In underwater observation using an autonomous underwater vehicle (AUV), a support vessel typically monitors the AUV to support the observation. The AUV have to be positioned to be tracked by the vessel and informed the positioning result for navigation with acoustic communication. This process is significant especially in deep water. The AUV uplinks to inform its status and transmit some observation data in real time, and sometimes be commanded to change the observation plan by downlink from the vessel. Authors planned to use an autonomous surface vehicle (ASV) to track and monitor multiple AUVs for efficient observation. An acoustic multi-access communication and positioning system have developed as one of the elemental technologies for the observation system.
The developed acoustic system achieves multi-access with frequency division multiple access (FDMA) method, and the ASV can monitor up to three AUVs simultaneously. Positioning is performed with super short baseline (SSBL) method. The acoustic device has operation mode in which positioning and communication functions are integrated, called as auto mode. In auto mode, uplink packets can be very close and the AUV can uplinks efficiently, and timing of downlink is calculated from positioning result appropriately.
Results of two sea trials are shown. The precision of positioning was enough to track the AUV. However random error was much larger than that of SSBL of the vessel, hence some precise method is necessary to keep the quality of observation data high. Communication and auto mode sequence worked well in the sea trial.