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To achieve covert underwater acoustic communication (UAC) at energies with a low probability of detection (LPD), a system should work at a low signal-to-noise ratio (SNR) with no repeating patterns in the transmitted signal. Not all covert UAC schemes can satisfy both requirements. The present paper studies a truly covert UAC scheme. The transmitted signal consists of both the initial signal (the previously recorded ambient noise of the ocean) and an information-bearing signal generated by multi-step operations on the initial signal (time-domain symmetrical division, phase rotation, time reversal, and counterpart exchange). As the two signals are transmitted together, the receiver does not require pre-knowledge of the initial signal for demodulation. Hence, each symbol can be assigned a different initial signal, thereby avoiding repeating patterns in the signal and improving the covertness of the system. After optimizing the rotation phase condition, the performance of the UAC system was analyzed in numerical simulations. The scheme achieved covert UAC and exhibited good performance at low SNR.

Minimum shift keying (MSK) shows better bandwidth efficiency than phase-shift keying due to its constant envelope property. The performance of block linear equalization (BLE), commonly used for MSK communication in radio communication systems, is limited in the complex time-varying underwater acoustic (UWA) channel because BLE cannot track channel changes within the data block. A time-domain adaptive decision feedback equalization (TD-ADFE) method based on Laurent’s decomposition is proposed to address issues pertaining to MSK communications over UWA communication. The method can effectively track the time-varying UWA channel through that the output of encoder updates the equalizer coefficient iteratively, which comprises matched filter, equalizer, differential decoding, and differential encoding. According to the numerical simulation results, the TD-ADFE performed better than FD-BLE over time-varying UWA channel by approximately 5.5 dB when the bit error rate (BER) was 10-3. The sea trial results over 28 hours demonstrated that the BER of TD-ADFE was 36.8% lower than FD-BLE.

Various line array configurations are evaluated for the source localization performance based on the analysis of mode decomposition matrix in this paper. The guideline of array shape design focuses on improving the localization performance of matched filed processing, meanwhile reducing the difficulty of deploying equipment in practical experiments. In the shallow water environment, when the environment is well known, the source localization result can be obtained by matched field processing algorithms effectively, but the source localization performance is affected by the array parameters, such as array length, the number of sensors, and the configurations of various horizontal and vertical line arrays. The modal decomposition method provides a useful insight into the questions of how many modes are needed and how to design the array to resolve the modes. Therefore, the method of utilizing a normal mode acoustic propagation model to decompose mode is proposed by vertical line array, horizontal line array and combined array respectively. Then we can evaluate the source localization performance of various line array configurations by studying the characteristic of normal mode decomposition matrix, thus establishing a qualitative or even quantitative relationship between each other. The more the normal mode decomposition matrix tends to be diagonalized, the better performance of line array localization will be obtained. Simulation results show that the localization performance of matched field processing with the combined arrays will be severely degraded when the mode amplitudes cannot be accurately deduced by one of the sub-arrays. Considering the requirements for the practical experiments and various environments, the source localization performance of short vertical line array and combined array are mainly discussed in this paper. The combined array can increase the azimuth and depth information of the source and realize three-dimensional target detection while the vertical array provides range-depth information and the horizontal array provides bearing information. Simulation result indicates that the design guidelines based on the normal mode decomposition are appropriate for arrays employed for matched filed processing. Meanwhile, the combined arrays perform better than the short vertical array, which is benefited by the horizontal array's suppressing the side lobes, which leads the ratio of peak to sidelobe to increase, and thus improving the location accuracy. The values of localization accuracy of combined arrays are all above 90% according to the simulation experiment. Take the practical application into account, the combined array is undoubtedly a compromise choice for the localization performance and the test complexity.

