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It is first elaborated that when the transmission time of underwater acoustic waves is estimated from the transmitting end to the receiving end, LFM pulse signal is often used, because after it is processed through matched filtering, a very important phenomenon will produce, which is pulse compression that a wide pulse in the time dimension is squeezed into a narrow pulse, so that transmission time can be accurately estimated; secondly it is explained that due to the multipath effect in the shallow water, reverberation signal in the propagation process and environmental noise and other factors, when a common detection method of fixed threshold is used, often the wrong detection can be caused, an adaptive detection method is presented for this problem, that is based on the pulse compression of the LFM signal, and the adaptive detection algorithm, the correlation peak of sound waves reach time is detected; finally through processing simulated data and measured data of the sea, it is verified that the this method is effective. Introduction It is a commonly used method of underwater acoustic positioning that the beacon is installed in the cooperation of underwater target, using acoustic path difference or phase difference from sound waves to the measuring array to locate[1]. But we know, in the communication process of actual ocean channel, signal detection and processing is a very important step, especially under the circumstances of the presence of multipath effect in shallow water, reverberation signal in the propagation process and the presence of environmental noise, it is very key how to accurately obtain the correlation peak of direct sound. In practical applications LFM signal is often used, we can first use the cross-correlation method of same frequency correlation and different frequency uncorrelation to filter the signal, in order to remove the interference effects of the noise, after signal is processed by matched filtering, a very important phenomenon will produce, that is pulse compression that a wide pulse in the time dimension is squeezed into a narrow pulse, so transmission time can be accurately estimated[2][3]. But in the actual ocean channel it is not easy to effectively detect the correlation peak, using common detection method of fixed threshold, wrong detection is offen generated. So this paper an adaptive detection method is proposed, which can effectively detect the correlation peak. Through the simulation and sea trial data, it is verified that this method is good and has certain practical value. Analysis of LFM pulse signal characteristics Mathematical expression of a linear frequency modulated pulse signal is given in Formula (1)[4], and its waveform is shown in Figure 1.    < < + = others T t t K t f j A t s L LFM 0 0 )]] * 2 / ( 2 exp[ Re[ ) ( 2 p (1) International Conference on Advances in Mechanical Engineering and Industrial Informatics (AMEII 2015) © 2015. The authors Published by Atlantis Press 290 Fig.1 LFM signal time-domain waveform It is a rectangular pulse that envelope width is T and frequency linearly changes with time. Among them, T f f K H L / ) ( − = is change rate of the signal frequency, or frequency rate, LFM signal FM range is ] , [ H L f f . After the LFM pulse signal is treated through self correlation matched filtering, a very important phenomenon will produce, that is pulse compression that a wide pulse in the time dimension is squeezed into a narrow pulse. The following is the analysis of this nature: the envelope of autocorrelation function absolute value of LFM pulse signal is given in Formula (2).      < < − − = others T t T K T K A RS 0 )] ( sin[ 2 ) ( 2

The Linear Frequency Modulation Pulse Signal Adaptive Detection Method