Active Sonar Detection Research Articles

Richard H. Bolt introduced me to acoustics, and I am still engaged

I arrived at the MIT Acoustics Laboratory in 1948, and quickly learned from Dick Bolt that U.S. submarines needed coatings to reduce vulnerability via active sonar detection. All who remember Dick will be familiar with his next step: He brought me to Robert Blizzard and A. Wilson Nolle, both then at the Lab, from whom I learned in detail how to measure and then how to understand the dynamic moduli of rubber-like materials. My path, of course, widened to include medical acoustics (under the leadership of Theodore Hueter) and scattering (Phillip Morse and Uno Ingard). Fast-forward to the present. Rubber-like materials are not only in use as submarine coatings to avoid detection by active sonars, but also to reduce noise radiated by submarines. Further, such materials cover submarine acoustic arrays to reduce flow–noise interference with signals received by such arrays. The problem that most engages me today is the stochastic nature of sound propagation in the ocean, a problem that has the practical consequence of degraded sonar performance. In the spirit of Dick, I will present snippets of my current work, with the hope of painting the big picture that he always did so well.

Active sonar detection in shallow water using the Page test

The use of active sonar in shallow water results in received echoes that may be considerably spread in time compared to the resolution of the transmitted waveform. The duration and structure of the spreading and the time of occurrence of the received echo are unknown without accurate knowledge of the environment and a priori information on the location and reflection properties of the target. A sequential detector based on the Page test is proposed for the detection of time-spread active sonar echoes. The detector also provides estimates of the starting and stopping times of the received echo. This signal segmentation is crucial to allow further processing such as more accurate range and bearing localization, depth localization, or classification. The detector is designed to exploit the time spreading of the received echo and is tuned as a function of range to the expected signal-to-noise ratio (SNR) as determined by the transmitted signal power, transmission loss, approximate target strength, and the estimated noise background level. The theoretical false alarm and detection performance of the proposed detector, the standard Page test, and the conventional thresholded matched filter detector are compared as a function of range, echo duration, SNR, and the mismatch between the actual and assumed SNR. The proposed detector and the standard Page test are seen to perform better than the conventional thresholded matched filter detector as soon as the received echo is minimally spread in time. The use of the proposed detector and the standard Page test in active sonar is illustrated with reverberation data containing target-like echoes from geological features, where it was seen that the proposed detector was able to suppress reverberation generated false alarms that were detected by the standard Page test.

Detection-tracking performance with combined waveforms

It is commonly understood that in radar or active sonar detection systems constant-frequency (CF) pulses correspond to good Doppler but poor delay resolution capability; and that linearly swept frequency (FM) pulses have the opposite behavior. Many systems are capable of both types of operation, and hence in this paper the fusion of such pulses is examined from a system point of view (i.e., tracking performance) via hybrid conditional averaging (HYCA), a new but increasingly accepted technique for evaluating tracking performance in the presence of missed detections. It is shown that tracking errors are highly dependent on the waveform used, and in many situations tracking performance using a good heterogeneous waveform is improved by an order of magnitude when compared with a scheme using a homogeneous pulse (CF or FM) with the same energy. Between the two types of FM considered (upsweep and downsweep), it is shown that the upsweep is always superior. Also investigated is an alternating-pulse system, which, while suboptimal, appears to offer robust performance.

Active sonar detection in multipath: A New bispectral analysis approach

Detecting active sonar returns in multipath media is a central underwater signal processing problem. This paper studies a new approach to active sonar detection using bispectral analysis of sonar data. Its sensitivity to non-stationarities is used to develop a threshold detector that can be applied to broad classes of signals and noise. Precise statistical descriptions of the underwater medium and noise are not required. Theoretical analyses predicting its performance as a function of signal-to-noise ratio and time-bandwidth product are presented. Computer simulation experiments verify the results and show that its performance compares favorably to that of conventional detectors.

Real time time-frequency active sonar processing: a SIMD approach

A paradigm for massively parallel processing of matched filters, replica correlators, ambiguity functions, and time-frequency distributions is presented, using a SIMD (single instruction stream, multiple data stream) programming methodology. It is shown that active sonar detection algorithms, as implemented by frequency domain processing, can be a natural match to a SIMD methodology, meeting the extensive computational needs of enhanced active sonar systems. The decomposition process is presented, and examples are given of the output of the computer program CMASP (Connection Machine Ambiguity Surface Processor). CMASP can provide real-time simultaneous multiple-beam, Doppler, and waveform replica correlations. Synthetic data are processed, and the corresponding CMASP outputs are displayed as three-dimensional ambiguity surfaces on networked graphic workstations. Because of efficient problem decomposition, other time-frequency processing can be exploited. Specifically, real-time instantaneous-like time-frequency distributions have been realized in which the data set is presented and processed as time-varying spectral representations. >

Active sonar and transient detection using polyspectra

The detection of transients is a special case of nonstationary time series. It encompasses the detection of transient sonar signals, including active sonar returns. The detection of active sonar returns is generally treated as a matched-filter problem. The matched filter requires total a priori knowledge of the signal which, ideally, is stored in the receiver in the form of a “replica.” Since both the sonar target and the propagation channel distort the signal, and the time of arrival is usually not known, this replica of the signal is never known exactly, and therefore the matched filter is always applied suboptimally in practice. In this paper, the application of polyspectra to the general problem of the detection of a deterministic transient signal in stationary but not necessarily Gaussian noise is examined. It is applicable to both transient and active sonar detection since the exact configuration of the signal does not need to be known a priori. This, coupled with the fact that there is no need to assume Gaussianity on the part of the stochastic part gives the method great power. Detection statistics are computed for the method and compared to those for the matched filter. Also, results using synthetic data will be shown.

Optimum frequencies for noise‐limited active sonar detection

The curves of optimum frequencies versus maximum range for active sonar detection under specific sets of assumptions are presented for the more recent expressions for attenuation given by Lovett [J. Acoust. Soc. Am. 58, 620–625 (1975)] for the eastern North Pacific and Thorp [J. Acoust. Soc. Am. 42, 270 (1967)] for the western North Atlantic as corrected at low frequencies by Kibblewhite et al. [J. Acoust. Soc. Am. 60, 1040–1047 (1976)].

A Theory of Active Sonar Detection

The results of a theory of echo detection developed for radar are extended to sonar for the case of narrow-banid transmitted signals. The exposition specifically omits discussion of the decision problem and the statistical nature of the whole process of detection. The theory assumes that the received signal is processed by cross correlation with a normalized replica of the expected signal. The problem of echo to noise is analyzed by sampling theory leading, in the case of Gaussian noise, to the energy principle which states that in all cases the output echo-to-noise ratio is the ratio of the input signal energy to the input noise power spectrum density. The threshold effect in detection is briefly discussed in conjunction with the problem of noise ambiguities. The problem of echo to reverberation (clutter) is treated by the use of the time and frequency shift correlation function leading to the ambiguity diagrams for various waveforms. The calculation of the relative reverberation power for short and long pulses of single frequency, FM and pseudorandom or noise-like waveforms for certain special cases is presented to illustrate the application of these principles. The paper concludes with a discussion of some limitations to the theory in terms of its narrow-band approximation, the instability of platforms and medium, and combined space-time ambiguities.