Measurements of temporal coherence of sound transmissions through shallow water

Abstract

In this paper we report the measurements of temporal coherence of acoustic signals propagating through shallow water using data from three experiments in three different parts of the world, with sound speed standard deviation (STD) varying from 0.3 to 5m∕s near the layer depth. Temporal coherence is estimated from the autocorrelations of broadband channel impulse functions, the latter are deduced from broadband signals transmitted through the ocean during the experiments. The measurements covered three frequency bands: low frequencies below 1.2kHz, midfrequencies between 2 and 5kHz, and high frequencies between 18 and 22kHz. The source-receiver range covers 3, 5, 10, and 42km. The signal coherence-time is defined and deduced from the data. Motivated by previous theoretical work in deep water on the signal coherence-time as a function of the signal frequency, the source-receiver range, and sound speed STD, a similar but empirical analysis is applied to the measured data in shallow water. While the range dependence agrees with the theory, the data exhibit a different dependence on the signal frequency than the theoretical prediction for deep water.

Doppler Frequency Estimation for Time-Varying Underwater Acoustic Communication Channel

“…음파가 전달되면서 흡수, 손실됨은 물론 해수면과 해저면에 의한 반사와 산란, 다양한 잡음원들, 해류에 의한 송수신기 이동으로 발생하는 도플러 효과까지 수많은 요소들이 복잡하게 작용한다. 그 중에서도 도 플러 효과는 실질 전송률과 관계된 채널의 상관시간 에 영향을 미친다 [5] . …”

Performance of Time Reversal Based Underwater Target Detection in Shallow Water

“…This simulation investigated the relationship between the coherence of the clutter channel [26] and the detection probability of the proposed approach for TCR varying from −20 dB to 20 dB. The false alarm probability P FA was set to 10 −4 and the threshold was adjusted according to the coherence values.…”

“…Note that retransmitting the time reversal signal from the received signal is a kind of matched filtering for the time-invariant components, while it is a kind of energy spreading for the time-varying components [25]. The time domain correlation analysis according to distance and frequency shows that the time-varying characteristics of the shallow water channel become stronger as the distance becomes longer and as the frequency increases [26]. For this reason, the time reversal technique can achieve good performance even if the scattering environment is complicated.…”

Measurement of Spatial Coherence of Active Acoustic Sensor Array Signal

“…해양에서 신호의 상관성에 영향을 미치는 주요 변 동 요인은 수온의 시간 변화와 해면파, 내부파 등으 로 알려져 있으며, 이와 관련하여 신호의 시간 상관 성과 해양학적인 요인의 상관관계 분석에 대한 많은 연구가 진행되었다 [3][4][5] . 장거리 심해에서 시간 상관 성에 영향을 주는 해양 요인은 내부파가 가장 잘 알 려져 있다 [6][7][8][9] .…”