Philip R. Atkins

A routing and channel-access approach for an ad hoc underwater acoustic network

In this paper, the concept of an ad hoc underwater acoustic network (UAN) is presented. It seeks to meet two critical needs: to establish a long-range and decentralised communication link, and to maintain a reliable on-demand communication link with minimal human intervention. Despite having the same goal, the operating characteristics of an ad hoc UAN usually differs from its radio counterpart due to the nature of its target applications and the environment in which it operates. The differences must be taken into account in order to achieve an optimal system. Such a scheme is based upon the ad hoc on-demand distance vector (AODV) algorithms developed for wireless radio networks. The considerations necessary for adopting these algorithms are discussed. The multiple-access with collision avoidance and acknowledgement (MACAW) scheme is implemented for the control of medium access. The issues and challenges faced, along with the viability of alternative techniques are discussed. Simulation results are presented.

Calibration sphere for low-frequency parametric sonars

The problem of calibrating parametric sonar systems at low difference frequencies used in backscattering applications is addressed. A particular parametric sonar is considered: the Simrad TOPAS PS18 Parametric Sub-bottom Profiler. This generates difference-frequency signals in the band 0.5-6 kHz. A standard target is specified according to optimization conditions based on maximizing the target strength consistent with the target strength being independent of orientation and the target being physically manageable. The second condition is expressed as the target having an immersion weight less than 200 N. The result is a 280-mm-diam sphere of aluminum. Its target strength varies from -43.4 dB at 0.5 kHz to -20.2 dB at 6 kHz. Maximum excursions in target strength over the frequency band due to uncertainty in material properties of the sphere are of order +/-0.1 dB. Maximum excursions in target strength due to variations in mass density and sound speed of the immersion medium are larger, but can be eliminated by attention to the hydrographic conditions. The results are also applicable to the standard-target calibration of conventional sonars operating at low-kilohertz frequencies.

Transmit-Signal Design and Processing Strategies for Sonar Target Phase Measurement

The majority of active sonar systems detect and classify a target based on the amplitude of the received echo strength or the induced Doppler shift. However, additional classification information is available from the phase shift introduced by some targets as a result of the acoustic boundary conditions. For example, reverberation returns from the sea surface and from the swimbladders of various fish introduce an additional phase shift that is not usually present in returns from the seabed or man-made targets. Techniques based on the use of sub-band correlators are presented for measuring the phase shifts associated with certain stationary and moving targets when insonified by broadband transmissions. The performance of the target-phase measurement technique is derived for transmit signals used in existing sonar systems such as continuous wave (CW), linear frequency-modulated (LFM), and hyperbolic frequency-modulated (HFM) pulses in the presence of target or platform motion. The use of HFM signal processing with sub-band correlators to measure the Doppler shift of very slowly moving targets is presented. Modifications to sinusoidal frequency-modulated (SFM) and HFM transmission types are proposed in order to maintain a good compromise between overall detection and target-phase classification performance, i.e., between range resolution and phase measurement. Field trial results are presented for a plankton classification sonar system and a collision-avoidance system designed for operation near the sea surface

Estimation of source bearings at a randomly deformed array by signal space transformations

A novel approach to array processing is presented which allows high resolution bearing estimation from arrays where the exact sensor locations are not known. It involves rotating weighted eigenvectors in the signal subspace until they resemble wavefront phase delay vectors. In the case of a deformed linear array the phase of these vectors may then be unwrapped and a least squares fit performed. The gradient represents the source wavenumber and the deviations from linear are due to the sensor displacements. The iterative method bears some resemblance to the maximum likelihood solution, but has reduced computational complexity. A simple algorithm involving an eigendecomposition at each iteration is given to present the ideal convergence characteristics. This decomposition is performed on a Hermitian symmetric M*M matrix where M is the number of sources.<<ETX>>

Phase calibration of sonar systems using standard targets and dual-frequency transmission pulses

The phase angle component of the complex frequency response of a sonar system operating near transducer resonance is usually distorted. Interpretation and classification of the received sonar signal benefits from the preservation of waveform fidelity over the full bandwidth. A calibration process that measures the phase response in addition to the amplitude response is thus required. This paper describes an extension to the standard-target calibration method to include phase angle, without affecting the experimental apparatus, by using dual-frequency transmission pulses and frequency-domain data processing. This approach reduces the impact of unknown range and sound speed parameters upon phase calibration accuracy, as target phase is determined from the relationship of the two frequency components instead of relying on a local phase reference. Tungsten carbide spheres of various sizes were used to simultaneously calibrate the amplitude and phase response of an active sonar system in a laboratory tank. Experimental measurements of target phase spectra are in good agreement with values predicted from a theoretical model based upon full-wave analysis, over an operating frequency of 50-125 kHz.

