Azmi A. Al‐Kurd

Source localization using a reference wave to correct for oceanic variability

This paper describes a low‐frequency, long‐range acoustic propagation method for source localization. It is based on imaging techniques that remove the effects of adiabatic oceanic variability between an acoustic receiving array and an unknown source by combining measured data from the unknown source with measured data from a known reference source. At the heart of the concept is the realization that both sets of measured data are contaminated in the same way by the range‐integrated effects of the ocean, for the stretch of ocean that they share between their positions and the receiving array. These contaminants can be made to cancel, leaving the integrated, and much smaller, effects of the oceanic variability between the reference source and the unknown source.

Performance analysis of the holographic array processing algorithm

This paper provides detailed analysis and performance evaluation of the holographic array processing (HAP) algorithm. The HAP is a source localization method that is based on medium calibration. Conventional array processing algorithms, such as matched‐field processing (MFP), require precise knowledge of the medium between the source and the receiving array, but the HAP method relaxes this stiff requirement. It calibrates the integrated effect of a great portion of the medium, and geoacoustic parameter estimation is needed only for a small portion of the ocean between the unknown source and the virtual array. The virtual array is constructed by moving a reference source to incremental depths of the water column near the target. Theoretical analysis is provided using the WKB approximation for a range dependent ocean. The numerical simulation is performed using a high‐order parabolic equation (PE) code for a range‐dependent analytical sound‐speed profile (SSP), and measured sound‐speed data from the North P...

Holographic array processing in a range‐dependent ocean.

Imperfect knowledge of the salient characteristics of the propagation medium limits the performance of acoustic array processors at long ranges in the ocean. Holographic and phase conjugation techniques can be used to diminish the range‐integrated effect of the medium and reconstruct the wave front in the vicinity of a scatterer or other signal source. Then, using a back‐propagation technique that focuses sound at the position of the unknown source, the location of the source can be determine. In this paper, the WKB approximation is implemented for a range‐dependent ocean with both N2 bilinear and exponential stratification of the sound‐speed profile. The results of the analysis and simulation show the possibility of localizing a source at large distance with great accuracy.

Temporal coherence: A critical function for phase coherent underwater acoustic communications

The temporal evolution of an acoustic channel can be characterized by its temporal correlation function from which temporal coherence (the value) and temporal correlation time (at coherence equal to, say, 0.5) can be deduced. Temporal coherence and correlation time have been experimentally measured in the past when the coherence time is much longer than the pulse duration. This is the intra‐packet coherence. As frequency increases to tens of kHz or above, the coherence time can be shorter than the packet length. Under such circumstances, the channel coherence can impede the equalizer performance. In the extreme case when the coherence time is shorter than the multipath delay time, the later arrivals may travel through a channel that is different from the earlier arrivals. This paper presents the simulation of communications signals through a time‐evolving channel including random media due to internal and turbulence for which temporal correlation time can be predicted for stationary source/receivers. For ...

The effect of diffused arrivals on the performance of phase coherent underwater acoustic communications

The delayed multipath arrivals cause intersymbol interference (ISI) in underwater acoustic communications, which can sometimes extend over tens to hundreds of symbols. The performance of the decision feedback equalizer (DFE) requires a reasonably accurate estimate of the number of tap coefficients as too few taps fail to remove the ISI and too many taps increases the mean square error (the processor noise). An initial estimate of the channel impulse response is often used to determine the number and positions of the taps. This decision is clear‐cut for well‐defined (usually high‐amplitude) arrivals but ambiguous for diffused arrivals that are weak and undistinguished. It should be noted that diffused arrivals can be a significant component of the signal, resulting usually from sound scattering from fluctuating surface/bottom and random media. Its amplitude and complexity are compounded and enhanced by platform motion. This work illustrates the effect of diffused arrivals on the performance of the equalize...

