Jea Soo Kim

Long-range multi-carrier acoustic communications in shallow water based on iterative sparse channel estimation

Long-range orthogonal frequency division multiplexing (OFDM) acoustic communications is demonstrated using data from the Kauai Acomms MURI 2008 (KAM08) experiment carried out in about 106 m deep shallow water west of Kauai, HI, in June 2008. The source bandwidth was 8 kHz (12-20 kHz), and the data were received by a 16-element vertical array at a distance of 8 km. Iterative sparse channel estimation is applied in conjunction with low-density parity-check decoding. In addition, the impact of diversity combining in a highly inhomogeneous underwater environment is investigated. Error-free transmission using 16-quadtrative amplitude modulation is achieved at a data rate of 10 kb/s.

Multi-static scattering characteristics of submerged objects with experimental investigation

The scattering characteristics of target echoes are essential for detecting and classifying the submerged objects. The target strength, which is widely used in mono-static sonar system, is also important in multi-static sonar system to identify the submerged target. In this presentation, a series of experiments in the acoustic water tank was conducted to measure the target echoes from submerged cylinder-shaped target with multi-static measurement system, which consists of a single transmitter and 16 receivers. The target strengths are presented in 2-dimensional plane as a function of receiver position according to target aspect angle. The numerical simulation results based on Kirchhoff approximation are presented to explain some characteristics of the measured multi-static target echoes. [Work supported by Agency for Defense Development, Republic of Korea.]

Cascade of blind deconvolution and array invariant for robust source-range estimation

The array invariant proposed for robust source localization in shallow water is based on the dispersion characteristics in ideal waveguides. It involves conventional plane-wave beamforming using a vertical array, exploiting multiple arrivals separated in beam angle and travel time, i.e., beam-time migration. The approach typically requires either a short pulse emitted by a source or the Greens function that can be estimated from a probe signal to resolve distinct multipath arrivals. In this letter, the array invariant method is extended to unknown source waveforms by extracting the Greens function via blind deconvolution. The cascade of blind deconvolution and array invariant for robust source-range estimation is demonstrated using a 16-element, 56-m long vertical array at various ranges (1.5-3.5 km) from a towed source broadcasting broadband communication waveforms (0.5-2 kHz) in approximately 100-m deep shallow water.

Investigation of Target Echoes in Multi-static SONAR System - Part I : Design for Acoustic Measuring System

The target echoes contain information on the target such as the orientation, kinematics, and internal structure, as well as the external geometrical shape of the target. In addition, the pattern of the target echoes depends on the arrangement of the transmitters and receivers in space. Therefore, the study of the target echoes in a multi-static SONAR system can be useful for detecting and tracking submerged objects using an underwater surveillance system. For this purpose, an acoustic measuring system for multi-static target echoes was designed and tested in an acoustic water tank. Some preliminary data are presented and discussed.

Investigation of Target Echoes in Multi-static SONAR system - Part II : Numerical Modeling with Experimental Verification

A multi-static SONAR system consists of the transmitters and receivers separately in space. The active target echoes are received along the transmitter-target-receiver path and depend on the shape and aspect angle of the submerged objects at each receiver. Thus, the target echo algorithm used with a mono-static system, in which the transmitter and receiver are located at the same position, has limits in simulating the target echoes for a multi-static SONAR system. In this paper, a target echo modeling procedure for a 3D submerged object in space is described based on the Kirchhoff approximation, and the SONAR system is extended to a multi-static SONAR system. The scattered field from external structures is calculated on the visible surfaces, which is determined based on the locations of the transmitter and receiver. A series of experiments in an acoustic water tank was conducted to measure the target echoes from scaled targets with a single transmitter and 16 receivers. Finally, the numerical results were compared with experimental results and shown to be useful for simulating the target echoes/target strength in a multi-static SONAR system.

Time delay estimation via zero-crossing of envelope

Time-delay estimation of an acoustic signal in an ocean waveguide is a challenging due to the received signal corrupted by noise and phase distortion caused commonly by reflection of the transmitted signal traveling to a remote receiver via more than one path. In this study, envelope-based Hilbert transform method is applied to enhance the accuracy of time-delay estimation. Through the mathematical interpretation and numerical simulations, the proposed method is compared to the conventional time-delay estimation methods (e.g., cross correlation and zero-crossing of Hilbert transform). It is found that the envelope-based Hilbert transform method more efficiently estimates the time-delay than the conventional methods and resolve ambiguity of time delay estimate.

