The simple transition from available navigation techniques based on electromagnetic signals for mobile robots or flying robots to underwater vehicles, is not applicable due to the peculiarities and constraints of the underwater environment, as the electromagnetic signals do not penetrate below the sea surface. The good propagation characteristics of sound waves in water makes acoustic positioning and navigation as a feasible candidate, and the related study of the implications of such methodology for the underwater vehicles has been conducted for a long time.
As fig. 1 illustrated, classic acoustic approaches for underwater vehicle positioning, include Long Baseline (LBL), Short Baseline (SBL), Ultra-short Baseline (USBL) Systems, and Long & Ultra Short Baseline (LUSBL), etc.
The application and performance of this challenging area have been investigated by many researchers (Vickery, 1998). In the LBL case, a set of acoustic transponders is pre-deployed on the seafloor with the geometry of interested vehicles centered. The vehicle position is achieved by the basis of the acoustic signal returns detected by the transponders with the required accuracy (Collin, 2000). In the SBL side, a dedicated ship follows the underwater vehicle at short range with a set of three hydrophones to determine the AUV position, and the AUV can also get its absolute position via the bidirectional communication among the
AUV and the mother ship (Storkensen, 1998). USBL systems are very similar to SBL principles except that the transducers are built into a single transceiver assembly or an array of transducer elements in a single transceiver. The distances are measured as they are in an SBL system but the time differences are replaced by the "time-phase" of the signal in each element with respect to a reference in the receiver. The "time-phase differences" between transducer elements are computed by subtraction and then the system is equivalent to an SBL system. The LUSBL system is a special case of a USBL system. It utilizes USBL hardware in a configuration similar to the one described for the LBL system. Range and bearing in an LUSBL system are still measured as described for a basic USBL system. However, because a larger number of beacons are deployed on the seabed, a considerable improvement in accuracy may be achieved.
Although all these classic acoustic methods have been used for a long time, there are still some disadvantages existed in practical utilization. LBL systems require long time with associated costs for deployment and comprehensive calibration at each deployment. SBL systems is installed on a dedicated ship so that they are in poor signal to noise ratio due to ship's self noise and the accuracy of acoustic positioning can only be achieved in calm weather and without ship motion which also lies on USBL and LUSBL systems.
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