Covert Underwater Research Articles

Dolphin Sounds-Inspired Covert Underwater Acoustic Communication and Micro-Modem.

A novel portable underwater acoustic modem is proposed in this paper for covert communication between divers or underwater unmanned vehicles (UUVs) and divers at a short distance. For the first time, real dolphin calls are used in the modem to realize biologically inspired Covert Underwater Acoustic Communication (CUAC). A variety of dolphin whistles and clicks stored in an SD card inside the modem helps to realize different biomimetic CUAC algorithms based on the specified covert scenario. In this paper, the information is conveyed during the time interval between dolphin clicks. TMS320C6748 and TLV320AIC3106 are the core processors used in our unique modem for fast digital processing and interconnection with other terminals or sensors. Simulation results show that the bit error rate (BER) of the CUAC algorithm is less than 10 when the signal to noise ratio is over ‒5 dB. The modem was tested in an underwater pool, and a data rate of 27.1 bits per second at a distance of 10 m was achieved.

벡터 하이드로폰을 위한 두께 전단형 진동자의 빔 패턴 해석

수중에서 은밀하게 움직이는 표적을 조기에 탐지하기 위한 선배열센서에 적용되는 통상의 하이드로폰은 표적신호의 크기만 측정할 뿐, 외부에서 들어오는 음향신호의 방향은 파악할 수 없는 한계가 있다. 이에 본 연구에서는 표적으로부터의 음향신호의 크기와 방향을 동시에 탐지할 수 있는 관성형 벡터 하이드로폰의 기본 구성품으로 두께 전단형 진동자를 제안하였다. 외력에 대한 진동자의 출력 전압을 해석할 수 있는 수식을 유도하였으며, PMN-PT 단결정 진동자에 대한 유한요소해석을 통해 수식의 타당성을 검증하였다. 본 연구에서 얻어진 해석 결과는 향후 두께 전단형 진동자로 구성된 관성형 벡터 하이드로폰 설계에 활용될 것이다. Typical hydrophones in line array sensors for early detection of covert underwater targets can measure only sound-pressure-magnitude with the limitation of being unable to identify the direction of an incoming wave. In this study, a thickness shear mode vibrator was proposed as the main component of an inertia type vector hydrophone to measure both magnitude and direction of acoustic signals from targets. The equation to analyze the output voltage of the vibrator to an external force was derived, and the validity of the equation was verified through finite element analysis of a PMN-PT single crystal vibrator. The analysis results from this study will be utilized in the future for the design of inertia type vector hydrophones made of thickness shear vibrators.

Covet underwater acoustic communication based on spread spectrum orthogonal frequency division multiplexing (OFDM)

A covert underwater acoustic communication (UAC) method is realized with a dolphin whistle as the information carrier. The proposed method jointly utilizes the spread spectrum and orthogonal frequency division multiplexing (OFDM) techniques. The original dolphin whistle is represented as the complex baseband OFDM transmitted signal and the phase of its discrete Fourier transform (DFT) coefficients are modulated by a spread spectrum code to carry information. The audio quality and waveform similarity are used as two covert effect criterion. In simulations, the influences of the code length and modulation parameters are investigated. It is verified that the communication performance and camouflage effect are a pair of contradictions. Therefore, the best compromise formula should be chosen according to the actual demands.

Biologically inspired covert underwater acoustic communication by mimicking dolphin whistles

Focusing on the need of covert underwater acoustic communication, a camouflage underwater acoustic communication approach is proposed in this paper. The idea is to deceive the interceptor to actively exclude the communication signal as it resembles the original whistle. Based on the time and frequency modulation feature of dolphin whistles, a series of segmented chirp rate modulated signals are designed to mimic the dolphin whistles by sound characteristics. A specialized communication frame structure is designed to facilitate the information demodulation, where the original whistle is used as the synchronization signal and the mimicry signal is followed to convey information. Besides, the original synchronization whistle is also used as a transmit reference to extract the whistle contour and furthermore to get the center frequency of each chirp. Virtual time reversal mirror technique is employed to mitigate channel effect. Fractional Fourier Transform and correlation approaches are adopted to realize demodulation respectively. The feasibility and efficiency of the proposed method are verified by both simulation and sea experiment. Bit error rate less than 10-3 with chirp duration of 20ms at a distance of 5.5km is achieved in the sea trial.

