Optimising Performance of a Long Range Ultra Short BaseLine Tracking and Telemetry System.

While Ultra Short BaseLine (USBL) is a well-established methodology for subsea positioning, there has usually been a trade-off between high precision short-range performance and low precision long-range performance based on choice of frequency. Furthermore, the susceptibility of low frequency systems to vessel generated noise can degrade the performance of such low frequency long range systems. As a result, the majority of USBL systems used in ocean science have utilised Medium Frequency (MF 19–34 kHz) operation as a compromise between precision and range, including the Ranger 2 USBL systems produced by Sonardyne International Ltd. These are widely used on a range of research vessels; however, increasing use of Autonomous Underwater Vehicles (AUV) and other subsea platforms have pushed the need for longer range systems with integrated telemetry. In response to this need, Sonardyne have developed a variant of Ranger 2, operating in the Lower Medium Frequency (LMF 14–19 kHz) Band. While this band is subject to a wider variety of noise sources on the vessel, the potential gains in range as a consequence of lower absorption are significant. Indeed reducing from a centre frequency of 24kHz to 16kHz effectively halves the absorption in the water path. The LMF transceiver head is outwardly identical to the MF transceiver head, with both being capable of positioning precision of up to 0.1% of range. During 2017, Sonardyne undertook two trials of the system: one onboard the Monterey Bay Aquarium Research Institute’s (MBARI) vessel, RV Rachel Carson; and, secondly one onboard the National Oceanography Centre’s (NOC) vessel, RRS James Cook. The RV Rachel Carson is a former oilfield support vessel acquired in 2011 by MBARI, while the RRS James Cook is a modern purpose built multidisciplinary research vessel built to the International Council for the Exploration of the Sea Cooperative Research Report 209 standard (ICES 209), which has become a de-facto standard for acoustically quietened research vessels. These trials comprised both positioning and telemetry trials at slant ranges increasing to over 11,000 metres and depths of up to 4,500 metres. In addition, a number of baselining trials were undertaken to characterize noise sources on the vessels, under a variety of propulsion configurations and in parallel to other scientific system operation. In particular, the RRS James Cook has twin USBL spars, so it was possible to undertake side-by-side comparison between MF and LMF systems in a variety of operational modes. In both cases, consistent slant range positioning was achieved at ranges in excess of 11,000 metres, although modelling results suggest that positioning up to 15,000 metres is possible using a directional beacon, which has a higher (7dB) source level than the beacon used in the trials. Telemetry trials, using Sonardyne’s standard schemes ranging from 200 – 9,000 bps were also undertaken at a variety of slant ranges, with actual achieved data transmission rates in excess of 2,000 bps (at 17mJ per bit) being achieved at a slant range of 7,200 metres during the RRS James Cook trial. This paper outlines both trials and discusses the modelled and field data for each: In particular, the impact of vessel configuration is discussed, noting especially the insignificant difference in system performance achieved between two very different vessels. While this is in some part due to the fact that ICES 209 is targeted at ship design considerations for frequencies below those being used by USBL, it was also noteworthy that there was little difference in radiated noise of the RRS James Cook in stationary, station-keeping (dynamic positioning) and underway modes. Most importantly though, the difference observed between noise received on the MF and LMF systems was ca. 5dB, which was more than offset by the increased received sound pressure level and enabled the observed gain in range. Overall, both trials demonstrated the capability to extend USBL performance well beyond ranges achievable using MF, with no loss in precision and the capability to support high rate acoustic communications.