Coherent underwater digital communication during LWAD 99‐1 experiment: Performance and analysis

Underwater acoustic communications data using phase‐modulated signals were collected during the Littoral Warfare Advanced Development (LWAD 99‐1) experiment. The signals were projected from a drifting and a towed source. Data of several band rates were transmitted and used to study the temporal and spatial variation of the acoustic impulse response of the ocean and to evaluate the performance of a phase coherent digital communication algorithm. Postexperimental analysis showed that the experiment site is a harsh environment for coherent digital communication. The oceanic channel impulse response is very complex and represents a dynamic and extended multipath structure (changes in seconds). Also, the first arrival is weaker than the later arrivals (most of the time). In addition to the environmental limitations the receiver has to overcome system limitations and operate in a low signal‐to‐noise ratio conditions. Spatial and temporal diversity are implemented to improve the algorithm performance. The object...

Experimental evaluation of the performance of coherent digital communications algorithm in littoral ocean

Underwater acoustic communications data using phase modulated signals were collected during the Littoral Warfare Advanced Development 98‐1 experiment, conducted in the Gulf of Mexico in Nov. 1998. CW pulse and quadrature phase shifted keying (QPSK) signals were projected from a towed source. Data of three baud rates were collected to study the temporal and spatial variation of the acoustic impulse response of the ocean and to evaluate the performance of a phase coherent digital communication algorithm. Post‐experiment analysis examined the Doppler shift, signal amplitude fluctuation, signal‐to‐noise ratio of the received signal, channel impulse response, temporal correlation function of the received signals, and signal fluctuation statistics during several segments of the experiment. The objective is to determine the characteristics of signal propagation in littoral environment, and to determine whether and how these characteristics affect the bit‐error‐rate of a coherent acoustic communication algorithm....

Sea‐surface roughness effect on underwater coherent acoustic communications

The ocean represents a band‐limited, temporally unstable multipath, and rapidly fading communication channel. Although the joint implementation of an adaptive decision feedback equalizer and phase‐locked loop synchronizer updates the receiver filter coefficients to offset the signal error introduced by the time‐varying channel transfer function, the effects of the temporal variation on the symbol rate and bit error rate have not been quantitatively investigated. This is because several oceanographic processes with inherently different temporal and spatial scales are simultaneously contributing to the amplitude and phase fluctuations of the signal; these factors are not easily isolated in the received data. In this study a full‐field wave propagation code is used to simulate the transfer function for a temporally varying oceanic waveguide. An exact and approximate methods are being used to compute the rough surface scatter. The signal intensity and phase fluctuation statistics are investigated, and the eff...

Wavefront Reconstruction Applied to Matched Field Processing in Variable, Multimode Waveguides

Signals in multimode waveguides such as the SOFAR channel are dispersed and distorted during propagation. Holographic processing techniques that use a reference source can unravel these signals. Processing reconstructs an image of the wavefront in the vicinity of the signal source without explicit knowledge of the sound speed structure. We use this image, as constructed across the axis of the oceanic waveguide, in a source localization scheme that is closely related to matched field processing. The main advantage of this procedure is that it is independent of the variability in the sound speed structure between the source (reference and target) region and the receiving array. We show localization results for modeled and measured range-dependent sound speed profiles.

Holographic array processing using truncated arrays

Imperfect knowledge of the salient characteristics of the propagation medium limits the performance of acoustic array processors at long ranges in the ocean. Holographic and phase conjugation techniques can be used to diminish the range integrated effect of the medium and reconstruct the wave front in the vicinity of a scatterer or other signal source. Then, using a backpropagation technique, which focuses sound at the position of the unknown source, the location of the source can be determined. In deep water, the idealistic situation of having a receiving array and reference source that span the water column is prohibitively complex. In this paper the effect of truncating both the receiving and virtual arrays on the performance of the holographic array processing algorithm is presented. The analysis was carried out using normal mode theory, and the simulation for a range‐dependent ocean was performed using a wide angle PE (parabolic equation) code, FEPE. It is shown that the holographic array processing ...