Diver Navigation
INTEGRATED SENSORS HELP SWIMMERS NAVIGATE, MAP AND RECORD THE UNDERWATER BATTLESPACE.
With the Mk 107 Mod 0 Hydrographic Mapping Unit (HMU), the U.S. Navy SEALS now have a standalone navigation aid for clandestine hydrographic reconnaissance, mine countermeasures and ship-attack missions. Used in place of the familiar combat swimmer’s tactical board, the handheld device integrates complementary sensors to navigate underwater accurately, map the sea floor, spot obstructions and record images of underwater objects. The state-of-the-art HMU was designed and integrated by the Applied Research Laboratories of the University of Texas at Austin and built by Teledyne RD Instruments in San Diego. Though production deliveries to the Navy concluded in September, Teledyne RDI continues development of a commercial Map-Tac navigator with some of the same autonomous navigation capabilities for other users.
The laptop-sized Hydrographic Mapping Unit enables the diver to plan a route with GPS coordinates and navigate underwater using either the Doppler log or a long baseline master/slave beacon set taken from the INSS. The complementary sensors make the HMU accurate and covert. After using the Doppler log to navigate from the GPS delivery point to the search box, the diver verifies GPS coordinates near the surface and places master and slave acoustic beacons to define two corners of the box. Once the long-line beacons are set, the diver can navigate accurately inside the search box for many hours with the help of the HMU pinger receiver. The Doppler navigator is used to exit the field and swim a distance to a predetermined extraction point.
The HMU chart display shows the diver’s position at all times. The HMU also records hydrographic data throughout the mission, records the position and acoustic image of underwater objects and overlays the information on nautical charts for later analysis. Collected bathymetry data and marked target positions facilitate follow-on clearance or assault missions. The transponder beacons may be left in place for future missions.
The Naval Diving and Salvage Training Center that trains all but Special Forces divers says it teaches no special navigation techniques or equipment. However, underwater dead reckoning by magnetic compass is taught by civilian dive schools, and the offshore oil industry and other commercial users have developed more advanced navigation technologies. Combat swimmers who must navigate long distances accurately and undetected face special challenges. Unlike commercial divers, covert operators generally cannot rely on pre-positioned acoustic “pingers” to establish navigation baselines. Signals from Global Positioning System satellites are cut off just below the water’s surface, and surfacing frequently for GPS updates is tactically undesirable. GPS receiver buoys towed through ocean swells on 10- to 20-meter cables are likewise unwanted complications.
COMPASS AND CLOCK
SEALs and other special operations divers have long relied on a tactical swim board (Tacboard) with a compass and timer for dead reckoning. With board and body aligned, the user swims along a planned compass bearing and counts fin kicks over time to determine distance traveled and current position. A depth gauge built into the board is especially important for Special Operators who often use closed circuit diving equipment. Early swim boards were homemade by the different Special Operations communities. RJE Technologies’ TAC-100 underwater navigation board standardized the configuration and has been adopted by special forces units around the world. Still used today, it combines an underwater magnetic compass with a digital or analog depth gauge and chronometer, and it has a chemical- light holder to illuminate the instruments.
Dead reckoning based on swimmer kicks can nevertheless generate navigation errors of hundreds of meters. Though trained divers calibrate themselves to kick consistently, even their accuracy could be thrown off by currents and other factors. Minor errors in even the most accurate electronic fluxgate compasses accumulate over time and cause significant navigation errors. A more sophisticated navigation solution will have to integrate multiple sensors in a package small enough for an individual swimmer.
The DNC-100 diver navigation console, developed and manufactured by Oceana Integrated Technologies and distributed by RJE Technologies, is a small and rugged platform which allows a diver to navigate underwater autonomously without the need for acoustic baselines or floating GPS antennas. Completely self-contained, the DNC-100 arrives at a navigation solution using internal sensors which supply geodetic position, depth and heading while velocity information is provided by an imbedded speed chart. Built-in firmware takes this sensor information and plots the diver’s position with the support of a preloaded dive plan. Distance and bearing to target waypoint, depth, and other navigation information is displayed in real time to the diver via a full color sunlight readable LCD display.
