Autonomous Ocean Sampling Network Research Articles

Power systems for autonomous underwater vehicles

In this paper, we examine the issues involved in designing battery systems and power-transfer (charging) techniques for Autonomous Underwater Vehicles (AUVs) operating within an Autonomous Ocean Sampling Network (AOSN). We focus on three different aspects of the problem, battery chemistry, pack management and in situ charging. We look at a number of choices for battery chemistry and evaluate these based on the requirements of maximizing power density and low temperature operation particular to AUVs. We look at the issues involved in combining individual cells into large battery packs and at the problems associated with battery monitoring, and the charging and discharging of packs in a typical AUV application. Finally, we present a methodology for charging an AUV battery pack in situ in support of long term deployments at remote sites.

Mooring developments for autonomous ocean-sampling networks

Two general-purpose mooring designs have been developed to support autonomous underwater vehicle (AUV) operations in autonomous ocean sampling networks (AOSNs). These moorings provide two-way communications between investigators and AUVs docked on the moorings or conducting survey operations some distance from the moorings. A deep-water design that incorporates an AUV dock and recharging station was built for use in the Labrador Sea during the winter of 1997/1998. This severe winter environment required a robust design that could operate unattended for six months while isolating the dock from surface wave motion. A much lighter, easier-to-deploy design was developed for use in coastal waters to extend the nearshore AOSN operating area by extending the communications network. This coastal design has been deployed without the dock component and has typically been configured for use in a small network of moorings maintained with a small research vessel. The deep-water mooring has been deployed successfully on two occasions, for short periods of time. The coastal moorings have been deployed a number of times and have proven to be quite effective. This paper describes the two moorings in detail and provides information on their performance so that interested investigators can utilize the technology where it meets their needs.

Analysis of channel effects on direct-sequence and frequency-hopped spread-spectrum acoustic communication

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.

Enabling technologies for REMUS docking: an integral component of an autonomous ocean-sampling network

A dock for an autonomous underwater vehicle (AUV) allows the vehicle to be left on station ready for deployment. However, it represents a significant engineering challenge, as docking requires an accurate navigation system so that the vehicle can find the dock, and complex mechanics to make the required underwater power and data connections. This paper describes the docking system built for the REMUS AUV. It outlines the basis for the design decisions, the as-built configuration, and its performance once deployed. It also delineates the lessons learned from the deployments, and the refinements in the vehicle that have been made since that time, that will improve the system's utility and reliability.

Underwater Vehicle Networks for Acoustic and Oceanographic Measurements in the Littoral Ocean

Recent progress in underwater robotics and acoustic communication has led to the development of a new paradigm in ocean science and technology, the Autonomous Ocean Sampling Network (AOSN). AOSN consists of a network of fixed moorings and/or autonomous underwater vehicles (AUV), tied together by state-of-the-art acoustic communication technology. The GOATS2000 (Generic Oceanographic Array Technology Systems) Joint Research Program is aimed toward the development of environmentally adaptive AOSN technology specifically directed toward Rapid Environmental Assessment in coastal environments. The research program combines theory and modeling of the 3-D environmental acoustics with experiments involving AOSN and sensor technology. To explore the potential of providing a reliable environmental forecasts of the coastal ocean, the observational capabilities are coupled with a nested ocean modeling and assimilation framework, consistently coupling basin scale oceanography with small scale ocean processes. The central element of the the program is a prototype underwater vehicle network managed remotely by a surface platform, including moored sources and arrays, AUVs equipped with a variety of acoustic and environmental sensors, and reliable navigation and communication system.

Shallow-water REA using autonomous ocean sampling networks

Environmental management and naval operations in coastal waters are severely limited by a lack of oceanographic predictive capability associated with the strong spatial gradients and temporal variability inherent to such environments. A key to improving the predictive capability is a measurement system which can provide data in close to real time for assimilation into oceanographic circulation models. In that regard, the Autonomous Ocean Sampling Network (AOSN) combines small, autonomous underwater vehicles (AUV) with an underwater communication and navigation network to provide an ocean measurement system with real-time data access and adaptive sampling capabilities. The acoustic communication capability of the AOSN allows for feedback to the sensor platforms of uncertainties of the field estimates, information which can then be used to redistribute the AOSN resources optimally. The development of a new observation and prediction capability combining the AOSN with efficient coastal circulation models is the objective of an ongoing multidisciplinary research effort, but the feasibility has already been demonstrated through a series of field deployments. The results of these experiments are reviewed, and the potential for this new rapid environmental assessment capability for littoral environments is discussed.