Seabed is an important part of the marine environment and it has a significant influence on sound propagation. Considering the fact that geoacoustic parameters are directly acquired with difficulty and complexity, a lot of researchers have focused on the inversion of them. The seabed attenuation coefficient is insensitivity to the matching field. However it has great effects on the transmission loss, mode amplitude ratios, etc. It can be inverted from measurements of these quantities. In this paper, we present an inversion scheme based on warping transform technique for estimating the seabed attenuation coefficient. It utilizes an equivalent seabed model which is constructed by using a prior and posterior knowledge. The dispersion characteristics of normal modes can be observed using the time-frequency analysis of the explosive signal recorded. The dispersion curve can be used to invert the seabed sound speed and density. The results presented by other scholars in the same circle are cited in this paper that focuses on how to obtain the seabed attenuation. Warping transform technique is used to separate and extract the normal modes. The main advantage of warping transform is that it can transform the time-frequency spectrogram into linear relationship which makes it easier to extract the normal modes. The feature of this paper lies in determining the distance normalized normal mode transmission loss. If the depths of receiving hydrophone and the explosion source are constant, the plot of normalized normal mode transmission loss versus distance is a straight line from the normal modes theory, which can be used to obtain the attenuation factor of real part of pressure. Then the seabed attenuation coefficient of the shallow water acoustic model can be calculated. In order to verify the effectiveness of this method, the warping transformation technology is used to separate and extract the first three modes from the simulated Gaussian pulse signal which is obtained in a simulated environment which is similar to the real marine environment. The extracted results are completely consistent with the numerical results. After that, the impulsive signal data collected in the Yellow Sea are analyzed according to the scheme process, and the relationship between the seabed attenuation and frequency is =0.581fk1.86(dB/m) in a range from 150 Hz to 550 Hz. The results are in good agreement with those obtained by other scholars in the same circle. On the other hand, the inversion results of seabed attenuation from different modes can be used for comparison at the same frequency, which can be a good support for the result.

It is found in the data processing of matched filed source localization experiment that in the case of the slight mismatch of sound speed profile, the louder source below disappears while the weaker source above comes out. Based on the experimental phenomenon, the influences of the sound speed profile on the source localization of different depths are investigated. Firstly, the simulation is conducted in order to extract the influence of the uncertain factors in the sea test, from which it is further concluded that the deeper source is greatly affected. Then another simulation about the mismatch of the depth of the thermocline is conducted in the typical shallow-water negative gradient environment, from which a preliminary conclusion is drawn that the most sensitive depth to the mismatch of the sound speed profile is around 10 meters below the lower boundary thermocline. Finally, this pheonomenon is reasonably explained through a theoretical analysis from two aspects of the normal mode theory.

To explain the change of the waveguide invariant β caused by the internal wave, the waveguide invariant is expressed as a stochastic distribution and the time-varying characteristics of β are analyzed; then the cause and principle of variation of β are discussed in terms of the normal modes theory of the acoustic field. It turns out that the waveguide invariant shows great frequency-dependent characteristics. And in the internal wave environment, when the sound is propagated parallel to the internal wave front, the sound field's composition of normal modes remains unchanged, but the turbulence in the group slowness and phase slowness of the main normal modes in the sound field may also change the distribution of β; when the sound is propagated perpendicular to the internal wave front, the coupling between acoustic normal modes might also cause obvious changes in the values of β.

Matched field processing techniques have been studied extensively in recent decades, and a lot of detail algorithms have been put forward for practical use. When the underwater target is obscured by the strong surface interferences, the performance of matched field processing localization will degrade severely. The now existing spatial filter technique can be used to suppress the surface interferences, but the burden of calculation is heavy and the memory usage is large. In this paper, a scheme of optimizing spatial filter design based on the compressed replica vectors is presented, and the broadband data are processed incoherently. In contrary to the existing spatial filter, the optimized spatial filter can effectively reduce the computational complexity and memory usage when the number of array elements N is greater than the number of the effective modes Q, meanwhile, it also retains the original performance of interference suppression. Numerical simulations in a littoral shallow water environment are performed to validate the performance of the spatial filter and the promotion of computation speed. Then, data processing results obtained from an experiment conducted in the littoral shallow water environment are presented. It follows from the results that the weak underwater target can be distinguished from the strong surface interference clearly by use of the incoherent matched field processing with the application of the spatial filter based on compressed replica vectors.