Description: Inverse problems in ocean acoustics are based on 2-D modeling of sound propagation, hence ignoring the effects of horizontal refraction, referred to as 3-D propagation effects. However, the acoustic propagation in shallow-water environments, such as the continental shelf, may be affected by 3-D effects requiring 3-D modeling to be accounted for. The aim of this work is to investigate the importance of the 3-D effects with respect to the performance and reliability of typical 2-D-model-based inversion procedures of ocean acoustics. The study is carried out on a well-established synthetic test case which exhibits well-known 3-D effects. A matched-field inversion procedure is implemented based on the exhaustive search over the parameter space. The feasibility and the limits of inverting low-frequency noisy 3-D synthetic data for some parameters describing the test case by matching replica from 2-D computations are explored. Both synthetic data and replica are generated using a parabolic-equation-based code. This approach highlights the relevance of using 2-D propagation models when inversions are performed at relatively short ranges from the source. On the other hand, important mismatch occurs when inverting at farther ranges, demonstrating that the use of fully 3-D forward models is required.

Description: In this paper, the context of several self-propelled, short-length cables, embedded with passive sensors for environmental diagnostics and swimming efficiently in formation over long duration and in shallow water, is considered. The basic problem of this volumetric diagnostic-namely, the low-speed motion control of a short-length, neutrally buoyant cable-is examined. More specifically, the constant-rate, circular turning of a 7-m-long cable held taut in a shallow-water basin using a biorobotic propulsor that has multiple flapping fins at one end, the other end being tied to a mooring post, is examined via modeling and laboratory and basin experiments. A drag analysis is used to estimate the fastest steady turning rate achievable while holding the cable taut. An axial tension and position controller, as well as a depth controller, is developed and evaluated in a quiescent laboratory tank accounting for the cycle-averaged hydrodynamic characteristics of a rigid cylinder to which six flapping fins are attached, three at each end. A small test range of 100-m scale, containing seven floor-mounted hydrophones in a hexagonal layout, is built in a stillwater basin to track the motion of the propulsor, to which a pinger is attached. The estimated overall resolution of the acoustic tracking system is 5 cm; it is possible to detect the imprint of the environmental unsteadiness on the cable and propulsor assembly. In the basin experiment, a mean radius of turning of 8.91 m can be achieved within a standard of deviation of 0.27 m, and a uniform turn rate of 22 min for one full revolution can also be maintained, when the applied turning force is 10% of the cable tension. The basin experiment has verified the drag analysis. This paper explores the value of a flapping fin propulsor (which is inspired by large swimming animals) as an alternative to conventional rotational propulsors for the low-speed maneuvering of a short cable.

Description: Registering sonar images to correctly describe seafloors and explain wide geological or biological phenomena is often achieved manually requiring significant human resources. This paper proposes an automatic intensity-based registration algorithm that relies on the optimization of a new similarity measure (SM), within a multiresolution block matching framework. Indeed, several SMs have been evaluated and ranked on real sidescan sonar data to determine the most relevant intensity dependencies between images for matching purposes. Correlation ratio (CR) and mutual information (MI) are then selected and because of their complementary behaviors, merged in a new SM (MI&CR), which performs better than CR or MI alone, to determine robust matching blocks between images. Thus, the proposed two-step registration algorithm uses MI&CR to match two sonar images: a single rigid translation globally matches the images, then a field of locally applied translations is computed for adjusting the final registration to remaining local distortions. Actual processing time can then be tuned according to the required registration accuracy. Due to a survey standard operating mode, only same-survey overlapping images are considered as candidates for matching. Moreover, building mosaics from registered images assumes a flat sea bottom as no global elevation information is provided by sidescan sonar images.

