Adaptive multiuser detection for underwater acoustical channels

An underwater acoustic local area network (ALAN) provides multipoint-to-point telemetry between many high-rate, ocean-bottom sensors and a central, surface-deployed receiver in the 10-30 kHz vertical acoustical channel. Ocean-bottom modems initiate the transmission process by requesting data channel time slots via a common narrow-band request channel. Request packets overlap in time and frequency in this channel, and the throughput and average transmission delay rely heavily on the successful resolution of the request packet collisions. This paper presents the design, analysis, and experimental demonstration of a request channel receiver capable of resolving collisions between several asynchronous and cochannel packets. The receiver algorithm differs from standard capture schemes (by demodulating the data from both strong and weak transmitters), conventional spread-spectrum receivers (by overcoming the near-far problem), and existing multiple-access demodulation techniques (by adapting to the number of interfering signals, and the unknown phase, Doppler, amplitude, and timing of each signal in the collision). The receiver demodulates the collided packets by decision-directed techniques through a novel method of estimating the interference for each user which minimizes error propagation due to inaccurate tentative decisions. An inwater experiment illustrates that this technique is extremely desirable for collision resolution in underwater acoustic local area networks, and also for underwater autonomous vehicles with both sidescan sonar as well as acoustic telemetry links     

Frequency Selectivity Analysis Based on Inductively Coupled Channel for Current Transmission Through Seawater

Inductively coupled channels are based on the electromagnetic induction principle and realize long-distance current signal transmission through seawater. Due to a few difficulties in performing actual experiments, it is unclear how the seawater medium affects the frequency selectivity of the current signal. In this paper, a dual dipole model of the inductively coupled seawater transmission channel is established for the traditional short-distance current field transmission mode. The transmission characteristics of electrical signals in seawater are theoretically derived. A platform is used to measure the amplitude-frequency and phase-frequency characteristics of the current signal transmission in seawater with transmission frequencies ranging from 30 kHz to 1 MHz, and transmission distances in the vertical range of 4 m. The COMSOL Multiphysics simulation and practical test analysis are carried out to analyze the frequency selectivity of seawater conductivity. It is proved that the seawater resistance increases as the frequency increases, which is the key problem that affects the current signal. This study provides an important theoretical support and experimental evidence for improving the transmission performance of long-distance underwater current signals.     

Cramer-Rao lower bounds for sonar broad-band modulation parameters

A role of passive sonar signal processing is the detection and estimation of the parameters associated with amplitude modulated broad-band signals. An example of such signals is propeller noise. Discrete frequency lines occur at the rotational frequency of the propulsion shaft and at the blade frequency. This correspondence provides expressions for the Cramer-Rao lower bounds for the estimates of broad-band signal power, modulation level, modulation frequency, and modulation phase. It is shown that for low broad-band-signal-to-broad-band-noise ratios, the estimates of power and modulation level are uncoupled from the estimates of modulation frequency and phase     

Source localization with broad-band matched-field processing inshallow water

Acoustic source localization using matched-field processing is presented for multitone signals from the Shallow Water Evaluation cell Experiment 3 (SWellEX-3). The experiment was carried out in July 1994 west of Point Loma, CA, in 200 m of water of complex bathymetry. The multitone signal (ten tones between 50 and 200 Hz) was transmitted from an acoustic source towed at various depths over tracks which produced complex propagation paths to a vertical line array receiver. Broad-band and narrow-hand processing, localization, and tracking results are compared with each other and with independent estimates of source position. With narrow-band processing, mismatch between the data and the predicted signal replica of ~1 dB reduced the mainlobe to levels equal to or below the sidelobes. Incoherently averaging the processing output over the multiple tones reduced range/depth sidelobe levels, allowing accurate source localization and tracking     

Deterministic three-dimensional analysis of long-range soundpropagation through internal-wave fields

