Real-time TDOA measurements of an underwater acoustic source

The direction of arrival of sound waves has been extensively used for passive acoustic tracking of underwater sound sources, such as marine mammals or ultrasonic electronic tags attached to animals or submerged equipment. This process can be automated by measuring the time difference of arrival (TDOA) of the sound wave arriving to two or more hydrophones and then calculating the relative direction of the acoustic source using those time differences. Although the generalized cross-correlation between the received signals is a common technique for determining the TDOA, the underwater environment introduces several distortions in amplitude and phase of the received sound waves due to reflections and reverberation, particularly in confined spaces. This is aggravated by the variation of the sound propagation speed with temperature, pressure and salinity. Because of this, the use of the cross-correlation method not effective to determine the TDOA especially when using single frequency pulses as the transmitted signal. In this work we propose an alternative method to calculate the TDOA, consisting in the analysis of the initial part of the received signals to discover a series of similar zero-crossing periods to identify their beginning, and then calculating the time difference between them. We have implemented this technique in a reconfigurable system-on-chip, attaching to an embedded ARM processor a custom designed digital signal processing system. This has been tested in a test tank and in outside environment. This system is capable of computing in real-time the 2D direction of an underwater acoustic transmitter, and combining the different directions resulting from the relative movement between transmitter and receiver it is possible to estimate the relative position of the acoustic source.