Description: In this paper, we derive bounds to the channel capacity of orthogonal frequency division multiplexing (OFDM) systems over the underwater (UW) acoustic fading channel as a function of the distance between the transmitter and the receiver. The upper bound is obtained under perfect channel state information (CSI) at the receiver. The lower bound is obtained assuming the input is drawn from phase-shift keying (PSK) constellation which results in non-Gaussian distribution of the output signal and no CSI. The reduction from the upper bound is due to limited mutual information that can be conveyed by PSK constellation and the linear minimum mean square prediction error. Our UW channel deviates from the wide sense stationary and uncorrelated scattering (WSSUS) model commonly used for small bandwidths. We incorporate frequency-dependent path loss due to the acoustic propagation into each arrival path between the transmitter and the receiver. This leads the UW channel to be modeled as a frequency-dependent doubly spread fading channel characterized by the wide sense stationary and correlated scattering (WSS-non-US) fading assumption. Both Rayleigh and Ricean fading assumptions are investigated in our model. Results from the model show a gap between the upper and lower bounds which depends not only on the ranges and shape of the scattering function of the UW channel but also on the distance between the transmitter and the receiver. Our model for the scattering function was suggested by Rescheduled Acoustic Communications Experiment (RACE08) experimental data, leading to a multilag autoregressive (AR- q ) model for the fading.

Description: At very high speeds, underwater bodies develop cavitation at the trailing edges of sharp corners or from contours where pressures are sufficiently low to allow the formation of cavities containing water vapor. At sufficiently high speeds, a properly designed cavitator at the nose of an underwater vehicle can induce the formation of a vaporous cavity that entirely envelops the vehicle. The injection of gas behind the cavitator can result in the creation of cavities of comparable size at lower vehicle velocity and drag than would be required for vapor cavities. The formation of the cavity results in a significant reduction in drag on the vehicle and so-called high-speed supercavitating vehicles (HSSVs) have been reported to operate at speeds in excess of 100 m/s. The first part of this paper presents a derivation of a model for the longitudinal or pitch-plane dynamics of an HSSV. The vehicle is characterized by its mass and moment of inertia relative to a reference frame fixed to the body. The cavitator is assumed to be a disk, with a scale parameter that can be adjusted to represent an acute cone having an opposite sign for its lift curve slope. The control surface lift curve is specified relative to the cavitator lift effectiveness. A force model for a planing afterbody is also presented. The planing force model is found to be a significant source of damping and depending on a number of vehicle characteristics, the longitudinal dynamics may be stable. This result is significantly different than the conclusions of a number of previously published works. The final section of the paper examines the longitudinal stability at equilibrium of a 170-mm diameter HSSV. Results of parametric studies show the variation of pole locations associated with the transfer function relating cavitator angle to body pitch rate. The varied parameters are length, speed, fin effectiveness, body density, and the load carried on the aft planing surface.

Description: This paper introduces two methods that use a coherence analysis framework to generate synthetic aperture sonar (SAS)-like images that display acoustic color (AC) information useful for the classification of buried and/or proud underwater objects. The first method is applicable to sonar backscatter collected with multiple hydrophones and involves forming two channels using the data of two hydrophone subarrays at each frequency and over several pings. The second method is intended for applications where sonar backscatter is collected using a single hydrophone and involves forming two channels using the data of two synthetic subarrays at each frequency. In both cases, the resulting SAS-like AC images display information in a ping-frequency plane, and hence convey information that is useful for the detection, localization, and classification of underwater objects based on properties that are typically not conveyed by conventional SAS images. The single-hydrophone version of SAS-like AC processing has the added benefit of generating 3-D images that also display range information for enhanced localization capabilities. Furthermore, this coherence-based SAS-like method does not require the elaborate platform motion estimation and compensation used in conventional SAS. The effectiveness of these methods is demonstrated on two real sonar databases both by comparing generated SAS-like AC images to those generated using conventional methods and by applying a simple but effective classification framework directly to the AC features in the SAS-like images.

Description: Underwater explosions with energy yields equivalent to less than 40 kg of TNT are detected at hydrophones in the deep-sound channel at distances up to 16 000 km. The arrival times and azimuths of the signals are compared with values based on known explosion locations and times. Arrival-time mismatches are of the order of 10 s. Azimuth mismatches are less than half a degree. Signal arrival times and azimuths are input to a seismological global association algorithm and the resulting estimates of explosion times and locations are compared with actual values. Estimated locations are shown to lie within tens of kilometers of the actual explosions and estimated explosion times to be correct to within tens of seconds. All location estimates have spatial error ellipses that contain the location of the relevant explosion.