A Munk profile and a set of propagating internal-wave modes are used to construct a three-dimensional time-varying ocean sound-speed model. Three-dimensional ray tracing is employed to simulate long-range sound propagation of a broadband acoustic signal. Methods are developed to convert three-dimensional ray-tracing results to acoustic time-domain amplitude and phase measurements. The ocean sound-speed model is defined deterministically, and the model acoustic receptions are analyzed deterministically. A single internal-wave mode that is “spatially synchronizes” to an arrival can coherently focus and defocus the acoustic energy. These internal waves can cause an arrival's amplitude fluctuation to mimic Rayleigh fading; however, the time-domain phase is stable, in contradiction to the classical Rayleigh fading environment where the received phase is uniformly distributed. For example, the received power attributed to an early arrival propagated over a 750-km range can fluctuate over 40 dB, while the time-domain phase remains within a quarter of a 75 Hz cycle. The characteristics of the time-domain phase are important for establishing coherent integration times at the receiver     

Underwater acoustic channel models for 4800 b/s QPSK signals

Two computer models are presented, one for short-range and one for long-range propagation of acoustic signals through an underwater channel from a transmitter to a receiver. In the short-range model, the received signal is due to a direct path (steady component) and a random path (diffused component) that could be the result of boundary scattering. For the long-range case, the received signal is the superposition of a number of time-delayed, randomly propagated components arriving by different paths. Both models assume perfect transmitter-receiver synchronization but use realistic channel time delays. They demonstrate the time-varying characteristics of underwater acoustic channels and are used in simulations to evaluate the performance of the detection technique     

Ocean-bottom-seismograph of the Institut für Geophysik,Hamburg

The ocean bottom seismograph (OBS) of the Institut für Geophysik, Hamburg (IfG) is designed for refraction seismic experiments and for recording microseismic noise. Hydrophone signals are recorded directly on a casette tape recorder with a band width of 3–60 Hz. Signals from three component 1 Hz seismometers are recorded on a 2nd casette tape recorder in FM for a frequency range of 0.1–1 Hz. A telemetering buoy at the surface is connected with the OBS by a polypropylene rope.     

A sound projector for acoustic tomography and global ocean monitoring

Long-range underwater acoustic systems, such as those used in ocean acoustic tomography, require low-frequency signals covering a broad frequency band. To meet this requirement, a novel design based on a tunable narrow-band high-efficiency sound projector has been used. The projector transmits a frequency sweep signal by mechanically tuning a resonator tube (or organ pipe) to match the frequency and phase of a reference signal. The resonator tube projector consists of a symmetrical pressure-balanced Tonpilz driver placed between two coaxially mounted tubes. The Tonpilz acoustic driver is composed of two pistons separated by preloaded ceramic stacks. The resonant tube is a simple, efficient, narrow-band, medium-output projector that operates at any ocean depth. Both projector tubes have slots (or vents) which are progressively covered or uncovered by sliding coaxial tubular sleeves. The frequency varies with the sleeve position. A computer-controlled electromechanical actuator moves the cylindrical sleeves along the tubes, keeping the projector in resonance at the instantaneous frequency of a swept frequency signal. The actuator smoothly tunes the resonator tube frequency in a bandwidth of 200 to 300 Hz during a 135-s transmission. A computer synthesizes the linear frequency-modulated signal; compares the phase between transmitted and reference signals; and, using a phase-lock loop (PLL) system, keeps the resonator tube frequency in resonance with the driver frequency. The estimated PLL precision is better than 3/spl deg/ phase error. The system was analyzed by means of finite element analysis and electrical equivalent circuit simulation. The projector prototype was first tested at the Woods Hole Oceanographic Institution (WHOI) dock in Woods Hole, MA and later in the Pacific Ocean during a voyage of the R/V "Point Sur" in November 2001.     

