imaging quality can be largely counteracted by reasonably allocating aperture spacing error according to the optimized spacing error matrix. The correctness and validity of the theoretical model are verified by a simulation experiment. The results show that the Strehl ratio of imaging quality index can be improved by about 100% through using the proposed method, the greater the aperture spacing error, the higher the Strehl ratio of imaging quality index obtained by the method will be. In addition, the method is easy to use practically and less costly as well. Finally, it is concluded that the proposed method can easily and effectively counteract the degrading effect of aperture spacing error on imaging quality. The research can provide a theoretical basis for improving imaging quality and reasonably assigning transmitter aperture spacing accuracy of coherent field imaging telescope.

스테레오 음향학적 에코 제거를 위한 Soft Decision 기반 필터 확장 기법

We propose a novel approach for stereophonic acoustic echo suppression using spectro-temporal filtering based on soft decision. Unlike the conventional approaches estimating the echo pathes directly, the proposed technique can estimate stereo echo spectra without any double-talk detector. In order to improve the estimation of echo spectra, the extended power spectrum density matrix and echo overestimation control matrix are applied on this method. In addition, this echo suppression technique is based on soft decision technique using speech absence probability in STFT domain. Experimental results show that the proposed method improves compared with the conventional approaches. 논문 / 스테레오 음향학적 에코 제거를 위한 Soft Decision 기반 필터 확장 기법

EChO spectra and stellar activity – I

Stellar activity is the major astrophysical limiting factor for the study of planetary atmospheres. Its variability and spectral characteristics may affect the extraction of the planetary signal even for moderately active stars. A technique based on spectral change in the visible band was developed to estimate the effects in the infrared due to star activity. This method has been purposely developed for the EChO mission which had the crucial characteristics of monitoring simultaneously a broadband from visible to infrared. Thanks to this capability the optical spectrum, whose variations are mainly due to stellar activity, has been used as in an instantaneous calibrator to correct the infrared spectrum. The technique is based on principal component analysis which significantly reduces the dimensionality of the spectra. The method was tested on a set of simulations with realistic photon noise. It can be generalized to any chromatic variability effects provided that optical and infrared variations are correlated.

Properties of Films Characterized by Scanning Acoustic Microscope

The scanning acoustic microscope is used to detect the properties of films. The ultrasonic wave propagates in the films with thickness h, acoustic impedance Z2 between medium with acoustic impedance Z1. The echoes from upper and lower interfaces overlap and interfere. The echoes are transformed by FFT. The interference phenomena are observed in amplitude spectrum of echoes. The spectrum has periodic extreme values at fn, fn=nc/2h. When thickness h is known, sound velocity c2 of film can be calculated. According to the principle, the properties of films such as thickness, acoustic impendence and elastic modulus are evaluated by scanning acoustic microscopy. The experimental results are good accorded with the actual properties of specimens.

The sky-wave radar detection performance computing based on the dynamic ionospheric model

The time-varying, dispersion and non-stationary ionospheric characteristics cause the sky-wave radar echo spectra broaden and the layer-structure characteristics make the multimode and multipath propagation occur, which seriously affect the detection performance of the OTHR. Therefore, in this paper, using the ionospheric model provided by CCIR as reference and the data from the Hainan ionosonde, a dynamic ionospheric model is proposed to assess the detection performance of the sky-wave radar. We will study the detection performance of the sky-wave radar by the quality of the propagation path from three aspects. Firstly, the coverage of the sky-wave radar under the given ionospheric parameters and the working frequency is calculated. Secondly, whether the multimode phenomenon appeared in the propagation is considered. Thirdly, whether the multipath phenomenon occurred is studied. Then a program is developed to assess the radar detection capability based on the propagation path analysis, which can provide the working parameters for the sky-wave radar.

Species classification of individual fish using the support vector machine

The fish species classification using echo-sounder is important for fisheries. In the case of fish school of mixed species, it is necessary to classify individual fish species by isolating echoes from multiple fish. A broadband signal, which offered the advantage of high range resolution, was applied to detect individual fish for this purpose. The positions of fish were estimated from the time difference of arrivals by using the split-beam system. The target strength (TS) spectrum of individual fish echo was computed from the isolated echo and the estimated position. In this paper, the Support Vector Machine was introduced to classify fish species by using these TS spectra. In addition, it is well known that the TS spectra are dependent on not only fish species but also fish size. Therefore, it is necessary to classify both fish species and size by using these features. We tried to classify two species and two sizes of schools. Subject species were chub mackerel (Scomber japonicas) and Japanese jack mackerel (Trachurus japonicus). We calculated the classification rates to limit the training data, frequency bandwidth and tilt angles. It was clarified that the best classification rate was 71.6 %. [This research was supported by JST, CREST.]

Radar Scattering From the Composite Ship-Ocean Scene: Doppler Spectrum Analysis Based on the Motion of Six Degrees of Freedom

In general, a ship moves with translation and rotation under the action of water. The investigation of the motion of ship is useful for the scattering echo simulation. In this communication, we firstly focus on modeling and simulation for the motion of ship in six degrees of freedom in two dimensional (2-D) sea surface based on the program public domain strip method (PDSTRIP). Then, some further simulations and the influence of various parameters (such as the polarization, incident angle and wind direction) on the Doppler spectrum are analyzed to prove that the facet-based asymptotical model is an effective model to investigate the EM scattering from the 2-D sea surface. Finally, in consideration of the motion of ship in six degrees of freedom, the radar echo and Doppler spectrum of the dynamic composite maritime scene are then studied based on the proposed specular reflection weighted four-path model. It is found that the motion of ship has the most significant effect on the RCS and Doppler spectrum.