Simultaneous estimation of array tilt and source range using broadband ship noise and a vertical array

The array invariant, a robust approach to source-range estimation in shallow water, is based on the dispersion characteristics of broadband signals in ideal waveguides. It involves time-domain planewave beamforming using a vertical line array (VLA) to separate multiple coherent arrivals (eigenrays) in beam angle and travel time. Typically, a probe signal (i.e., a cooperating source) is required to estimate the Green’s function, but the array invariant has been recently extended to a ship of opportunity radiating random signals using a ray-based blind deconvolution. Still, one major drawback is its sensitivity to the array tilt, shifting the beam angles and adversely affecting the array invariant parameter that determines the source range. In this paper, a simple optimization algorithm for simultaneous estimation of the array tilt and the source range is presented. The method is applied to a ship of opportunity (200–900 Hz) circling around a 56-m long VLA at a speed of 3 knots (1.5 m/s) at ranges of 1.8-3.6 km in approximately 100-m deep shallow water. It is found that the standard deviation of the relative range error significantly reduces to about 4%, from 14% with no compensation of the array tilt.The array invariant, a robust approach to source-range estimation in shallow water, is based on the dispersion characteristics of broadband signals in ideal waveguides. It involves time-domain planewave beamforming using a vertical line array (VLA) to separate multiple coherent arrivals (eigenrays) in beam angle and travel time. Typically, a probe signal (i.e., a cooperating source) is required to estimate the Green’s function, but the array invariant has been recently extended to a ship of opportunity radiating random signals using a ray-based blind deconvolution. Still, one major drawback is its sensitivity to the array tilt, shifting the beam angles and adversely affecting the array invariant parameter that determines the source range. In this paper, a simple optimization algorithm for simultaneous estimation of the array tilt and the source range is presented. The method is applied to a ship of opportunity (200–900 Hz) circling around a 56-m long VLA at a speed of 3 knots (1.5 m/s) at ranges of 1.8-3....

Tracking a surface ship via cascade of blind deconvolution and array invariant using a bottom-mounted horizontal array

The array invariant developed for robust source-range estimation typically requires a priori knowledge about the source signal (i.e., cooperating source) to estimate the Greens function. However, the array invariant method has been recently extended to a surface ship radiating random signals (200-900 Hz) by extracting the Greens function via blind deconvolution using a vertical array [J. Acoust. Soc. Am. 143. 1318–1325 (2018)]. In this paper, the blind deconvolution is applied to a 60-m long, bottom-mounted horizontal array to extract the Greens function of the same ship circling around the array in a square spiral pattern at ranges of 300–1500 m. The overall tracking performance shows good agreement with GPS measurements except when the ship is towards the broadside with respect to the horizontal array. Further, simultaneous localization of multiple ships is discussed.The array invariant developed for robust source-range estimation typically requires a priori knowledge about the source signal (i.e., cooperating source) to estimate the Greens function. However, the array invariant method has been recently extended to a surface ship radiating random signals (200-900 Hz) by extracting the Greens function via blind deconvolution using a vertical array [J. Acoust. Soc. Am. 143. 1318–1325 (2018)]. In this paper, the blind deconvolution is applied to a 60-m long, bottom-mounted horizontal array to extract the Greens function of the same ship circling around the array in a square spiral pattern at ranges of 300–1500 m. The overall tracking performance shows good agreement with GPS measurements except when the ship is towards the broadside with respect to the horizontal array. Further, simultaneous localization of multiple ships is discussed.

Frequency-wavenumber analysis with a sparse array

In underwater acoustics, there has been many studies for finding the target direction using beamforming technique. When receiving a signal with a frequency higher than the design frequency of the array, it is difficult to estimate the direction of the signal due to spatial aliasing. In this study, we propose a method of estimating the direction of the frequency signal higher than the design frequency of the array by using frequency-wavenumber analysis. When the frequency-wavenumber analysis is performed, the striation pattern appears, and it is confirmed that the slope of the striation remains constant even if the spatial aliasing occurs. The direction of the signal was estimated by visual inspection and it was verified with SAVEX15 data.

Frequency-difference wavenumber analysis with a sparse array

Frequency (f)- wavenumber (k) analysis can be used to estimate the direction of arrival (DOA) [J. Acoust. Soc. Am. 69, 732–737 (1980)]. When the receiver is a sparse array that is not suitable for conventional plane-wave beamforming, it adversely causes aliasing error due to spatial sampling, thus many striation patterns can emerge in f-k domain. In this study, we propose frequency-difference wavenumber analysis that is motivated frequency-difference beamforming [J. Acoust. Soc. Am. 132, 3018–3029 (2012)]. It is found that this approach can mitigate (or eliminate) such aliasing effect, which extends its applicability to the robust DOA estimation. Numerical simulation and experimental results are presented, and a major drawback is discussed.Frequency (f)- wavenumber (k) analysis can be used to estimate the direction of arrival (DOA) [J. Acoust. Soc. Am. 69, 732–737 (1980)]. When the receiver is a sparse array that is not suitable for conventional plane-wave beamforming, it adversely causes aliasing error due to spatial sampling, thus many striation patterns can emerge in f-k domain. In this study, we propose frequency-difference wavenumber analysis that is motivated frequency-difference beamforming [J. Acoust. Soc. Am. 132, 3018–3029 (2012)]. It is found that this approach can mitigate (or eliminate) such aliasing effect, which extends its applicability to the robust DOA estimation. Numerical simulation and experimental results are presented, and a major drawback is discussed.