The interference characteristics of platform and towed body noise in shallow water for active/passive towed array sonar

The interference characteristics of towed platform noise resulted from propeller and towed body for active/passive towed array is analyzed. It is shown that, in shallow water environment, the direct wave and bottom/sea surface reflected wave will seriously affect the performance of sonar system. The formula for calculating the direction of arrival (DOA) of interference in terms of various parameters, such as array depth, length of tow cable, is derived. The effect of interference noise for the detection performance of sonar system is described. The results of system simulation provide the method for reducing the effect of these kind of interferences.     

Fractional beamforming of dense spacing array for time-frequency mix

1 IntroductionIn the last twenty years) the presence of towed line array and flank array indicates the maindevelopment of low frequency and large aperture techniques in underwater acoustics. But, thereare many difficulties in the engineering realization of towed line array. The towed ship noise,flow noise and array shape distortion are main limitational factors of its performance. Thetowed ship noise can be taken as an interference in the fixed direction, and suppressed by theelements with dir…     

Bearing splitting and near-surface source ranging in the direct zone of deep water

Sound multipath propagation is very important for target localization and identification in different acoustical zones of deep water. In order to distinguish the multipath characteristics in deep water, the Northwest Pacific Acoustic Experiment was conducted in 2015. A low-frequency horizontal line array towed at the depth of around 150 m on a receiving ship was used to receive the noise radiated by the source ship. During this experiment, a bearing-splitting phenomenon in the direct zone was observed through conventional beamforming of the horizontal line array within the frequency band 160 Hz–360 Hz. In this paper, this phenomenon is explained based on ray theory. In principle, the received signal in the direct zone of deep water arrives from two general paths including a direct one and bottom bounced one, which vary considerably in arrival angles. The split bearings correspond to the contributions of these two paths. The bearing-splitting phenomenon is demonstrated by numerical simulations of the bearing-time records and experimental results, and they are well consistent with each other. Then a near-surface source ranging approach based on the arrival angles of direct path and bottom bounced path in the direct zone is presented as an application of bearing splitting and is verified by experimental results. Finally,the applicability of the proposed ranging approach for an underwater source within several hundred meters in depth in the direct zone is also analyzed and demonstrated by simulations.     

The port/starboard discrimination based on line-spectrum character in towed line array with hydrophone triplets

The energy ratio of broadband signal is utilized for port/starboard discrimination in towed line array with hydrophone triplets in tradition.It works well in most conditions except for two cases:One is low signal to noise ratio(SNR),and the other is that there is a stronger interference lied on the other side of the line array with approximate bearing angles. A method called line-spectrum character extracting is proposed to discriminate port/starboard for these two cases in this paper,and the SNR needed for port/starboard discrimination is reduced significantly.Simulation and the sea-trial data are analyzed to show that the method is stable and valid,and is expected to be applied in real towed line array sonar.A line-spectrum enhancing technology with lower calculation complexity and stable validity is proposed to ensure the application of this method in lower SNR condition.     

On problem of the determination of operational mode of towed line array with distorted shape

The distortion of the array shape is one of the main factors which resultthe performance degeneration from the ideal situation of towed line array(TLA).Based on the ordinary array shape distortion,the directivity function ofTLA is presented in this paper.An algorithm for precisely determning thecoordinates of each element by means of the inverse elliptic function is derived.Afast approximation of recursive formula for solving distorted array shape isgiven.According to the comparison between ideal directivity and the directivityof distorted array,the criterion for making decision of operational mode of TLAsonar is presented.So that the performance prediction problem in TLA issolved.The results of system simulation in computer show a good agreement withthe theoretical analysis.     

Beamforming of random line array

In order to solve the problem of the sensor location uncertaintyfor a towed line array,the sample matrix of the signal and time-spacecurve equations of the simulated array shape are presented in this paper.Average beampatterns of randomized processing are given too.The quan-titative results of array shape variation and distortion losses are obtainedfrom the comparison between the computer simulation and the experimen-tal results.In addition,a model for estimating array shape is offered.Anexperiment on dynamical beamforming is carried out.The results showthat SNR improvement of about 4dB may be obtained by dynamic com-pensation for a small array when the distortion is medial and the towspeed is low.     

Phase-coupling characteristics of ship radiated-noise demodulation spectrum and application

To solve blade number characteristic extraction problem of low noise ships,phase characteristics of ship radiated-noise demodulation spectrum harmonic components using higherorder spectrum analysis were considered.Through analyzing a large number of ship radiated noise data,it points out the existing phase coupling relationship between two or three harmonic components of demodulation spectrum.The phase difference between line spectrum of blade frequency and the sum of line spectrum at shaft frequency and its some harmonic commonly is zero or other constant.Higher-order cumulant is given and higher-order spectrum is simulated when above phase difference is a non-zero constant.The application of phase coupling relationship is studied.According to the higher-order spectral analysis of some weak modulated ship radiated noise,it is indicated that the phase coupling relationship and higher-order spectral analysis can improve the ability to extract propeller parameters,especially for the weak modulation.     

Preliminary analysis of left-right ambiguity resolution performance for twin-line array

It is well known that the single line array sonar can't identify whether the target comes from left or right. Multi-line array provides the possibility to solve the left-right ambiguity problem. The performance of left-right resolution for twin-line array is described in this paper. It is shown that the suppression ratio, which is defined as the ratio of the response in the steering direction to opposite symmetrical direction, can be considered as an index for evaluating the ability of solving left-right ambiguity. The characteristics of suppression ratio is discussed. The theoretical expression for suppression ratio and some numerical results are illustrated. The result of theoretical analysis can be used in design of twin-line array sonar.     

Flow induced noise calculations for non-axially distributed hydrophones in towed arrays

Two improvements are put forward on the analyses of flow induced noise in towed arrays. First, the differences between Corcos/Carpenter pressure fluctuation models have been discussed at length, as well as flow induced noise calculated with these two models. Second, flow induced noise received by the finite hydrophones distributed non-axially is discussed and the relevant power spectrum is deduced. The results show that there are some disparities between the wavenumber spectrums and the responses of flow induced noise of these two models. Flow induced noise is closely related with the tow speed, the tube radius and the off-axis distance. The numerical analyses with Carpenter model indicate that the power spectrum of flow induced noise will increase 24 dB approximately with the tow speed doubled, decrease with the radius of the tube, and increase with the off-axis distance. The tube radius and the off-axis distance have greater influence on the high-frequency components than on the low-frequency components.     

Estimating the sound speed of the surface layer of the seabed using ocean ambient noise in shallow water

To improve the inversion efficiency, a method for quickly and directly estimating the sound speed of the surface layer of the seabed is proposed. According to the energy flux theory, the ocean ambient noise data received by the vertical line array can be used to extract the bottom loss(BL) passively. The BL curve has an effect on the critical angle, which is used to estimate the sound speed of the surface layer of the seabed. Based on the ray model,the difference between the BL calculated from t...