On shallow-water bottom reverberation frequency dependence

Determinations of bottom scattering strength in the decade below 1 kHz under downward refracting conditions have been made using acoustic reverberation and transmission data from the 2001 East China Sea Asian Seas International Acoustic Experiment (ASIAEX). The measurements were performed using explosive sources and receiving hydrophones in ship-suspended vertical-line arrays. The focus of this paper has been the dependence of bottom scattering strength on the frequency and characterization of the uncertainties associated with the extraction of scattering strength from reverberation. The derived bottom scattering strength gradually rises with frequency from 100-300 Hz and then more rapidly above 300 Hz. A potential explanation suggests that the frequency variation results from two scattering mechanisms, rough layer scattering at the low end of the band and sediment near-surface volume scattering at the high end. The spatial extrapolation of these results is explored by comparing them with similarly derived scattering strengths using data obtained under the Navy's Harsh Environments Program at a somewhat separated site (56 km) under environmental conditions similar to those during ASIAEX. In the ASIAEX analysis, it has been found that the largest source of uncertainty in the scattering-strength frequency dependence arises from persistence of finite-amplitude effects associated with the source signal.     

Pasisar: Performances of a High and Very High Resolution Hybrid Deep-Towed Seismic Device

The Pasisar seismic acquisition system combines a source at the sea surface and a deep-towed single channel streamer. This unconventional device geometry reduces the width of the first Fresnel zone which increases the lateral resolution. However, the device acquisition geometry generates artifacts on seismic profiles and induces large incidence angles of the seismic signal. A specific processing sequence must be applied to the data to obtain a readable seismic section. Penetration of the seismic signal depends on the energy of the signal reaching the seafloor and on its incidence angle. Because of smaller source energy, 800 Joules Sparker data cannot be acquired in water depth larger than 1500 m for example, whereas there is no depth limit for the use of this system with air gun sources. Differential acoustic absorption of seismic frequencies (below 1000 Hz) in the water column is negligible when compared with wave fronts expansion. Thus, the horizontal resolution of any seismic system strongly depends on the frequency spectrum of the seismic source and on the travel distances. Pasisar and conventional seismic profiles being usually simultaneously recorded, we illustrate the interest of using a hybrid seismic device by comparing horizontal resolutions as well as signal-to-noise ratio obtained with both the Pasisar and conventional systems. In addition, by carefully picking time arrivals of a reflection on simultaneously recorded surface and deep-towed seismic records, it is possible to estimate the average interval seismic velocity. We present the simplified example of a horizontal reflector.     

XCTD剖面仪数据传输电路的设计与实现

针对XCTD(投弃式温盐深)剖面仪数据传输速率低、可靠性差的缺点,提出基于曼彻斯特编码的基带传输技术。在系统结构设计的基础上,着重讨论了基于FPGA的曼彻斯特编码器、位定时提取电路和自动增益控制电路的设计。通过测试表明,编码后的系统不仅可以使用基带信号直接传输,而且能够满足系统的通信速率要求。     

An underwater towed electromagnetic source for geophysical exploration

Low-frequency electromagnetic methods are used in geophysical exploration to detect the magnetic field distortion between a transmitter and receiver produced by locally conductive bodies. Both ground and airborne systems are in current use. It is possible to similarly conduct underwater geophysical exploration by using an underwater towed source of electromagnetic radiation and a receiving magnetic or electric field detector. The receiver can be towed on an auxiliary cable, mounted on a boom on the towing platform, or land based. An underwater towed electromagnetic source suitable for ocean-bottom exploration has been constructed, and its underwater propagation characteristics at low frequency have been studied. This underwater calibrated source (UCS) is 4 m long, weighs 383 kg in air, and can produce vertical and horizontal magnetic dipoles and a horizontal electric dipole. Powered by a current-feedback-controlled, high-power, modified sonar amplifier, the UCS can produce 9710 ampereturn.m2 of magnetic dipole or 200 A.m of electric dipole at 50 A at frequencies up to 200 Hz without significant attenuation from coil inductance. This paper concentrates on the mechanical, hydrodynamic, and magnetic design details of the UCS and the electrical system, consisting of the high-current drive power system and the shipboard monitoring system for attitude and depth detectors.     