A covert underwater acoustic communication method based on spread spectrum digital watermarking

The covert underwater acoustic communication is necessary under some circumstances when the contents of transmission or location of transmitter are required to be concealed. In this paper, the signal transmitter and the information detector are designed, which hides the spread spectrum digital watermark (SSDW) into the log-spectrum of dolphin whistles that are as a carrier and detects SSDW by matched filtering in the same log-spectrum. Various factors of noise, carrier and multipath in channels may degrade the performance of the method. In order to analyze their effects, corresponding channel models in log-spectrum domain are derived from the additive Gaussian noise channel and the underwater time-varying acoustic multipath channel. Results suggests that the effects of channel noise and the carrier are equivalent to that of two additive Gaussian noise terms attached to the correct decision and the proposed method also has a good multipath extension tolerance performance. Preliminary results obtained form at-sea experiments showed that the data rate can achieve 6 bps and bit error rate (BER) under 10-3 in the distance of 5 km.

Covert underwater acoustic communication using dolphin sounds

In November 2012, an experiment demonstrating biological mimicry method for covert underwater acoustic communication (UAC) was conducted at Lianhua Lake in Heilongjiang China. Dolphin whistles were used for synchronization while dolphin clicks were used as information carrier. The time interval between dolphin clicks conveys the information bits. Channel estimates were obtained with matching pursuit (MP) algorithm, which is useful for sparse channel estimation. Adaptive RAKE Equalization was employed at the receiver. Bit error rates were less than 10(-4) with 37 bits per second data rate in the lake trial.

Research on bionic underwater acoustic communication technology based on differential Pattern time delay shift coding and dolphin whistles

Regarding the performance degradation caused by the traditional method of covert underwater acoustic communication, in this paper we propose a bionic underwater acoustic communication technology on the basis of differential Pattern time delay shift coding system and dolphin whistles. The signal band of dolphin whistles is narrow and the intervals between information signals are different, and the cross-correlation is very weak. Dolphin whistles are used for synchronization and Patterns, for the time interval between dolphin whistles conveys the information bits. The mimicked version of communication signal is not easy to detect and intercept, and the special encoding method of differential Pattern time delay shift also makes the information not easy to decipher, so this bionic underwater acoustic communication technology has strong covert and confidential nature and it also has an excellent performance in anti inter-symbol interference and anti Doppler effect. A tank experiment is conducted for this system. At SNR 0 dB and relative movement, the user message is recovered at an effective data rate of 67 bit/s with low bit error. It is proved that the system has an effectiveness, robustness and covert nature.

Covert underwater acoustic communications

Low probability of detection (LPD) communications are conducted at a low received signal-to-noise ratio (SNR) to deter eavesdroppers to sense the presence of the transmitted signal. Successful detection at intended receiver heavily relies on the processing gain achieved by employing the direct-sequence spread-spectrum (DSSS) technique. For scenarios that lack a sufficiently low SNR to maintain LPD, another metric, referred to as low probability of interception (LPI), is of interest to protect the privacy of the transmitted information. If covert communications take place in underwater acoustic (UWA) environments, then additional challenges are present. The time-varying nature of the UWA channel prevents the employment of a long spreading waveform. Furthermore, UWA environments are frequency-selective channels with long memory, which imposes challenges to the design of the spreading waveform. In this paper, a covert UWA communication system that adopts the DSSS technique and a coherent RAKE receiver is investigated. Emphasis is placed on the design of a spreading waveform that not only accounts for the transceiver structure and frequency-selective nature of the UWA channel, but also possesses a superior LPI. The proposed techniques are evaluated using both simulated and SPACE'08 in-water experimental data.