High-precision acoustic localization of dolphin sonar click transmissions using a modified method of passive ranging by wavefront curvature.

When configured as a wide aperture array, only three hydrophones are required to localize dolphin sonar transmissions with unprecedented precision, even when the underwater sound scene of their natural habitat is complicated by many of them emitting echolocation "click" signals at the same time. Given the sensor position coordinates and speed of sound travel, the passive ranging by the wavefront curvature algorithm estimates the source range and bearing, using range difference measurements between signals, arriving at two adjacent pairs of widely spaced sensors. If the sensor positions are not strictly collinear, then the source range estimates are biased. This problem is overcome by modifying the input parameters to the basic passive ranging algorithm. The experimental results for the estimated source positions are found to agree with the predicted localization performance for a wide aperture array passive ranging sonar. The precision of the source bearing estimates is 0.005°, which is independent of the source range. The precision of the source range estimates degrades a hundredfold (from 2.5 cm to 2.6 m) for a tenfold increase in source range (33-318 m). A lower bound for the peak-to-peak source levels of Indo-Pacific bottlenose dolphins (Tursiops aduncus) is 183 ± 2 dB re 1 µPa for regular click pulses.

Acoustic ranging of small arms fire using a single sensor node collocated with the target.

A ballistic model-based method, which builds upon previous work by Lo and Ferguson [J. Acoust. Soc. Am. 132, 2997-3017 (2012)], is described for ranging small arms fire using a single acoustic sensor node collocated with the target, without a priori knowledge of the muzzle speed and ballistic constant of the bullet except that they belong to a known two-dimensional parameter space. The method requires measurements of the differential time of arrival and differential angle of arrival of the muzzle blast and ballistic shock wave at the sensor node. Its performance is evaluated using both simulated and real data.

Chalcogenide fiber-based distributed temperature sensor with sub-centimeter spatial resolution and enhanced accuracy.

We demonstrate a sub-centimeter spatial resolution fiber-based distributed temperature sensor with enhanced measurement accuracy and reduced acquisition time. Our approach employs time domain analysis of backscattered Stokes and anti-Stokes photons generated via spontaneous Raman scattering in a chalcogenide (ChG) As2S3 fiber for temperature monitoring. The sensor performance is significantly improved by exploiting the high Raman coefficient and increased refractive index of the ChG fiber. We achieve a temperature uncertainty of ± 0.65 °C for a short measurement time of only 5 seconds; whilst the detection uncertainty is less than ± 0.2 °C for a longer integration time of 2 minutes. We also investigate the optimum Stokes and anti-Stokes bands for optimal sensing performance. Our theoretical analysis shows that a small detuning frequency regime from a pump is more suitable for rapid measurements while a large detuning regime provides higher temperature resolution.

Localization of small arms fire using acoustic measurements of muzzle blast and/or ballistic shock wave arrivals.

The accurate localization of small arms fire using fixed acoustic sensors is considered. First, the conventional wavefront-curvature passive ranging method, which requires only differential time-of-arrival (DTOA) measurements of the muzzle blast wave to estimate the source position, is modified to account for sensor positions that are not strictly collinear (bowed array). Second, an existing single-sensor-node ballistic model-based localization method, which requires both DTOA and differential angle-of-arrival (DAOA) measurements of the muzzle blast wave and ballistic shock wave, is improved by replacing the basic external ballistics model (which describes the bullet's deceleration along its trajectory) with a more rigorous model and replacing the look-up table ranging procedure with a nonlinear (or polynomial) equation-based ranging procedure. Third, a new multiple-sensor-node ballistic model-based localization method, which requires only DTOA measurements of the ballistic shock wave to localize the point of fire, is formulated. The first method is applicable to situations when only the muzzle blast wave is received, whereas the third method applies when only the ballistic shock wave is received. The effectiveness of each of these methods is verified using an extensive set of real data recorded during a 7 day field experiment.

Application of acoustic reflection tomography to sonar imaging.

Computer-aided tomography is a technique for providing a two-dimensional cross-sectional view of a three-dimensional object through the digital processing of many one-dimensional views (or projections) taken at different look directions. In acoustic reflection tomography, insonifying the object and then recording the backscattered signal provides the projection information for a given look direction (or aspect angle). Processing the projection information for all possible aspect angles enables an image to be reconstructed that represents the two-dimensional spatial distribution of the object's acoustic reflectivity function when projected on the imaging plane. The shape of an idealized object, which is an elliptical cylinder, is reconstructed by applying standard backprojection, Radon transform inversion (using both convolution and filtered backprojections), and direct Fourier inversion to simulated projection data. The relative merits of the various reconstruction algorithms are assessed and the resulting shape estimates compared. For bandpass sonar data, however, the wave number components of the acoustic reflectivity function that are outside the passband are absent. This leads to the consideration of image reconstruction for bandpass data. Tomographic image reconstruction is applied to real data collected with an ultra-wideband sonar transducer to form high-resolution acoustic images of various underwater objects when the sonar and object are widely separated.

Locating far-field impulsive sound sources in air by triangulation.

The firing of a gun generates an acoustic impulse that propagates radially outwards from the source. Acoustic gun-ranging systems estimate the source position by measuring the relative time of arrival of the impulse at a number of spatially distributed acoustic sensors. The sound-ranging problem is revisited here using improved time-delay estimation methods to refine the source position estimates. The time difference for the acoustic wavefront to arrive at two spatially separated sensors is estimated by cross correlating the digitized outputs of the sensors. The time-delay estimate is used to calculate the source bearing, and the source position is cross fixed by triangulation using the bearings from two widely separated receiving nodes. The variability in the bearing and position estimates is quantified by processing acoustic sensor data recorded during field experiments for a variety of impulsive sound sources: artillery guns, mortars, and grenades. Imperfect knowledge of the effective speed of sound travel results in bias errors in the source bearing estimates, which are found to depend on the orientation of the sensor pair axis with respect to the source direction. Combining the time-delay estimates from two orthogonal pairs of sensors reduces these bias errors.

Passive ranging errors due to multipath distortion of deterministic transient signals with application to the localization of small arms fire.

A passive ranging technique based on wavefront curvature is used to estimate the ranges and bearings of impulsive sound sources represented by small arms fire. The discharge of a firearm results in the generation of a transient acoustic signal whose energy propagates radially outwards from the omnidirectional source. The radius of curvature of the spherical wavefront at any instant is equal to the instantaneous range from the source. The curvature of the acoustic wavefront is sensed with a three-microphone linear array by first estimating the differential time of arrival (or time delay) of the acoustic wavefront at each of the two adjacent sensor pairs and then processing the time-delay information to extract the range and bearing of the source. However, modeling the passive ranging performance of the wavefront curvature method for a deterministic transient signal source in a multipath environment shows that when the multipath and direct path arrivals are unresolvable, the time-delay estimates are biased which, in turn, biases the range estimates. The model explains the observed under-ranging of small arms firing positions during a field experiment.