Measurements of high-frequency shallow-water acoustic phasefluctuations

A shallow-water high-frequency (HF) acoustic propagation experiment was conducted just off shore in Panama City, FL. Several broad-band high-resolution sources and receivers were mounted on stable platforms and deployed in water depths of 8-10 m. Signals covering the frequency range from 20 to 200 kHz were transmitted from the sources to two spatially separated receivers. The data were analyzed to provide estimates of the signal phase variances as a function of frequency and source-to-receiver range. These phase variabilities are correlated with small-scale water column thermal variabilities and ocean swell conditions     

Modeling the generation and gravity-capillary waves using an underwater sound source

This is an experimental study of the acoustic method of surface-wave excitation using an underwater source of high-frequency (950 kHz) sound. The surface waves are excited at the sound-beam modulation frequency (3–55 Hz). For a normal fall onto the free surface, the modulated sound beam efficiently generates waves in the gravity-capillary range. This provides flexible electronic control of the main wave parameters (frequency and amplitude) in the packet and continuous modes. The amplitude-frequency characteristics of the process of surface-wave generation were obtained by numerical calculations (based on equations for the rate of acoustic flux and propagation of gravity-capillary surface waves) and by experiments (based on surface wave measurements by optical and contact methods). Both values are very consistent: on the background of a similar monotonic attenuation with frequency, they have a local dip near the minimum of the phase velocity and oscillation in the frequency range above 20 Hz. The experiments on the excitation of wave packets by single acoustic messages with varying lengths and powers, as well as by falling water drops, indicated that, in all cases, the phase characteristics are satisfactorily consistent with one another and the time needed for the signal to arrive at the measurement point is determined by the group velocity.     

Analysis of EMD algorithm for identification and extraction of an acoustic signal in underwater channel against wind driven ambient noise

Sonar generated acoustic signals transmitted in underwater channel for distant communications are affected by numerous factors like ambient noise, making them nonlinear and non-stationary in nature. In recent years, the application of Empirical Mode Decomposition （EMD） technique to analyze nonlinear and non-stationary signals has gained much attention. It is an empirical approach to decompose a signal into a set of oscillatory modes known as intrinsic mode functions （IMFs）. In general, Hilbert transform is used in EMD for the identification of oscillatory signals. In this paper anew EMD algorithm is proposed using FFT to identify and extract the acoustic signals available in the underwater channel that are corrupted due to various ambient noises over a range of 100 Hz to 10 kHz in a shallow water region.Data for analysis are collected at a depth of 5 m and 10 m offshore Chennai at the Bay of Bengal. The algorithm is validated for different sets of known and unknown reference signals. It is observed that the proposed EMD algorithmidentifies and extracts the reference signals against various ambient noises. Significant SNR improvement is alsoachieved for underwater acoustic signals.     

Spatial diversity processing for underwater acoustic telemetry

A large increase in the reliability of shipboard or stationary underwater acoustic telemetry systems is achievable by using spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, are combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver and is shown to perform reliability in a variety of non-Gaussian noise and jamming environments and reduce to the traditional optimal diversity system in a Gaussian environment. The dynamics of the quality estimator allow operation in the presence of high-power impulsive interference by exploiting the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger-scale inhomogeneities in the acoustic field. A number of data transmission experiments were carried out to demonstrate system performance in realistic underwater environments     

A Novel Method for Real-Time Monitoring of Channel Siltation Based on Bistatic Scattering Theory

