Echo Arrival Times Research Articles

Improved measurement of the glue layer in composite material by using sparse deconvolution.

Due to its powerful penetration, and greater spatial resolution than microwaves and ultrasonic waves, the terahertz technique stands out as being particularly useful in identifying thin glue layers in multilayered materials. However, the arrival times of echoes are challenging to pinpoint from the experimental data because of the temporal form of the incident pulse and the system noise. Here, two terahertz signal sparse deconvolution algorithms are studied to more accurately identify the times of the echoes. Using the circulant structure of the convolution matrix, the method's computation time can be lowered to hundreds of milliseconds. In addition, a method based on group velocity dispersion is investigated to reduce the impact of time-varying pulses with minimal computational expense. The presented algorithms have the potential to be employed in real-time inspection in production lines due to their quick speed and high confidence.

Astrophysical gravitational-wave echoes from galactic nuclei

ABSTRACT Galactic nuclei (GNs) are dense stellar environments abundant in gravitational-wave (GW) sources for the Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo, and Kamioka Gravitational Wave Detector (KAGRA). The GWs may be generated by stellar-mass black hole (BH) or neutron star mergers following gravitational bremsstrahlung, dynamical scattering encounters, Kozai–Lidov-type oscillations driven by the central supermassive black hole (SMBH), or gas-assisted mergers if present. In this paper, we examine a smoking gun signature to identify sources in GNs: the GWs scattered by the central SMBH. This produces a secondary signal, an astrophysical GW echo, which has a very similar time–frequency evolution as the primary signal but arrives after a time delay. We determine the amplitude and time-delay distribution of the GW echo as a function of source distance from the SMBH. Between ${\sim} 10{{\ \rm per\ cent}}\hbox{ and }90{{\ \rm per\ cent}}$ of the detectable echoes arrive within ${\sim} (1\hbox{--}100)M_6\, \mathrm{s}$ after the primary GW for sources between 10 and 104 Schwarzschild radius, where $M_6=M_{{\rm SMBH},z}/(10^6\, \mathrm{M}_{\odot })$, and MSMBH, z is the observer-frame SMBH mass. The echo arrival times are systematically longer for high signal-to-noise ratio (SNR) primary GWs, where the GW echo rays are scattered at large deflection angles. In particular, ${\sim} 10{{\ \rm per\ cent}}\hbox{--}90{{\ \rm per\ cent}}$ of the distribution is shifted to ${\sim} (5\hbox{--}1800)M_6\, \mathrm{s}$ for sources, where the lower limit of echo detection is 0.02 of the primary signal amplitude. We find that ${\sim} 5{{\ \rm per\ cent}}\hbox{--}30{{\ \rm per\ cent}}$ (${\sim} 1{{\ \rm per\ cent}}\hbox{--}7{{\ \rm per\ cent}}$) of GW sources have an echo amplitude larger than 0.2–0.05 times the amplitude of the primary signal if the source distance from the SMBH is 50 (200) Schwarzschild radius. Non-detections can rule out that a GW source is near an SMBH.

Terahertz Superresolution Stratigraphic Characterization of Multilayered Structures Using Sparse Deconvolution

Terahertz sparse deconvolution based on an iterative shrinkage algorithm is presented in this study to characterize multilayered structures. With an upsampling approach, sparse deconvolution with superresolution is developed to overcome the time resolution limited by the sampling period in the measurement and increase the precision of the estimation of echo arrival times. A simple but effective time-domain model for describing the temporal pulse spreading due to the frequency-dependent loss is also designed and introduced into the algorithm, which greatly improves the performance of sparse deconvolution in processing time-varying pulses during the propagation of terahertz waves in materials. Numerical simulations and experimental measurements verify the algorithms and show that sparse deconvolution can be considered as an effective tool for terahertz nondestructive characterization of multilayered structures.

Target range discrimination by the bottlenose dolphin

In order to measure range accuracy of the dolphin’s sonar, a range discrimination experiment was conducted. To prevent simultaneous exposure of both targets to each dolphin’s click, targets were submerged under water at different ranges in sequence with a 0.3–0.5-sec delay. Otherwise, if presented simultaneously, the targets could be distinguished by difference in the echo arrival times, without measuring the target ranges. Couples of identical spheres as well as targets of a different size and shape were tested. Nevertheless, it proved to be impossible to make the dolphin’s measure the distance from themselves to targets. Both dolphins solved the task by measuring a time interval between the target echo and an echo from a reference object, which could be the pool walls or even a net. There was no consistent change in the range accuracy with the change of the target range whereas it regularly increased when the targets were moved closer to the reference object. The dolphins did not keep the distance from the targets as short as possible. Instead, by backing off from the targets they tried to position the target echoes as close as possible to the reference echo. It appeared that the dolphins were incapable of measuring a target range.

Real time mobile robot sonar with interference rejection

A new approach to rejecting interference between sonar systems is presented in this paper. The approach is based on identifying a transmitter by sending a double pulse with known separation. Using simple delayed pulse subtraction, a sonar receiver can test echo pulses quickly and simply for acceptance. This approach is implemented on a sonar system that is capable of accurately tracking objects at measurement rates exceeding 10 Hz. Two sonar sensors, each consisting of a transmitter and two receivers, are independently controlled to track objects from bearing and range measurements. Matched filtering is used to optimally estimate echo arrival times from which range and bearing angle are derived. Bearing accuracy is typically better than 0.1 degrees but degrades if there is air turbulence and temperature gradients. Bearing errors are shown experimentally to have significant autocorrelation at times of the order of seconds. The ability to reliably reject interference is demonstrated experimentally by both sensors simultaneously tracking the same plane object.

