An Architecture for High Data Rate Very Low Frequency Communication

Very low frequency (VLF) communication is used for long range shore-to-ship broadcasting applications. This paper proposes an architecture for high data rate VLF communication using Gaussian minimum shift keying (GMSK) modulation and low delay parity check (LDPC) channel coding. Non-data aided techniques are designed and used for carrier phase synchronization, symbol timing recovery, and LDPC code frame synchronization. These require the estimation of the operative Eb/N0 for which a kurtosis based algorithm is used. Also, a method for modeling the probability density function of the received signal under the bit condition is presented in this regard. The modeling of atmospheric radio noise (ARN) that corrupts VLF signals is described and an algorithm for signal enhancement in the presence of ARN in given. The BER performance of the communication system is evaluated for bit rates of 400 bps, 600 bps, and 800 bps for communication bandwidth of ~200 Hz.Defence Science Journal, 2013, 63(1), pp.25-33, DOI:http://dx.doi.org/10.14429/dsj.63.376

Automatic Classification of Diving Sperm Whales by Analysis of Click Time Delay using Two Hydrophones

特集1 海中工学研究センタ

Estimation of Diving Behavior of Multiple Sperm Whales by a Pair of Hydrophone Arrays

特集1 海中工学研究センタ

Estimation of Dive Behavior and Bio-sonar Characteristics of Free-ranging Baiji(Lipotes vexillifer) Based on Acoustic Data

特集1 海中工学研究センタ

Computer model of a high-resolution imaging sonar

A computer model of a high-resolution sector-scanning sonar used for imaging objects against the sea bottom is presented. The model accounts for the sonar parameters, bottom backscatter, reflections from the target's visible surface, and the target's acoustic shadow. A variety of imaging scenarios can be simulated including the type of target, the target's orientation relative to the sonar and the bottom, and the backscatter statistics. The acoustic images are presented in the conventional B-scan format. A new display format that is useful for visualizing the target's silhouette is also presented. Visual perspective images of the scene are presented to serve as a reference for subsequent image reconstruction work. Preliminary results for 3-d reconstruction of the confining volume of the visible target surface are also presented. The simulation model and the results of the 3-d reconstruction demonstrate the promise of the application of image processing techniques for classification of objects using acoustic images. (kr)Naval Postgraduate School, Monterey, CAhttp://archive.org/details/computermodelofh00bahlNational Research CouncilThis report was prepared in conjunction with a National Research Council Senior Research Associateship awarded to Rajendar Bahl.Approved for public release; distribution is unlimited

Microphone Based Acoustic Vector Sensor for Direction Finding with Bias Removal

The acoustic vector sensor (AVS) is used to measure the acoustic intensity, which gives the direction-of-arrival (DOA) of an acoustic source. However, while estimating the DOA from the measured acoustic intensity the finite microphone separation (d) in a practical AVS causes angular bias. Also, in the presence of noise there exists a trade off between the bias (strictly increasing function of d) and variance (strictly decreasing function of d) of the DOA estimate. In this paper, we propose a novel method for mitigating the angular bias caused due to finite microphone separation in an AVS. We have reduced the variance by increasing the microphone separation and then removed the bias with the proposed bias model. Our approach employs the finite element method (FEM) and curves fitting to model the angular bias in terms of microphone separations and frequency of a narrowband signal. Further, the bias correction algorithm based on the intensity spectrum has been proposed to improve the DOA estimation accuracy of a broadband signal. Simulation results demonstrate that the proposed bias correction scheme significantly reduces the angular bias and improves the root mean square angular error (RMSAE) in the presence of noise. Experiments have been performed in an acoustic full anechoic room to corroborate the effect of microphone separation on DOA estimation and the efficacy of the bias correction method

Direction Estimation and Tracking of Coherent Sources Using a Single Acoustic Vector Sensor

A single acoustic vector sensor (AVS) cannot be used to find the direction-of-arrival (DOA) of two or more coherent (fully correlated) sources. We have proposed a technique for estimating DOAs (in 2D geometry) of two simultaneous coherent sources using single AVS under the assumption that acoustic sources enter in the field sequentially. The DOA estimation has been investigated with two different configurations of AVS, each consisting of three microphones in a plane. The technique has been also applied in tracking (a) an acoustic source in the presence of stationary interfering coherent source and (b) two coherent sources when the sources are changing their locations alternatively. The experimental environment has been generated using the Finite-Element Method tool viz. COMSOL to corroborate the proposed scheme

Studies on underwater acoustic vector sensor for passive estimation of direction of arrival of radiating acoustic signal

213-219<span style="font-size:9.0pt;font-family: " times="" new="" roman","serif";mso-fareast-font-family:"times="" roman";mso-bidi-font-family:="" mangal;mso-ansi-language:en-gb;mso-fareast-language:en-us;mso-bidi-language:="" hi"="" lang="EN-GB">A passive underwater surveillance system performs the function of detection and localisation by utilising the acoustic emissions from the source, whilst itself remaining concealed. The conventional technique employs an array of hydrophones (measure the scalar pressure), wherein the size of the array is dependent on the desired angular resolution and the wavelength of the signal to be monitored. The “vector” sensors, on the other hand, measure the  scalar pressure as well as the orthogonal components of particle velocity using a co-located sensor and offer decided advantages over the traditional solution due to their smaller size and frequency independent performance (over the frequency range specified for constituent particle velocity sensors). This paper presents studies on a prototype underwater acoustic vector sensor. The present approach employs a tri-axial accelerometer as a co-located particle acceleration sensor, and along with a hydrophone it provides four sets of measurements of an acoustic field. The parameter (Direction of Arrival) estimation has been carried out using both subspace based methods as well as beamforming technique for demonstration of the acoustic vector sensor system in the lab environment and its performance analysis.</span