Description: Two different synchronous downlink multiple-access schemes, interleave-division multiple access (IDMA) and code-division multiple access (CDMA), are considered for pragmatic underwater communications channels exhibiting extended multipath spread and time variability. Two single-element DFE-IDMA and DFE-CDMA receivers are proposed that utilize chip-level adaptive decision feedback equalization (DFE) and carrier phase tracking along with iterative interference cancellation (IC) and channel coding. The receiver equations describing the detection algorithms are derived and their performance is investigated and compared. To track and compensate for the channel effects, the DFE and carrier phase tracking units are jointly optimized based on the mean square error (MSE) criterion and adapted iteratively by exchanging soft information in terms of log-likelihood ratio (LLR) estimates with the turbo processing stage. The detection is implemented by using soft chip cancellation to remove multiple-access interference (MAI) effects between users. The performance of the proposed receiver structures is investigated and compared in short-range shallow-water acoustic channels using offline processing of signals acquired during sea trials in the North Sea. Results for synchronous multiuser scenarios, with two and four users at an effective rate of 439.5 b/s per user, demonstrate that the proposed DFE-based IDMA and CDMA receivers can provide bit error rate (BER) performances of approximately 10 -5 at an average signal-to-interference-and-noise ratio (SINR) of 11 dB . The experimental results also demonstrate that these direct adaptive receivers have better performance and significantly mitigate the bit errors associated with the channel estimation (CE)-based IDMA and CDMA receivers with Rake reception while maintaining lower complexity. Furthermore, in some cases, the receivers with partial knowledge of the interleavers' patterns or codes can still achieve performance compa- - rable to those with full knowledge.

Description: The hypothesis tested is that internal gravity waves limit the coherent integration time of sound at 3709 km in the Pacific ocean at 133 Hz and a pulse resolution of 0.06 s. Five days of continuous transmissions at 2-min intervals are examined. The source and the receiver are mounted on the bottom of the ocean with timing governed by atomic clocks. Measured variability is only due to fluctuations in the ocean. A model for the propagation of sound through fluctuating internal waves is run without any tuning with data. Excellent resemblance is found between the model and data's probability distributions of integration time up to a day, which is the largest lag explored. The probability that the integration exceeds a day or more is about 0.15. The model underpredicts the probability of occurrence of integration times shorter than 10 min. However, the overwhelming agreement at longer times supports the conclusion that the standard spectrum of internal waves accurately explains almost all of the distribution of measured integration time.

Description: In this paper, we introduce the addition of an iterative, successive interference cancellation (SIC) process to improve on a multiuser, single-input-multiple-output (SIMO) communications receiver using passive time reversal as a space-time preprocessor. Time reversal has been shown to apply the spatial degrees of freedom to enhance the signal-to-noise ratio (SNR) and suppress interference for a target user. With the introduction of SIC, the receiver can remove the residual interference experienced by each user while preserving the SNR gain achieved by time-reversal preprocessing. The SIC process is a decision-directed approach for removing multiuser interference at the receiver and is similar to the decision-feedback equalizer (DFE) for intersymbol interference (ISI) channels. The interference experienced by each user is estimated at the receiver using previously decoded symbols from interfering users. This estimate is scaled and synchronized before subtraction from the target user's signal after time-reversal combining. Since SIC is applied before symbol decoding, symbol estimates are improved as the process is allowed to iterate until a stationary point is reached. Following time-reversal combining and SIC, a DFE can mitigate any remaining ISI before symbol decisions are made. Data collected from two Focused Acoustic Fields experiments (FAF-05 and FAF-06) are used to demonstrate the performance of the proposed interference cancellation scheme. During the FAF-05 experiment, three users transmitted 16-quadrature amplitude modulation (QAM) symbols simultaneously over the 3-4-kHz frequency band to a 20-element receiving array deployed in 120-m-deep water at a range of 20 km. The FAF-06 experiment included the simultaneous transmissions of 8-QAM symbols from two users over the 9-21-kHz band to a 16-element receiving array in 92-m-deep water at a range of 2.2 km. For both of the examples, SIC is shown to improve the output SNR in the presence of strong interference ove- - r time-reversal processing alone. This translates to a significant bit error rate (BER) reduction from 1.53 × 10 -2 to 8.80 × 10 -4 for the FAF-05 data and from 1.77 × 10 -3 to error-free decoding for the FAF-06 data.