Monitoring the thickness changes of channel siltation is paramount in safeguarding navigation and guiding dredging.This paper presents a novel method for realizing the field monitoring of channel siltation in real time.The method is based on the bistatic scattering theory and concerned more with the receiving and processing of multipath signal at high-frequency and small grazing angle.By use of the multipath propagation structure of underwater acoustic channel,the method obtains the silt thickness by calculating the relative time delay of acoustic signals between the direct and the shortest bottom reflected paths.Bistatic transducer pairs are employed to transmit and receive the acoustic signals,and the GPS time synchronization technology is introduced to synchronize the transmitter and receiver.The WRELAX (Weighted Fourier transform and RELAX) algorithm is used to obtain the high resolution estimation of multipath time delay.To examine the feasibility of the presented method and the accuracy and precision of the developed system,a series of sea trials are conducted in the southwest coast area of Dalian City,north of the Yellow Sea.The experimental results are compared with that using high-resolution dual echo sounder HydroBoxTM,and the uncertainty is smaller than ±0.06 m.Compared with the existing means for measuring the silt thickness,the present method is innovative,and the system is stable,efficient and provides a better real-time performance.It especially suits monitoring the narrow channel with rapid changes of siltation.     

A direct recording ocean bottom seismometer

We have designed a simple, cheap and reliable ocean-bottom seismometer. Signals from three-component geophones are recorded directly on magnetic tape running continuously at a speed of 1 mm s¹. Time reference is derived from a temperature-compensated quartz crystal oscillator and encoded on a fourth channel as an amplitude modulation of a 20 Hz carrier. A bipolar square-root signal-compression scheme doubles the tape dynamic range to 80 db, and the available bandwidth is 2 to 100 Hz. Tape and batteries are capable of 500-hr operation, and the unique magnetic release comes close to being a fail-safe system. A heavy, high-drag concrete anchor shaped like a flower-pot provides easy launching, fast stable descent and good coupling to the ocean floor. We have had numerous successful field emplacements which have yielded good earthquake and shot-refraction data.     

MIMO Underwater Acoustic Communication in Shallow Water with Ice Cover

Although multiple-input multiple-output(MIMO) underwater acoustic(UWA) communication has been intensively investigated in the past years, existing works mainly focus on open-water environment. There is no work reporting MIMO acoustic communication in under-ice environment. This paper presents results from a recent MIMO acoustic communication experiment which was conducted in Bohai Gulf during winter. In this experiment, high frequency MIMO signals centered at 10 kHz were transmitted from a two-element source array to a four-element vertical receiving array at 1 km range. According to the received signal of different array elements, MIMO acoustic communication in under-ice environment suffers less effect from co-channel interference compared with that in open-water environment. In this paper, time reversal followed by a single channel decision feedback equalizer is used to process the experimental data. It is demonstrated that this simple receiver is capable of realizing robust performance using fewer hydrophones(i.e. 2) without the explicit use of complex co-channel interference cancelation algorithms, such as parallel interference cancelation or serial interference cancelation.     

Effects of East China Sea shallow-water environment on acoustic propagation

Operational environmental acoustics experiments were conducted over the frequency range of 25 to 800 Hz in September 1997 in the East China Sea, where the water depth was about 100 m. Objectives of the data analysis reported here are to characterize this environment and to assess its complexities as they may impact acoustic propagation as measured by its transmission loss (TL). Conductivity-temperature-depths and expendable bathy-thermographs sampled the ocean, such that its spatial and temporal variability could be approximately separated. The sound-speed profiles are downward refracting, involve two water masses associated with the Kuroshio Current and Taiwan Warm Current, and have thermocline variations caused by internal tides. The bottom geoacoustic characteristics, presumed to be approximately horizontally isotropic, were based on data atlases and were estimated from the measured TL, for some interpretations. The TL data were obtained in octave bands from explosive signal underwater sound sources and sonobuoy receivers, both deployed at a depth of about 18 m. Tests were conducted in directions approximately normal and parallel to the bathymetric contours and the measured TL was, to zero order, independent of the direction of propagation. To higher order, directional differences in the TL were observed and ascribed to anisotropies in bottom properties. A state-of-the-art TL model was adopted, based on environmental idealizations typical of operational forecasting and compared with the measured TL. The comparison yields a probability density function that quantifies the uncertainty of such a TL model, caused by the stochastic variability of the environment, typically unknown a priori. For the model used, the pdf has a standard deviation of about 2 dB from 50 to 800 Hz and larger below 50 Hz.