Sarah E. Webster

Advances in single-beacon one-way-travel-time acoustic navigation for underwater vehicles

This paper reports the formulation and evaluation of a centralized extended Kalman filter designed for a novel navigation system for underwater vehicles. The navigation system employs Doppler sonar, depth sensors, synchronous clocks, and acoustic modems to achieve simultaneous acoustic communication and navigation. The use of a single moving reference beacon eliminates the requirement for the underwater vehicle to remain in a bounded navigable area; the use of underwater modems and synchronous clocks enables range measurements based on one-way time-of-flight information from acoustic data-packet broadcasts. The acoustic data packets are broadcast from a single, moving reference beacon and can be received simultaneously by multiple vehicles within acoustic range. We report results from a simulated deep-water survey and real field data collected from an autonomous underwater vehicle survey in 4000 m of water on the southern Mid-Atlantic Ridge with an independent long-baseline navigation system for ground truth.

Preliminary deep water results in single-beacon one-way-travel-time acoustic navigation for underwater vehicles

This paper reports the development and experimental evaluation of a novel navigation system for underwater vehicles that employs Doppler sonar, synchronous clocks, and acoustic modems to achieve simultaneous acoustic communication and navigation. The system reported herein, which is employed to renavigate the vehicle in post-processing, forms the basis for a vehicle-based real-time navigation system. Existing high-precision absolute navigation techniques for underwater vehicles are impractical over long length scales and lack scalability for simultaneously navigating multiple vehicles. The navigation method reported in this paper relies on a single moving reference beacon, eliminating the requirement for the underwater vehicle to remain in a bounded navigable area. The use of underwater modems and synchronous clocks enables range measurements based on one-way time-of-flight information from acoustic data packet broadcasts. The acoustic data packets are broadcast from the single, moving reference beacon and can be received simultaneously by multiple vehicles within acoustic range. We report experimental results from the first deep-water evaluation of this method using data collected from an autonomous underwater vehicle (AUV) survey carried out in 4000 m of water on the southern Mid-Atlantic Ridge. We report a comparative experimental evaluation of the navigation fixes provided by the proposed synchronous acoustic navigation system in comparison to navigation fixes obtained by an independent conventional long baseline acoustic navigation system.

Preliminary Results in Decentralized Estimation for Single-Beacon Acoustic Underwater Navigation.

We report a decentralized, vehicle-based extended information filter designed to enable single-beacon acoustic navigation of multiple underwater vehicles. In single-beacon navigation, ranges from a single reference beacon to a moving underwater vehicle are used in addition to the vehicle’s inertial navigation sensors to perform absolute (as opposed to relative) localization and navigation. For this implementation we assume a moving reference beacon from which range measurements are calculated using asynchronous acoustic data broadcasts that also contain information about the reference beacon’s position and recent sensor measurements. We assume that other than these acoustic data transmissions the vehicle has no knowledge of the beacon’s position or sensor measurements. This implementation allows the simultaneous navigation of multiple vehicles within acoustic range of the reference beacon. Within this framework we show that the decentralized information filter yields identical results to a centralized extended Kalman filter at the instant of each range measurement; in addition we show that between range measurements the results from the two filters differ only by linearization errors. We compare the state estimation results of the decentralized information filter to that of a centralized extended Kalman filter using a simulated data set.

The Nereus hybrid underwater robotic vehicle for global ocean science operations to 11,000m depth

This paper reports an overview of the new Nereus hybrid underwater vehicle and summarizes the vehicles performance during its first sea trials in November 2007. Nereus is a novel operational underwater vehicle designed to perform scientific survey and sampling to the full depth of the ocean of 11,000 meters - almost twice the depth of any present-day operational vehicle. Nereus operates in two different modes. For broad area survey, the vehicle can operate untethered as an autonomous underwater vehicle (AUV) capable of exploring and mapping the sea floor with sonars and cameras. For close up imaging and sampling, Nereus can be converted at sea to operate as a tethered remotely operated vehicle (ROV). This paper reports the overall vehicle design and design elements including ceramic pressure housings and flotation spheres; manipulator and sampling system; light fiber optic tether; lighting and imaging; power and propulsion; navigation; vehicle dynamics and control; and acoustic communications.

