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Abstract-Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sampling network (AOSN). Multiuser communication allows vehicles,moorings, and bottom instruments to interact without human intervention to perform adaptive sampling tasks. In addition, multiuser communication may be used to send data from many autonomous users to one buoy with RF communications capability, which will then forward the information to shore. The two major signaling techniques for multiuser acoustic communication are phase-shift keying (PSK) direct-sequence spread-spectrum (DSSS) and frequency-shift keying (FSK) frequency-hopped spread-spectrum (FHSS). Selecting between these two techniques requires not only a study of their performance under multiuser conditions, but also an analysis of the impact of the underwater acoustic channel. In the case of DSSS, limitations in temporal coherence of the channel affect the maximum spreading factor, leading to situations that may be better suited to FHSS signals. Conversely, the multipath resolving properties of DSSS minimize the effects of frequency-selective fading that degrade the performance of FSK modulation. Two direct-sequence receivers potentially suitable for the underwater channel are presented. The first utilizes standard despreading followed by decision-directed gain and phase tracking. The second uses chip-rate adaptive filtering and phase tracking prior to despreading. Results from shallow water testing in two different scenarios are presented to illustrate the techniques and their performance.

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Abstract. This paper evaluates the dynamics of two faunal assemblages colonizing high‐ and low‐temperature habitats of the TAG hydrothermal mound on the Mid‐Atlantic Ridge. Timelapse video and photographic transects were used to monitor changes in shrimp and anemone populations from June 1994 to March 1995. During this period, holes were drilled in the vicinity of the target populations by the Ocean Drilling Program (ODP). The a priori expectation of a rapid redistribution of motile alvinocaridid shrimp to occupy the optimal thermal and chemical habitat in response to changes in the pattern of hydrothermal activity was met by the observation of an increase in hydrothermal activity and shrimp population density at the site where a timelapse video system was deployed. The importance of hydrothermal activity as a control on the distribution of vent shrimp is suggested by tidal variations in the distribution of shrimp on a minidiffuser chimney. Sediment deposition during ODP drilling resulted in a temporary disturbance of the shrimp occupying the flat part of the mound surface seen by the timelapse video. In contrast to the changes observed in shrimp distribution, little change was observed in the distribution of anemones across the mound in two photographic transects 10 months apart, with the exception of a localized disturbance where anemones were buried by sediment around an ODP hole. Differences in the response of the shrimp and anemones to changes in the hydrothermal system may result from differences in their motility and rates of biological activity, or reflect a lower trophic position in the case of the shrimp.

This paper describes the design and use of two new autonomous underwater vehicles, Jaguar and Puma, which were deployed in the summer of 2007 at sites at 85• N latitude in the ice-covered Arctic Ocean to search for hydrothermal vents. These robots are the first to be deployed and recovered through ice to the deep ocean (> 3500m) for scientific research. We examine the mechanical design, software architecture, navigation considerations, sensor suite and issues with deployment and recovery in the ice based on the missions they carried out. Successful recoveries of vehicles deployed under the ice requires two-way acoustic communication, flexible navigation strategies, redundant localization hardware, and software that can cope with several different kinds of failure. The ability to direct an AUV via the low bandwidth and intermittently functional acoustic channel, is of particular importance. Based on our experiences, we also discuss the applicability of the technology and operational approaches of this expedition to the exploration of Jupiter's ice-covered moon Europa.