James A. Theriault

Volume reverberation and target echo calculations using normal modes

A shallow-water model for calculating boundary reverberation using normal modes and ray-mode analogies has been previously reported. The authors describe extensions for handling target echo, and volume reverberation within the water column and subbottom. The equations and sample results are presented for a shallow-water environment. As specific examples, subbottom reverberation is compared to boundary reverberation using Lamberts rule; and, the effects of time spreading on target echo and signal excess are illustrated.

Improved Active Sonar Performance Using Costas Waveforms

Costas waveforms are a class of waveforms having the form of frequency-hopped pulse trains. When used as a transmit waveform in an active sonar system they may provide superior performance to conventional waveforms such as continuous-wave (CW) and frequency-modulated (FM) pulses, as each Costas waveform provides both range and speed information for a target echo. Matched-filtering identifies individual Costas waveforms from among a set of Costas waveforms that are received. This characteristic allows for more frequent ensonification of a water mass for targets at moderate ranges, giving a higher rate of return echoes for improved target tracking. Alternatively, it offers the potential for overlapping bandwidths and waveform types to be used in a multisonar environment. In this work, the Costas wideband ambiguity, cross- and autocorrelation functions are derived. The range and speed resolutions of Costas waveforms are compared to CW and FM pulse resolutions both via a broadband pulse propagation model and using data obtained during the Defence Research And Development Canada-Atlantic (DRDC Atlantic) Towed Integrated Active-Passive Sonar (TIAPS) deep sea trials in September 2003. Costas waveforms are shown to provide simultaneous range and speed resolution in deep- and shallow-water environments, and Costas waveforms are shown to be individually identifiable at the receiver. The performance of Costas waveforms in reverberation is also briefly examined, and found to be similar to that of FM pulses.

Shallow-water low-frequency active sonar modelling issues

Developing models to predict the detection performance of low frequency active sonars requires consideration of each of the parameters in the traditional sonar equation. In many cases, it may be desirable to investigate and develop models that produce high fidelity estimates of parameters such as total reverberation energy. With each of the sonar-equation parameters, there are some fundamental modelling issues: for the purpose at hand, would ray theory, normal-mode theory, or yet another approach be appropriate? Is it acceptable to model only range-independent environments, N/spl times/2D environments, or is it necessary to model fully 3D varying environments? Furthermore, requirements on output products, graphical user interfaces (GUI), and availability of environmental information must be considered. This paper gives an overview of the issues in developing such models with examples drawn from DREAs Shallow Water Active-sonar Modelling Initiative (SWAMI).

Time-series modeling using the waveform transmission through a channel program

DRDC Atlantic has developed a coherent transmission loss model that simulates the propagation of acoustic pulses through an ocean environment. Using a set of input eigenrays and an input waveform, the WATTCH (waveform transmission through a channel) model can generate a set of output time series. Each time series represents the expected signal corresponding to a given range and depth. Assuming the required eigenrays can be generated, WATTCH can simulate the effects of complex environments. This is illustrated by examples comparing measured data to modeled data in two environments. In adddition, an example is provided using WATTCH to model data for testing a target detection algorithm.

Detection and classification of marine mammal clicks

Detection of marine mammals within an influence zone of episodal anthropogenic noise source is critical to insure the safety of the animals. Marine mammal clicks are closely modeled by AM/FM signals. The Teager-Kaiser energy operator followed by a threshold detector provides an effective means of detecting AM/FM signals. Classification of the species generating the click is done by finding the maximum cross-covariance between the power spectral density (PSD) of the received click and the PSDs of clicks which have been identified. The classification algorithm will create a new library entry when the cross-covariance is below a predefined threshold. Five species of marine mammals were classified using data from the 3rd International Workshop on Detection, Classification and Localization of Marine Mammals (DCL). This paper presented a computationally-inexpensive marine mammal detection and classification algorithm with high probabilities of detection and correct classification.

Two-way time spreading and path loss in shallow water at 20-40 kHz

Two-way time spreading and path-loss measurements were collected in water 100 m deep, off the coast of Nova Scotia. Data were collected at frequencies of 20-22 kHz, 27-29 kHz, and 35-37 kHz using linear FM pulses 0.160 s in duration. The source-receiver was an anchored, high-frequency active sonar, and the target was a free-drifting echo repeater. Sonar and target positions were recorded using a portable tracking range. In the paper, two-way time spreading and path loss measurements are compared with modeled estimates obtained using an enhanced version of the generic sonar model (GSM). The GSM estimates of time spreading due to multipath propagation compare favorably with the experimental data. The model indicates that the path loss for individual eigenrays was extremely sensitive to fluctuations in the sound-speed profile. This led to substantial variation in the model output depending on the choice of profile. In place of the model, an empirical estimate of path loss was computed from the data. We obtained a two-way spreading loss of 2[18.4log/sub 10/(R)] where R is the range from sonar to target. The data were also used to compute the standard deviation of the received echo intensity at each frequency. The standard deviation was computed two different ways. First it was computed using the peak echo level from each of the pulses at a given frequency. Then, it was computed from the total energy received from each of the pings. At all frequencies, the standard deviation was 1-2 dB lower when computed from the total received energy.

Directivity measurements of the horizontal line array projector

Defence Research and Development Canada - Atlantic (DRDC Atlantic) has completed a technology demonstration project that integrated active and passive shipborne towed array capabilities. One of the source arrays developed by the project was a 32-element horizontal projector array (HPA). HPA directivity data were recorded during a number of sea trials. This paper presents the system configuration, experimental geometries, and directivity measurements.

Resolution of Azimuthal and Vertical Arrival Angles at a Rotatable Horizontal Line Array

A Horizontal Line Array (HLA) receiver is subject to left-right bearing ambiguity. It is also susceptible to bearing bias caused by non-zero vertical arrival angles. The true bearing angle and the vertical arrival angle may both be estimated if a second observation is made, with the HLA rotated to a new heading. A closed-form solution is presented for the case of stationary source and receiver. For small observation errors, the resulting errors in the estimated angles can also be expressed in closed form.

2005 synopsis on Canadian environmental legislation and policies affecting active sonar research based in Nova Scotia

Defence Research and Development Canada (DRDC) Atlantic has a number of sonar projects that require the underwater transmission of acoustic energy. Because of potential adverse environmental effects, a number of international and Canadian laws as well as Department of National Defence (DND) policies impact on these research activities. Of particular interest for this paper are the Canadian legislation and departmental policies relating to the potential impact of active sonar research sea-tests on marine mammals.