A prototype of such acoustic navigation system proposed in this chapter consists of a set of light surface buoys usually more than three of them. The physical characteristics of the buoys with acoustic module(hydrophone), in terms of size, weight and autonomy, will tend to those of a standard sonobuoy, with however the capability of micro-controller, processing of acoustic signals, local data storage, and online transmission by a radio modem. The most peculiar characteristic is a DGPS receiver integrated on board, the hydrophone and the ARM CPU is synchronized before deployment with the DGPS clock datum. One possible simplified schematic of a sonobuoy with DGPS is shown on fig. 3. We call it as a DGPS Intelligent Sonobuoy system.
The core of the DIS is ARM7 CPU named as S3C44B0. An extended 16M memory is for data storage as well as a data buffer. An acoustic module based on MOTOROLA DSP. A two way radio link with 200M Radio Frequence (RF) allows for communication within ranges up to 40/50 km. The DGPS board for 1us precision timing and localization of the buoy. The system was designed to be deployed switch off and auto-activate in order to avoid electronic equipment damage by severe deck and banging during deployment and recover. The DIS system usually consists of four surface buoys which use the hydrophones to receive the acoustic impulses and record their times of arrival (TOA). The pinger onboard the vehicle emits two successive acoustic pulses during each emission cycle so that the pinger/vehicle depth is proportional to the time delay between the two pulses. The ranges or distances between the buoys and the pinger/vehicle can be translated into the times of arrival, and the value of sound speed in the water is assumed to be known. The buoys transmit the information of the DGPS position and the time of arrivals to the a Autonomous Surface Vehicle as a central station via radio link, where the true position of underwater target is computed by triangulation or by using more sophisticated algorithms. However, the dropouts and outlier of the acoustic positioning system should be carefully dealt with due to acoustic path screening, partial system failure, and multipath effects. See (Vagannay et al., 1996)for a treatment and discussion on this challenging topic, and (Alcocer et al., 2004) also gives an intensive study to deal with the dropouts and outliers in real time.
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