An approach for processing sonar signals with the ultimate goal of ocean bottom sediment classification and
underwater buried target classification is presented in this paper. Work reported for sediment classification is
based on sonar data collected by one of the AN/AQS-20's sonars. Synthetic data, simulating data acquired by
parametric sonar, is employed for target classification. The technique is based on the Fractional Fourier Transform
(FrFT), which is better suited for sonar applications because FrFT uses linear chirps as basis functions. In the
first stage of the algorithm, FrFT requires finding the optimum order of the transform that can be estimated based
on the properties of the transmitted signal. Then, the magnitude of the Fractional Fourier transform for optimal
order applied to the backscattered signal is computed in order to approximate the magnitude of the bottom
impulse response. Joint time-frequency representations of the signal offer the possibility to determine the timefrequency
configuration of the signal as its characteristic features for classification purposes. The classification
is based on singular value decomposition of the time-frequency distributions applied to the impulse response.
A set of the largest singular values provides the discriminant features in a reduced dimensional space. Various
discriminant functions are employed and the performance of the classifiers is evaluated. Of particular interest
for underwater under-sediment classification applications are long targets such as cables of various diameters,
which need to be identified as different from other strong reflectors or point targets. Synthetic test data are
used to exemplify and evaluate the proposed technique for target classification. The synthetic data simulates
the impulse response of cylindrical targets buried in the seafloor sediments. Results are presented that illustrate
the processing procedure. An important characteristic of this method is that good classification accuracy of an
unknown target is achieved having only the response of a known target in the free field. The algorithm shows an
accurate way to classify buried objects under various scenarios, with high probability of correct classification.
This paper discusses recent studies on Chirp Slope Keying (CSK) as a scheme suitable for underwater communications
and presents a new study on the performance of a time-frequency receiver in a Rayleigh fading environment. As
expected, CSK proves to be a digital modulation scheme inferior in the AWGN channel as compared to traditional
schemes but very promising in more detrimental channels present in underwater communications. In effect, while most
schemes' performances decay abruptly with the addition of new disturbances, a time-frequency CSK receiver's
performance deteriorates slowly with increasing Rayleigh fading. Intuition dictates that CSK will overpower other
schemes as channel conditions continue to worsen. The receiver first uses the Wigner distribution to transform the
incoming signal to the joint time-frequency domain and then computes the Radon transform to determine the binary digit received.
In this paper we present a time-frequency approach for acoustic seabed classification. Work reported is based on sonar data collected by the Volume Search Sonar (VSS), one of the five sonar systems in the AN/AQS-20. The Volume Search Sonar is a beamformed multibeam sonar system with 27 fore and 27 aft beams, covering almost the entire water volume (from above horizontal, through vertical, back to above horizontal). The processing of a data set of measurement in shallow water is performed using the Fractional Fourier Transform algorithm in order to determine the impulse response of the sediment. The Fractional Fourier transform requires finding the optimum order of the transform that can be estimated based on the properties of the transmitted signal. Singular Value Decomposition and statistical properties of the Wigner and Choi-Williams distributions of the bottom impulse response are employed as features which are, in turn, used for classification. The Wigner distribution can be thought of as a signal energy distribution in joint time-frequency domain. Results of our study show that the proposed technique allows for accurate sediment classification of seafloor bottom data. Experimental results are shown and suggestions for future work are provided.
This paper presents several receiver structures for Chirp Slope Keying (CSK), a digital broadband modulation scheme we propose to use for underwater acoustical communications. In its simplest form, the binary information modulates the slope of a linear chirp, with up-chirps representing ones and down-chirps representing zeros. A time-domain receiver and a novel time-frequency receiver structure based on the Wigner distribution and the Radon Transform are discussed and evaluated in terms of the probability of error versus Signal-to-Noise (SNR) performance. Simulation results and plots are presented for the Additive White Gaussian Noise (AWGN) channel. Results show that if the detector at the receiver operates directly on the slope of the received signal, performance is improved at the expense of computational complexity.
This paper presents a novel broadband modulation method for digital underwater communications: Chirp Slope Keying (CSK). In its simplest form, the binary information modulates the slope of a linear chirp, with up-chirps representing ones and down-chirps representing zeros. Performance evaluation in the form of probability of error vs. SNR show that the system performs as expected for AWGN environments and very well for more realistic models for underwater acoustical communications, such as the Raylegih channel with Doppler, delays, phase offset, and multipath.
KEYWORDS: Optical signal processing, Fourier transforms, Linear filtering, Signal processing, Deconvolution, Fractional fourier transform, Convolution, Radon transform, Commercial off the shelf technology, Time-frequency analysis
In this paper we present an approach for signal enhancement of sonar signals. Work reported is based on sonar data collected by the Volume Search Sonar (VSS), as well as VSS synthetic data. The Volume Search Sonar is a beamformed multibeam sonar system with 27 fore and 27 aft beams, covering almost the entire water volume (from above horizontal, through vertical, back to above horizontal). The processing of a data set of measurement in shallow water is performed using the Fractional Fourier Transform algorithm. The proposed technique will allow efficient determination of seafloor bottom characteristics and bottom type using the reverberation signal. A study is carried out to compare the performance of the presented method with conventional methods. Results are shown and future work and recommendations are presented.