Ultrasonic classification and location of 3D room features using maximum likelihood estimation - Part I

Current mobile robot ultrasonic localisation techniques use sensor systems which rely on features in a horizontal plane. The implicit assumption is that the room boundary on the horizontal plane is not obstructed by objects such as furniture. This assumption is often not realistic and restricts the versatility and portability of these systems. The solution proposed in this paper is the provision of sensing flexibility to use other 3D room boundaries (e.g. ceiling-wall intersections) as 3D natural beacons. We propose a 3D ultrasonic sensor array that uses a Maximum Likelihood Estimator to match the echo arrival times to different object classes and to determine the location of the 3D target. This method does not require fast data acquisition or powerful computing. It has been implemented on a robot localisation application with the Extended Kalman Filter. This paper is the first of two parts, and presents theoretical results on target classification and minimum transducer requirements. The second part, in the next issue of Robotica, presents experimental results on the characterisation of the sensor and its application to robot localisation, and includes the references for the both papers.

Signal and echo arrival‐time estimation using wavelets

A wavelet‐based method is introduced for estimating the arrival times of signals and echoes observed in noisy multipath environments. This problem arises when trying to model any signal containing overlapping segments of direct and reflected components. The method presented is more general than cepstrum‐based techniques since it is not limited to echoes that are scaled replicas of the direct signal. The statistics of the wavelet coefficients are derived for pyramid‐algorithm wavelet decompositions, and these are employed in a statistical detection algorithm for obtaining the arrival‐time estimates. The method will be used to estimate signal and echo arrival times in data observed during underwater transducer calibration experiments conducted in small pressurized tanks.

Experimental study of sound scattering by elastic spheroids, and the excitation of their resonances

Experimental studies of ultrasonic scattering by submerged steel spheroids, both oblate and prolate, and of steel spheres have been carried out using incident short, sinusoidally modulated pulses. The time waveform and frequency responses of the scattering objects are obtained directly from experimental data via numerical processing. The sound pulses are incident in the axial direction and also in discrete oblique directions at every 5 deg from the axis. The sound paths of surface waves generated on the objects, causing a chain of echo pulses, are identified, echo arrival times are shown to agree with the surface wave interpretation, and resonanc spectra are extracted using the Numrich–de Billy method. For explaining the resonance spectrum, the phase-matching principle is employed. The target strength of the objects is extracted from experiment using various methods of interpretation (specular echoes, elastic-wave echoes, total echo energy, or entrance windows), and compared to the geometrical target strength. It is shown that specular echoes alone are not sufficient to explain the entire target strength, and the presence of elastic surface waves makes itself felt here.

HF multipath dispersion measurements using cepstral signal processing

A technique for measuring ionospheric echo arrival times and strengths is described. The technique uses ordinary HF‐SSB transceivers and an easy to generate probe signal. It is based on the cepstral signal processing tool for deconvolution. Identification of ionospheric echoes in a received sky wave signal was viewed as a deconvolution problem, and the cepstral transformation was used to obtain information about the multipath profile of the channel. Field tests were carried out where multipath characteristics of the ionospheric channel was measured using this technique.

Correction of refraction and other angle errors in beam tracking speed of sound estimations using multiple tracking transducers

The beam tracking approach to the estimation of the speed of sound has shown potential for making unbiased estimates in tissues. The speed of sound in a medium can be found from the arrival times of echoes as a function of the position of a tracking transducer. There is a problem in this approach if the angle between the direction of tracked beam and the direction of tracking translation is not zero due to refraction or other effects. An angle error as small as 1 degree would result in an error that is too large for diagnostic applications. A modified technique using three or more tracking transducers is described. This yields a corrected speed of sound estimate, and calculates the angle error. A simulation program has shown that this modified technique could indeed correct for the angle errors.

Localization of sounds and targets in air and water by echolocating animals

Mammals that use echolocation, or biological sonar, to perceive the locations of objects in their environments have unusually acute capacities for localizing sound (echo) sources and for perceiving reverberation since they rely upon echolocation for spatial perception. Both bats in air and porpoises in water determine the distance to targets from the arrival time of echoes. The sonar signals which they emit are directional, with the possibility that porpoises may transmit different waveforms as well as different signal amplitudes in different directions. The hearing of echolocating animals is also directional, with reception through the external‐ear system in bats and through the lower jawbone in porpoises. Directional hearing provides binaural cues for horizontal localization of targets. In bats, the external ear has directional amplitude and phase responses which the bat uses for vertical localization of targets. In porpoises it is possible that the transmitted waveform contributes to vertical localizat...

Backscattering of short ultrasonic pulses by solid elastic cylinders at large k a

Backscattering measurements from solid brass, aluminum, and lucite cylinders in water have been performed at a ka of approximately 240 using short acoustic pulses. Numerous discrete echoes are received due to multiple internal reflections of the acoustic pulse at the boundary of the cylinders. Excellent agreement between the measured echo arrival times and the echo arrival times predicted by the theory of Brill and Überall [J. Acoust. Soc. Am. 50, 921 (1979)] is obtained for all of the cylinders. The amplitudes of the backscattered echoes from the brass and aluminum cylinders are also compared with theory and fair agreement is obtained.

Sonar mapping of the underside of pack ice

An underice survey was carried out in a pack ice field near Fletcher's Ice Island (T-3) by means of a 48-kHz Kelvin Hughes transit sonar. The transducer was lowered through a hole, and discrete underice scans were made through 360°. Five sets of data have been collected and transformed into polar coordinate mosaics. The echo length, the shadow zone, the arrival time of echoes, and the sounding depth provide the basic information for interpretation. A complete underice map has been constructed to show the distribution of the underice features. The keel depths of the ridges have been estimated. A surface survey that included a cross-ridge survey was carried out. The top and bottom features have been compared. The average ratio of the peak top elevation to the keel depths is 1:7.6.