Four‐and‐one‐half years' experience in monitoring of reproducibility of an MR spectroscopy system — application of in vitro results to interpretation of in vivo data

The primary purpose of this work was to assess long‐term in vitro reproducibility of metabolite levels measured using 1H MRS (proton magnetic resonance spectroscopy). The secondary purpose was to use the in vitro results for interpretation of ‘H MRS in vivo spectra acquired from patients diagnosed with Canavan disease. 1H MRS measurements were performed in the period from April 2006 to September 2010. 118 short and 116 long echo spectra were acquired from a stable phantom during this period. Change‐point analysis of the in vitro N‐acetylaspartate levels was exploited in the computation of fT factor (ratio of the actual to the reference N‐acetylaspartate level normalized by the reciprocity principle). This coefficient was utilized in the interpretation of in vivo spectra analyzed using absolute reference technique. The monitored time period was divided into six time intervals based on short echo in vitro data (seven time intervals based on long echo in vitro data) characterized by fT coefficient ranging from 0.97 to 1.09 (based on short echo data) and from 1.0 to 1.11 (based on long echo data). Application of this coefficient to interpretation of in vivo spectra confirmed increased N‐acetylaspartate level in Canavan disease. Long‐term monitoring of an MRS system reproducibility, allowing for absolute referencing of metabolite levels, facilitates interpretation of metabolic changes in white matter disorders.PACS numbers: 87.19.lf, 87.61.Tg, 87.64.K‐, 87.64.kj

Biosonar auditory model for target shape perception and clutter rejection

Big brown bats (Eptesicus fuscus) emit widely beamed biosonar sounds that contain two prominent harmonics (FM1, FM2). They exploit the ripple spectrum of multiple-glint echoes to perceive target shape and the relative weakening of higher harmonic FM2 frequencies in lowpass echoes from the surrounding scene to suppress clutter. Delay discrimination experiments using electronically generated echoes show that (1) flat-spectrum or ripple-spectrum echoes are perceived as having sharply focused delay images, (2) lowpass filtering causes defocussing of perceived delay, and (3) masking release occurs between defocused images of lowpass echoes and focused images of full-band echoes. A time-frequency auditory model for the decomposition of FM echoes into temporal and spectral dimensions, followed by reconstitution of object images in terms of delay alone accounts for both shape perception and the release from clutter masking. The model's structure resembles a neuronal spectral pattern-recognizing network grafted onto a neuronal delay-line/coincidence-detection delay-determining network. The categorical segregation of focused target shape from defocused clutter emerges from a novel anticorrelation process that depends on the auditory system's restricted range of lateral interactions across adjacent frequencies. [Work supported by ONR, NSF, BIBS, and JSPS.]

E region radar echoes from low‐latitude field‐aligned irregularities due to gravity waves and tides: A case study using radar, lidar, and radiosonde observations and simulations

AbstractOn the night of 5 February 2011, the height‐time plot of echo from E region field‐aligned irregularities (FAI) received by the Indian mesosphere‐stratosphere‐troposphere radar at Gadanki (13.5°N, 79.2°E) shows a slow‐descending layered structures separated by nearly 8 km in the height region 80–100 km and vertically elongated structures with quasiperiodic variations at higher heights. The stratospheric thermal structure also shows similar slowly descending structures with nearly 8 km separation. Hodograph of 7–9 km band‐pass filtered radiosonde zonal and meridional winds over Gadanki form clearly an ellipse, and the dominant period obtained from the ratio of major to minor axes of the ellipse is nearly 24 h indicating the presence of a nonmigrating diurnal tide with vertical wavelength of 8 km. The two‐dimensional spectrum of temperature perturbations, which are assumed to be due to gravity waves, shows dominant periods near 1 h and 1.5 h. The spectrum of FAI echo also shows similar dominant periods. Altitude‐time cross section of vertical ion velocity is computed using winds composed of observed gravity wave and tidal parameters. As the diurnal tide over Tirunelveli (8.7°N, 77.8°E) shows weaker amplitudes above 90 km, the removal of tidal contribution clearly reveals the observed FAI echo structure of vertically slanting plasma blob structures at higher heights and slowly descending continuous structures separated by 8 km at lower heights. The present study clearly demonstrates the role of tides and gravity waves in the formation of descending echo structures and quasiperiodic echoes respectively.

Improving the sensitivity of conventional spin echo spectra by preservation of initial signal-to-noise ratio

A simple processing strategy is introduced to enhance the spectral quality and signal-to-noise ratio in conventional J resolved spectra. The idea of pseudo echo filtering is extended to conserve the primary signal-to-noise, predominating at the beginning of the FID in the indirect dimension.This is achieved by matching the maximum amplitude of the FID with that of the sine window function. Practically, the FID is right shifted by the number of acquired points in the indirect dimension; missing data points are backward predicted and finally multiplied with the unshifted sine window function. Standard processing tools are employed for this purpose.The results of data processing using different window functions with and without right shifts and back predictions are discussed. The signal-to-noise ratio of the J resolved spectrum is increased by a factor of 6 compared to standard data processing using pseudo echo filtering alone.