Using piezo ceramic switches an operator can access all functions of the operational firmware to a depth of 30 meters. The sealed anodized aluminum housing provides protection for all internal components while rechargeable batteries allow the DNC-100 to be continuously operated for up to eight hours. Once activated, the built-in GPS receiver immediately provides accurate geodetic position to the on-board computer. Customized architecture allows for dive plans to be created using the DiveNav software suite.
DOPPLER VELOCITY LOG
Accurate velocity measurements are key to underwater navigation. Doppler velocity logs measure both velocity and altitude above the sea floor by reflecting acoustic signals off the ocean bottom. The frequency shift in the acoustic return provides a measure of speed and distance traveled and a profile of the ocean bottom. In the early 1990s, the Office of Naval Research awarded Teledyne RDI, then RD Instruments, a contract for CLAM—the Clandestine Littoral Acoustic Mapper that combined a Doppler velocity log with sonar mapping capability.
Although the CLAM demonstrator proved too big and too costly for the Navy, Teledyne RDI developed a commercial product based on its patented BroadBand Doppler technology in the Cobra-Tac diver navigation and mapping console. The Cobra-Tac enables swimmers to key-in a starting point based on GPS, LORAN or other surface navigation aids. It generates an underwater navigation display based on Doppler log updates. Four acoustic beams with temperature/pressure compensation track movement over the ocean bottom. An embedded computer integrates data from the Doppler sensor, fluxgate compass, and pressure transducer to map the topography of the ocean bottom. Divers can record bathymetry and navigation tracks and mark the locations of underwater objects.
Doppler navigation devices coupled with electronic compasses are still prone to errors. A Doppler device that momentarily loses its bottom return introduces errors to the position calculation. Submarines and remotely operated or autonomous underwater vehicles use Inertial Measurement Units (IMUs) with gyroscopes and threeaxis accelerometers to supplement their Doppler sensors. While magnetic compasses may suffer errors around 1 degree, highquality IMUs may hold a heading accuracy to 0.001 degree. However, the cost and size of high-quality IMUs are generally prohibitive in diver-navigation systems. The Hydrographic Mapping Unit for SEALs integrates Doppler technology with GPS and acoustic beacon receivers and forwardlooking imaging sonar.
INTEGRATED SOLUTION
The Applied Research Laboratories of the University of Texas (ARL:UT) have a long history in high-resolution sonar technology. ARL:UT developed the Underwater Imaging Sonar (UIS) with pinger beacon receiver and the Integrated Navigation and Sonar System (INSS) to detect and picture mines and other underwater objects. The INSS is used by Naval Special Clearance Team One including SEALS, Navy explosive ordnance disposal divers and Marine Corps force reconnaissance divers. In 2002, the Office of Naval Research funded the Hydrographic Reconnaissance Littoral Mapping Device (HRLMD) to integrate hardware and software from the INSS with a GPS receiver and the BroadBand Doppler velocity log. ONR generated the performance requirements for the HRLMD prototype developed at the University of Texas. Naval Surface Warfare Center Panama City also provided technical and programmatic support for the HRLMD. The HRLMD was later redesignated the Hydrographic Mapping Unit—today’s HMU—and RD Instruments received a build-to-print contract for 80 systems from the Naval Sea Systems Command in 2003. The company had previously won a buildto- print contract for the UIS.
WHAT NEXT?
Other underwater navigation and reconnaissance systems are in service or under development. BAE has supplied a Divers Electronic Beach Reconnaissance Aid (DEBRA) with electronic compass, Doppler velocity log, and GPS to the Royal Marines. QinetiQ in the U.K. has developed a diver reconnaissance system with a forward-looking sonar and long-baseline acoustic beacons similar to UIS.
Commercially available diver-navigation systems do not incorporate imaging sonar, but Teledyne RDI integrated GPS, Doppler and a magnetic compass into the Map-Tac diver mapping console. The device is accurate within two yards per 1,000 yards along the diver’s track or 3.5 yards per 1,000 yards cross-track. Collected bathymetric data is stored in the embedded computer and can be transferred to personal computers using either a USB memory stick or VHF radio. Map-Tac with color display, flash memory removable under water, and other refinements was offered to the Marine Corps Systems Command to satisfy requirements for Tactical Hydrographic Survey Equipment (THSE). With THSE requirements evolving, Teledyne RD Instruments plans to offer the Map-Tac as part of its diver-navigation and mapping-product line. ♦