Navigation and control of the Nereus hybrid underwater vehicle for global ocean science to 10,903 m depth: Preliminary results

This paper reports an overview of the navigation and control system design for the new Nereus hybrid underwater robotic vehicle (HROV). Vehicle performance during its first sea trials in November 2007 near Hawaii, and in May and June 2009 in the Challenger Deep of the Mariana Trench is reported. During the latter expedition, the vehicle successfully performed scientific observation and sampling operations at depths exceeding 10,903 m. The Nereus underwater vehicle is designed to perform scientific survey and sampling to the full depth of the ocean — significantly deeper than the depth capability of all other present-day operational vehicles. For comparison, the second deepest underwater vehicle currently operational worldwide can dive to 7,000 m maximum depth. Nereus operates in two different modes. For broad-area survey, the vehicle can operate untethered as an autonomous underwater vehicle (AUV) capable of exploring and mapping the sea floor with sonars and cameras. Nereus can be converted at sea to become a tethered remotely operated vehicle (ROV) to enable close-up imaging and sampling. The ROV configuration incorporates a lightweight fiber-optic tether (for high-bandwidth, real-time video and data telemetry to the surface), an electro-hydraulic manipulator arm, and sampling instruments. The Nereus vehicle is designed to render all parts of the Earths seafloor accessible to oceanographic science.

Advances in decentralized single-beacon acoustic navigation for underwater vehicles: Theory and simulation

This paper reports the theory and implementation of a decentralized navigation system that enables simultaneous single-beacon navigation of multiple underwater vehicles. In single-beacon navigation, each vehicle uses ranges from a single, moving reference beacon in addition to its own inertial navigation sensors to perform absolute localization and navigation. In this implementation the vehicles perform simultaneous communication and navigation using underwater acoustic modems, encoding and decoding data within the acoustic broadcast. Vehicles calculate range from the time of flight of asynchronous acoustic broadcasts from the reference beacon. Synchronous clocks on the reference beacon and the vehicles enable the measurement of one-way travel-times, whereby the time of launch of the acoustic signal at the reference beacon is encoded in the acoustic broadcast and the time of arrival of the broadcast is measured by each vehicle. The decentralized navigation algorithm, running independently on each vehicle, is implemented using the information form of the extended Kalman filter and has been previously shown to yield results that are identical to a centralized Kalman filter at the instant of each range measurement. We summarize herein the architecture and design of the acoustic communications (Acomms) system consisting of an underwater acoustic modem, synchronous clock, and the software necessary to run them, and salient results from the validation of the decentralized information filter using a simulated data set.

Feasibility analysis of an 11,000 m vehicle with a fiber optic microcable link to the surface

This paper discusses the feasibility of an 11,000 m underwater system that utilizes a battery-powered vehicle connected to the surface by a single channel fiber optic microcable. The proposed system has several benefits over conventional remotely operated vehicle (ROV) solutions for ultra deep applications. It avoids the prohibitively expensive tether and winch otherwise necessary for 11, 000 m work. Due to its relatively small size and low weight, the system allows for fast mobilizations and use on a variety of platforms. This concept also allows the vehicle to travel tens of kilometers away from the surface ship, which would be beneficial in applications where the surface ship has limited maneuverability such as under-ice work in the Arctic. This analysis focuses on the feasibility of using a microcable tether with a 450 N (100 lb) breaking strength housed in a constant force payout canister. Tension in the cable is modeled for current conditions in three areas of interest to the scientific community - a deep Western Pacific trench, the Arctic and the Juan de Fuca Ridge - and found to be well within the microcables working limit.

The predictability of acoustic receptions on gliders in the Arctic

Acoustic transmissions from shallow sources in the Arctic Ocean can propagate several hundred kilometers due to the presence of an Arctic acoustic duct. Receptions of these long-range transmissions are complex patterns of arrivals which are strongly dependent upon upper ocean sound-speed structure. In addition to measuring sound-speed parameters, gliders equipped with acoustic recorders can measure these arrivals and can complement moored receptions, providing data at many ranges with respect to the moored sources. There is a higher degree of uncertainty in glider position compared with moored receivers, but localization can be improved in post-processing with enhanced acoustic predictability. Two acoustic Seagliders were deployed for a short pilot study in late Summer 2016 in the vicinity of an array of acoustic tomography sources with frequencies on the order of 250 Hz in the Arctic Ocean in anticipation of a longer deployment in Summer 2017. Source receptions recorded on the gliders are compared with a...