Broadband Ultrasonic Target Strengths of Hollow Ceramic Flotation Spheres

The target strengths of hollow ceramic flotation spheres manufactured by DeepSea Power and Light, Inc., and tested to a pressure equivalent of 12,000 m have been measured as a function of frequency over the bands 10-150 kHz. The spheres are made of alumina, purity 99.9%, with diameter 91.44 plusmn 0.2 mm, nominal thickness 1.3 mm, mass 140 plusmn 1 g. The target strength spectra have also been calculated theoretically assuming a mass density of 3.984 g/cm3, longitudinal-and transverse-wave sound speeds of 11000 and 6510 m/s, respectively. Measurements and computations are compared. Potential applications of the sphere as a standard acoustic target are noted.

Sonar target‐phase measurement and effects of transducer‐matching

Active sonar systems normally detect and classify a target based on the amplitude of the received echo or the induced Doppler shift. However, additional classification information may be available from the phase shift introduced by some targets as a result of the boundary conditions. For example, reverberation from the sea surface and scattering from fish swimbladders introduce an additional phase shift that may not be present in returns from an acoustically stiffer seabed or synthetic target. Algorithms based on the use of subband correlators are presented for measuring the phase shifts introduced by the boundary conditions on stationary and moving targets when insonified by broadband transmissions. These techniques are used to remove the phase shifts introduced by the unknown target. However, the unknown phase characteristics of the transducer, matching circuit, and electronic circuitry of a sonar system imply that target‐phase measurements are very difficult to make in any practical system. The effects of adding a Butterworth‐derived matching circuit to a Reson TC2130 transducer are presented for the case of sinusoidal frequency‐modulated excitation of solid elastic and thin elastic‐shelled hollow spheres. It is concluded that target‐phase measurements can enhance the classification performance of a suitably calibrated sonar system.

Robust underwater imaging with fast broadband incoherent synthetic aperture sonar

Synthetic aperture techniques have been of interest to the sonar imaging community, but suffer from the problems of maintaining phase coherence and a slow mapping rate. The need for rigorous and accurate platform trajectory monitoring often increases the design and implementation costs as well as the hardware complexity. The paper examines incoherent processing techniques and shows their potential for use in designing a robust and practical underwater imaging system. A novel, fast, incoherent, time-domain processing technique is presented. The advantages of using incoherent processing for three dimensional imaging are discussed. Broadband signals are used for achieving high range-resolution. Results from simulations and processing of sea-survey data are included.

Standard target calibration of broad-band active sonar systems in a laboratory tank

High-frequency, scientific active sonar systems are frequently calibrated using standard target spheres. Spot frequency calibration accuracies (using carefully optimized sphere diameters) of 0.1 dB have been previously established as theoretically and technically feasible. The premise of this work is that a tungsten-carbide sphere may be used to calibrate an arbitrary sonar system in a laboratory water tank. In order to achieve an accurate calibration it is necessary to obtain exact knowledge of the characteristics of the immersion medium and the target. With this aim, the temperature and salinity of the water was accurately determined, resulting in an estimate of the sound speed of the water. The density and composition of the spheres were examined using laboratory equipment. Moreover, various other sources of variability have been addressed. For the purposes of this work, seven tungsten carbide spheres of different sizes were used to perform inter-comparison calibration estimates. Measured and modeled target strength curves were compared across the bandwidth of 50 to 150 kHz. Comparative results are presented for all the spheres, with the best-case average precision of 0.22 dB being obtained for the 25 mm sphere.

Calibration of broadband sonar systems using multiple standard targets

A seven‐octave active sonar system spanning the nominal frequency range 25‐3200 kHz was deployed in Norwegian waters for the purpose of measuring the acoustic scattering characteristics of a range of marine organisms. This system transmitted linear frequency‐modulated (LFM) signals in order to achieve good range resolution and to obtain spectral information on resolved targets. Total system performance was variously measured in situ and ex situ, depending on the particular octave band, using standard‐target spheres. This enabled the frequency response of the entire system to be determined and the sidelobe level of the matched‐filter receiver to be reduced. The effects of the deep nulls encountered in the backscattered spectrum of target spheres were partially reduced by using a string of up to six spheres of different sizes and material properties. Typical results will be presented showing that such calibration procedures are sensitive the relative alignment of the sonar‐target and to sound‐speed profile ...