An acoustic positioning system is an essential element for navigating underwater vehicles and it usually achieves underwater positioning by acoustic communication between acoustic transceiver and transponder. We are developing an underwater acoustic transceiver which is capable of localizing a remote autonomous underwater vehicle as well as communicating data with it. It is based on quadrature phase-shift keying (QPSK) and adopts a linear equalizer to compensate the underwater acoustic channel distortion. For positioning of remote transponder, the transceiver has a two-dimensional acoustic sensor array which consists of four receiving and one transmitting transducers. It can estimate the transponder position by time of arrival and incident angles obtained from phase of cross spectrum. In order to obtain the spatial diversity gain of multiple receiving elements, equal gain combining (EGC) is applied to the equalized data and information bits are recovered after the combining. To evaluate its performance, we carried out an experiment in an anechoic water tank which has two main signal transmission paths of line-of-sight path and surface reflected path. Through the experiment, we compare two different positioning methods; one using time of arrival only and the other using incident angles estimated from phase of cross spectrum. We also analyze the effect of channel equalizer on positioning accuracy. The results demonstrate that positioning by the incident angle estimation is superior to the other when the channel equalizer is used and EGC greatly reduces bit-errorrate of data communication.
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