Summer 2008

The 21st century battlefield has moved predominantly from open environments to cities, towns and villages. Military Operations on Urban Terrain (MOUT) testing facilities on bases around the country allow the services to prepare to fight in these surroundings.

photo by Gary Meek GTRI researchers adjust an acquisition waveform of the local positioning system.

While soldiers test equipment, tactics and procedures for clearing buildings and patrolling urban streets, videos capture their movement in selected MOUT structures. However, the cameras cannot capture every decision made and action taken during the test.

A team led by Georgia Tech Research Institute (GTRI) principal research engineer Dinal Andreasen aims to change that. The team is developing a wideband local positioning system to track the location of soldiers every second during an exercise, even when they're inside buildings.

"You can't just hand each soldier a global positioning system (GPS) device and track his or her movement," says Barry Sharp, a GTRI senior research engineer. "GPS loses accuracy and tracking ability in buildings, so it's not helpful if you're monitoring inside structures."

So Sharp, Andreasen and senior research engineers Mike Baden and Andre Lovas chose to transmit radio frequency waveforms that degrade more slowly as they pass through buildings. With four transmitters located up to a football-field-length away from the buildings, the GTRI researchers plan to demonstrate that they can locate a receiver-carrying soldier to within six inches.

To track the soldier, each transmitter will send out very long signals at two wavelengths – 915 megahertz (MHz) and 5.49 gigahertz (GHz). Since the higher 5.49 GHz signal can avoid most interference, including wireless networks, microwaves and walkie-talkies, it would be optimal to find a soldier's position with that signal alone. However, the Federal Communications Commission (FCC) limits the power that can be transmitted with the 5.49 GHz signal to 0.1 milliwatts.

"If the receiver searched for the 5.49 GHz signal, it would take almost 10 minutes to find its position," Baden notes.

Alternatively, the lower 915 MHz signal can be transmitted at a much higher power, allowing for better penetration through walls. The use of two frequencies is necessary to overcome the limitations of each. The researchers divide the 915 MHz signal into 128 different segments so they can quickly find an approximate position of the receiver.

Then they search that approximate location in the 5.49 GHz signal to find the receiver's exact location, reducing the search time to approximately five seconds. Once the receiver knows its distance from four transmitters, trilateration – similar to triangulation – determines the exact location.

Beyond tracking soldiers, the GTRI researchers believe their positioning system could be helpful in tracking firefighters inside burning buildings. If four emergency vehicles arriving at a fire had GPS and a transmitter, the local positioning system could automatically configure itself to track the firefighters inside the building.

"If a firefighter is in distress and not moving, our system could guide another firefighter to the distressed firefighter's location to save him or her," Sharp explains.

GTRI researchers are collaborating with James Buxton at the Aberdeen Test Center in Maryland and local company Research Network Inc. on this project. This research was funded by the Test Resource Management Center's Test and Evaluation/Science and Technology Program through Naval Undersea Warfare Center contract N66604-06-C-2335.

– Abby Vogel