Saturday, November 18, 2017
Published: Nov 2015
Authors
  • Peter Spain
    • Peter Spain
    • Company: Teledyne RD Instruments
Categories
  • Navigation Systems

AUVs are on track with BroadBand DVLs / Teledyne RD Instruments celebrates two decades of navigating unmanned vehicles

Do you remember navigating with a street atlas? What a difference GPS makes. And now it seems every other new gadget includes a GPS chip for supplementary input. Similarly, better navigation tools optimized operations that use unmanned underwater vehicles (UUVs). Plus better navigation permitted expanded capabilities and greater cost-effectiveness. Together these advantages delivered a major gain for users of tethered and untethered UUVs. These are Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs).

  • explorer_pa_pd.jpg
  • Hydroid REMUS at surface.jpg
  • gavia pic.jpg
  • nav-family-0315.jpg
  • explorer_pa_pd.jpg Fig. 1. Teledyne RDI’s Explorer DVL. This 600 kHz sonar provides navigation for small underwater platforms that require reduced size and weight. Explorer has been used on AUVs, gliders, ROVs, and diver platforms.
  • Hydroid REMUS at surface.jpg Fig. 2. Hydroid’s REMUS 100. This portable AUV carries a range of sensors. Operating to 100 m depth, this vehicle has seen action in a diverse range of surveys. Other Hydroid AUVs can reach much greater depths.
  • gavia pic.jpg Fig. 3. Teledyne Gavia’s AUV. This vehicle is made up of interchangeable modules that can be configured in the field. This AUV is intended for various types of survey work to 1000 m depth.
  • nav-family-0315.jpg Fig. 4. Teledyne RDI’s full family of DVL products

During the last decade, AUVs have rapidly advanced from experimental prototypes. They are now considered reliable tools that have operated in diverse undersea activities. AUVs can complete a mission independent of topside support and then return home reliably.

An essential springboard for this new-found autonomy has been much improved navigational systems. A key part of better navigation came from using BroadBand Doppler Velocity Logs (DVLs). In this article we review how they have contributed to the groundswell in using AUVs.

Increased Value to Users

AUVs see action in varied marine industries: offshore engineering, defense, and research. Low-noise Doppler velocity logs (DVLs) enable improved control of these AUVs — motion and positioning. Due to this improved control, other onboard sensors can now realize their true effectiveness.

Direct improvements affect productivity. These include better vehicle position, control, and stability. One example is long-term station keeping and the associated hover footprint. Others are better cruise control and more efficient and exact tracking and surveys.

Indirect improvements include superior output products, like mapping and visualization. One upside is more and higher-quality data during inspections and surveys. Another is making full use of high-resolution imaging sensors and cameras. Examples range from detecting and identifying submerged mines to inspection of structures and pipeline, as well as scientific exploration of fields of hydrothermal vents.

Any external control of a submerged AUV or the setup of its sensors relies on acoustic communication. For the user, this link affects both the productivity of the AUV and its output products. Teledyne Benthos provides a reliable acoustic link with an AUV across a mile underwater.

Productivity improves when the AUV does not need to surface routinely and send data through the air. Output products are better if sensors can be reconfigured while still on the job. For example, an AUV can repeat one survey line with different settings of the sensors. On seeing results, operators can send the optimal setup for the survey.

Doppler Velocity Logs

The motion and position of AUVs can change in horizontal and vertical directions. Doppler Velocity Logs (DVLs) are sonar systems that use sound to measure these changes.

Key advantages stem from using sound. Most important, it can measure speed-over-ground from a remote distance. Plus there are no rotating parts or electromagnetic sensors to foul. And sound changes predictably with ocean conditions. If the seabed is too far, the DVL can still provide measurements of speed-through-water.

Like the radar gun used to catch speeding cars, DVLs use the Doppler Effect to measure motion. This is the change in pitch of returning echoes compared to the transmitted sound. Echoes are measured along several beams. Combining data from these echoes tells how fast the vehicle is moving and in what direction.

These velocity measurements can be used to navigate the AUV. Integrating them over time shows the trajectory or path followed by the vehicle. Conversely, if an AUV is hovering, the motion data can aid the control system to hold position.

As a navigation system, the DVL can be stand-alone or part of an integrated system. The latter delivers even better performance. The DVL’s data stream includes velocity, altitude, depth, heading, pitch and roll, and temperature.

Teledyne RD Instruments (TRDI)

Teledyne RDI’s DVLs are the industry standard for Doppler-aided undersea navigation. They operate on most of the world’s scientific, commercial, and military AUVs. TRDI has a unique, track-record in using BroadBand Doppler processing. This signal processing opened a new era in low-noise and accurate Doppler velocity data. Real-time navigation and control systems benefited from improved update rates and precise positions. So too did users of onboard imaging sensors that operate with high resolution in time.

