With the underwater DGPS concept and the underwater acoustic navigation approach, the position of the AUV can be achieved with high precise given a set of range measurement from the AUV to known sonobuoys locations. In the set-up adopted for vehicle positioning the underwater pinger of an AUV carries a high precision clock that is synchronized with those of the sonobuoys (and thus with DGPS) prior to AUVs deployment. The pinger emits an acoustic signal every T seconds, at precisely known instants of time. As the underwater positioning of AUVs is done on the remote control station which uses upwards acoustic flow of data, one natural strategy for the undersea exploration is to use an Autonomous Surface Vessels (ASV) to control multiple AUVs motivated by the "reverse" underwater GPS method. With this method, the ASV can conveniently get all of the AUVs' positions in a heterogenous coordinated AUV team, because the pre-deployed four sonobuoys as a set transmit the DGPS positions themselves and the TOA of AUVs in a team to the central control ASV via radio link, and then generate and allocate the waypoints to the AUVs to implement the coordinated search or rescue scenario. As fig. 4 illustrated, a typical description of such heterogenous vehicle team is a ASV as a central control vehicle combined with three types low-cost AUVs based on DIS to provide navigation information. One type of them may be a vehicle with side-scan sonar to build maps of the ocean bottom and get rough list of interested objects with on-board computer-aided detection and classification. It will be followed by another vehicle with forward-looking high resolution sonar or camera for further identification of the object. The last type of vehicle with a dexterous underwater manipulator maybe intended to perform some sampling inspection operations.
The ASV communicates with AUVs via 3~5 kHz acoustic modem for long range bidirectional communication. Communications from the ASV to the AUVs in the heterogeneous team will support navigation and control information. Control commands include performing wide-scale mapping, making detailed acquisition or further identification over targets, drilling and sampling of interested objects, and so on. Navigation messages include the waypoint generated by the ASV as well as the positioning information for the specific AUV which will be accurately provided by the acoustic modem. AUVs will upload useful sensor information and compressed high resolution segments of data to the central ASV, and this function will be unnecessary if the vehicles are to be retrieved with all datum transferred on shore.
Without the INS/GPS onboard, the AUV with DIS need not periodically come up to the surface to calibrate their positions however seriously disturb the underwater vehicle team coordination and existed formation. In the side of underwater vehicles for navigation, just a good flux gate compass is the most important requirement, and a Doppler velocity sensor associated with a high quality attitude sensor can improve the overall system accuracy. From time to time, the AUVs will receive new heading settings via an acoustic link. The central control ASV will also move above the central of mass of the AUVs in order to keep a desirable distance between the ASV and AUVS to ensure the high quality acoustic communication. Thus the whole team with heterogenous vehicles could conveniently implement the coordinated search or rescue scenario as a whole base on the novel acoustic navigation scheme. Compared with traditional acoustic positioning methods, the high precision could also be conveniently achieved by the DIS without installation and calibration constraints and operational limitations. With this novel acoustic navigation scheme, it is possible for us to easily handle with heterogeneity in coordinated control task for underwater activities.
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