In the Swim

April 1, 2005 By: Marty Whitford GPS World

GPS Guides Autonomous Underwater Vehicles

It resembles a distant cousin of the torpedo. And sometimes it launches from submarine torpedo tubes. But the similarities end there. Among its many other roles, this cigar-shaped device noses about in hostile waters to identify mines and other explosives, to secure passageways for military missions.

U.S. Navy mine warfare intelligence teams deployed Remote Environmental Monitoring UnitS (REMUS) autonomous underwater vehicles (AUVs) during the 2004 invasion of Iraq. The Iraqi military had planted several mines by the country's port of Umm Qasr, and a line of REMUS AUVs named SAHRVs (Semi-Autonomous Hydrographic Reconnaissance Vehicles) answered the call to perform wide-area surveys and underwater mine reconnaissance in waters up to 100 meters (330 feet) deep.

An underwater drone, REMUS 100 locates suspected mines so dolphins and military divers together can confirm mine status, and safely detonate and clear the explosives away. Today, 80 percent of REMUS AUVs carry GPS receivers.

Tech-Savvy Swimmers

Dead-reckoning sensors and a Doppler Velocity Log (DVL) guide REMUS AUVs underwater. With an inertial navigation system (INS) tightly integrated with a 12-channel, WAAS-enabled GPS receiver — leveraged at AUV launch and upon each resurfacing — the REMUS 100 can search programmed tracks and pinpoint potential mines using side-scan sonar.

Proprietary correlator technologies in this Bluefin AUV’s GPS sensor help mitigate multipath errors and common signal delays of 37 meters caused by military ships' superstructures.

After processing of the sonar images, specially trained dolphins can inspect potential targets to distinguish between natural and man-made objects, so valid mines can be tagged and later safely neutralized using charges. In Umm Qasr, the Navy's AUV-dolphin-diver trio cleared a passageway in a mine-seeded port channel so coalition forces could transport 232 tons of food, water, blankets, and other supplies.

In the first Persian Gulf War, the U.S. Navy (USN) reportedly recovered more than 1,300 water mines. During the 2004 invasion, mine forces investigated a total of 478 contacts, 86 of which were determined to be mine-like objects. The REMUS AUVs did what no man or mammal alone could do, locating potential targets in silt-filled waters with visibility of less than 1 foot and currents of 2–5 knots.

"The hard sandy bottom at Umm Qasr was perfect for REMUS, and it went out first to do the tedious work of 'mowing the waters.' REMUS works for up to 20 hours without tiring, so it performs well at surveying large areas and locating potential targets," said Christopher von Alt, leader of the Woods Whole Oceanographic Institution (WHOI) engineering team that developed the 19-by-160-centimeter REMUS AUV.

REMUS underwater drones are the most widely used autonomous underwater vehicles (AUVs) for military applications worldwide.

GPS Plus Dead Reckoning

Following a license agreement with WHOI, Hydroid Inc. now manufacturers REMUS AUVs. More than 70 of the systems are in use today, mostly by the military for water mine countermeasure (MCM) operations; for other applications, see "Naval Commands" sidebar.

Prior to offering GPS and INS capabilities, REMUS AUVs navigated using Doppler Velocity Log (DVL)-assisted dead reckoning with a flux gate compass; Ultra Short Baseline (USBL) navigation; and Long Baseline (LBL) navigation using bottom-deployed transponders.