As well as their superior data quality, these DVLs are reliable, even in rough terrain. TRDI’s transducer configuration Janus) includes a redundant beam. This permits not only more robust operation but data QC on a ping-by-ping basis. Plus the bottom-tracking algorithms have a  legacy of refinement.

DVL products from TRDI are available in two styles of transducer: Phased Array and Piston. Explorer DVLs that operate at 600 kHz are available in either style. See Figure 1. This flexible design was developed for customized products.

Navigator DVLs are piston only; they come in higher frequencies from 1200 to 300 kHz. The piston design came first and measures lower-noise velocity data.

Pioneer and PAVS products are supplied in phased array configuration. There are several high (Pioneer) and low (PAVS) frequency models. They have greater range to bottom for any given transducer size. Plus this style has some size & weight advantages. They can be compelling for smaller vehicles and for achieving maximal range.

Recent projects

Deep-Sea Science

It seems much of the AUV industry stems from the Woods Hole Oceanographic Institution. Among their supply of vehicles is the AUV Sentry. Sentry’s navigation system includes a TRDI DVL. Using its science payload, the AUV explores both and near-seabed depths to 6000 m depth. Equipped with several acoustic imaging systems and cameras, Sentry has captured unique deep-sea terrains. Even while on the bottom, this AUV can be retasked using acoustic communications. These deep-sea scientists have been enthusiastic about improved navigation tools. They cite the precise positioning and reliable operation provided by the BroadBand DVL as key elements of better surveys.

Environmental Assessment

horizontal coverage of subsurface oceanographic data has not been common. AUVs have been widely perceived as a means to address this shortcoming. One example is a recent study in the northern Adriatic Sea by the Institute of Marine Sciences in Italy. They used a Hydroid REMUS-100 (Fig. 2) to examine spatial changes in coastal waters during floods.

Off one flooding river, the AUV ran a survey pattern that included several sections across the outflow’s edge. The researchers examined the dispersal patterns of suspended sediments and fresh water. They found an unexpected result across the visible edge of the river plume. Suspended sediments showed marked decline whereas the deviation in salinity remained unchanged. It seems different mixing models will apply for the dispersion of different river properties.

Resource Mapping

One invasive species of long-spine sea urchin strips reefs bare of living forms. Via this environmental destruction, these urchins can undermine important fisheries. The pest is extending its range along Tasmania’s east and south coast. A response strategy to the urchin spread requires a detailed map of their existing habitat.

A Teledyne Gavia AUV (Fig. 3) performed an exploratory mission using bathymetric mapping sonar. The AUV was deployed by scientists of the Australian Maritime College and Institute for Marine and Antarctic Studies. Their goal was to map urchin barrens and classify bottom types. They also identified where vegetation was reduced. Using the AUV permitted 100 times greater areal coverage than prior methods. At the same time, the mapping resolution is 10 times finer due in part to the precise navigation provided by the TRDI DVL.

A different use for the Teledyne Gavia AUV was to survey and picture sub-bottom conditions in Narragansett Bay USA. The AUV carried a sub-bottom profiler (SBP) from Teledyne Benthos. This SBP operates at high frequency and uses chirp technology. During the mission, the SBP setup and status of the Gavia AUV were controlled using an acoustic modem from Benthos.

These surveys revealed unseen details in near-surface sedimentation. Vertical resolution of the images was 20 cm. These results required a stable low-noise platform that can fly at low and constant altitude above the bed. This capability resulted in part from the BroadBand navigation of the AUV

Search Missions

Until recently, searching for lost vessels has been the preserve of ROVs. Now AUVs have become active wreck hunters. Earlier this year, a Bluefin 12 AUV was used to find a sunken Japanese battleship from World War II. The wreck was at 1000 m depth off the Philippines. In the Indian Ocean, a Bluefin 21 AUV had searched for the missing Malaysia Airlines MH370 plane.

Last year, a Hydroid REMUS 100 surveyed off southern UK where two U.S. Landing Ship Tanks (LSTs) were lost. They had been sunk during a training exercise before The AUV captured high-resolution imagery of the two LST wreck sites, located in 50 m depth. The low-noise Doppler velocity data enabled the steady navigation and control of these AUVs during the imaging surveys.

Looking Ahead

Trends in the industry point to increasing demand for smaller size, longer tracking range, improved power consumption, and more integrated technologies. Delivering on these challenges will no doubt add to the bright future of AUVs. Partnered with other Teledyne Marine companies, TRDI will continue to expand its product offerings for better navigation tools to optimize AUV operations. Recent additions include lower frequency DVLs to achieve longer range.

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