Kam W. Lo

Broadband passive acoustic technique for target motion parameter estimation

A nonlinear least-squares method is formulated to estimate the motion parameters of a target whose broadband acoustic energy emissions are received by a ground-based array of sensors. This passive technique is applied to real acoustic sensor data recorded during the passage of a variety of ground vehicles past a planar cross array and its effectiveness verified by comparing the results with the actual values of the target motion parameters. The technique cam also be applied to airborne targets.

Turboprop and rotary-wing aircraft flight parameter estimation using both narrow-band and broadband passive acoustic signal-processing methods

Flight parameter estimation methods for an airborne acoustic source can be divided into two categories, depending on whether the narrow-band lines or the broadband component of the received signal spectrum is processed to estimate the flight parameters. This paper provides a common framework for the formulation and test of two flight parameter estimation methods: one narrow band, the other broadband. The performances of the two methods are evaluated by applying them to the same acoustic data set, which is recorded by a planar array of passive acoustic sensors during multiple transits of a turboprop fixed-wing aircraft and two types of rotary-wing aircraft. The narrow-band method, which is based on a kinematic model that assumes the source travels in a straight line at constant speed and altitude, requires time-frequency analysis of the acoustic signal received by a single sensor during each aircraft transit. The broadband method is based on the same kinematic model, but requires observing the temporal variation of the differential time of arrival of the acoustic signal at each pair of sensors that comprises the planar array. Generalized cross correlation of each pair of sensor outputs using a cross-spectral phase transform prefilter provides instantaneous estimates of the differential times of arrival of the signal as the acoustic wavefront traverses the array.

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.

Aircraft flight parameter estimation using acoustic multipath delays

The signal emitted by an airborne acoustic source arrives at a stationary sensor located above a flat ground via a direct path and a ground-reflected path. The difference in the times of arrival of the direct path and ground-reflected path signal components, referred to as the multipath delay, provides an instantaneous estimate of the elevation angle of the source. A model is developed to predict the variation with time of the multipath delay for a jet aircraft or other broadband acoustic source in level flight with constant velocity over a hard ground. Based on this model, two methods are formulated to estimate the speed and altitude of the aircraft Both methods require the estimation of the multipath delay as a function of time. The methods differ only in the way the multipath delay is estimated; the first method uses the autocorrelation function, and the second uses the cepstrum, of the sensor output over a short time interval. The performances of both methods are evaluated and compared using real acoustic data. The second method provides the most precise aircraft speed and altitude estimates as compared with the first and two other existing methods.

Aircraft flight parameter estimation using acoustical Lloyd's mirror effect

A model is developed for the acoustical Lloyds mirror effect observed in the output time-frequency distribution of a microphone located near the ground during the transit of a jet aircraft. The feasibility of using this effect for flight parameter estimation is assessed by a simple Cramer-Rao lower bound analysis. The nonlinear least-squares method and the generalized Hough transform method are formulated for flight parameter estimation. The performances of both methods are evaluated and compared using real acoustic data.

Adaptive array processing for wide-band active sonars

A high-frequency sector scan sonar can be used to detect and classify man-made objects located on the sea floor. If the hydrophone array of the sonar is to be mounted on a miniaturized autonomous underwater vehicle, the aperture size of the array must be small enough so that it will not affect the vehicles maneuverability. However, a small aperture size of the array results in poor angular resolution when using the conventional delay and sum beamforming technique. This paper shows that, with the aid of spatial resampling, narrow-band adaptive beamforming algorithms can be applied to improve the angular resolution of wide-band active sonars with a small aperture uniform linear hydrophone array. Owing to the motion of the vehicle and the highly nonstationary environment, only single-snapshot adaptive beamforming algorithms can be used. In this paper, spatial smoothing is used in conjunction with the minimum variance distortionless response and multiple signal classification adaptive beamforming algorithms to produce a high-resolution image using a single ping. Real sonar images generated using these two algorithms are compared with those generated using another single-snapshot adaptive beamforming algorithm, spatial processing: optimized and constrained, and the conventional delay and sum beamforming algorithm.

Flight path estimation using frequency measurements from a wide aperture acoustic array

A narrowband technique based on the acoustical Doppler effect is proposed for estimating the trajectory of a turbo-prop aircraft in level flight with constant velocity as it transits over a ground-based passive acoustic sensor array. The basic principle is to measure the temporal variation of the instantaneous frequency (IF) of the acoustic signal received by each sensor and then to minimize the sum of the squared deviations of the IF estimates from their predicted values over a sufficiently long period of time for all sensors. The technique provides estimates of the propeller blade rate and the five source motion parameters that describe the aircraft trajectory. The six dimensional minimization problem is reduced to a five dimensional maximization problem, which is solved numerically using the quasi-Newton method. A simple method is described that provides the initial parameter estimates required for the numerical maximization. The effectiveness of the motion parameter estimation technique is verified using real acoustic data recorded from a wide aperture microphone array during various transits of a turbo-prop aircraft.

Passive wideband cross correlation with differential Doppler compensation using the continuous wavelet transform

An estimate of the time difference for the signal emitted by a stationary source to arrive at two spatially separated sensors is given by the time displacement that maximizes the cross-correlation function. For a fast moving source, however, this estimate is found to be in error because the time scales of the received signals are different for the two sensors. The correct time delay can be extracted by evaluating the continuous wavelet transform, which has the same functional form as the wideband cross-ambiguity function. When the signal-to-noise ratio is high, the coordinates of the ambiguity surface’s global maximum provide reliable estimates of both the differential time of arrival (or time delay) and the ratio of the time scales of the signals received by the two sensors. The continuous wavelet transform is computed using the one-step chirp z-transform method, the cross-wavelet transform method, and the two-step methods where multirate sampled replicas of the sensor waveforms are cross correlated, or ...

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.

Transiting aircraft parameter estimation using underwater acoustic sensor data

Sound from an airborne source travels to a receiver beneath the sea surface via a geometric path that is most simply described using ray theory, where the atmosphere and the sea are assumed to be isospeed sound propagation media separated by a planar surface (the air-sea interface). This theoretical approach leads to the development of a time-frequency model for the signal received by a single underwater acoustic sensor and a time-delay model for the signals received by a pair of spatially separated underwater acoustic sensors. The validity of these models is verified using spatially averaged experimental data recorded from a linear array of hydrophones during various transits of a turboprop aircraft. The same approach is used to solve the inverse time-frequency problem, that is, estimation of the aircrafts speed, altitude, and propeller blade rate given the observed variation with time of the instantaneous frequency of the received signal. Similarly, the inverse time-delay problem is considered whereby the speed and altitude of the aircraft are estimated using the differential time-of-arrival information from each of two adjacent pairs of widely spaced hydrophones (with one hydrophone being common to each pair). It is found that the solutions to each of the inverse problems provide reliable estimates of the speed and altitude of the aircraft, with the inverse time-frequency method also providing an estimate that closely matches the actual propeller blade rate.