A mining crew is trapped deep underground after a cave-in. Firefighters run into a smoke-spewing high-rise to battle a violent blaze. A team of soldiers breaches a door and storms into a dark building.

In any of these life-threatening scenarios, the direst question on the lips of rescuers or supervisors is, “Where are they right now?”

Dr. Alonzo Kelly, a professor at Carnegie Mellon University’s National Robotics Engineering Center, has a solution to answer that query. He and his team have figured a way to accurately locate people in places like buildings and mines where there is no GPS signal to help out.

And the answer fits inside the heel of a shoe.

Finding the way with each footfall
They have created a wearable suite of sensors that measure footsteps to analyze a person’s location. The system uses dead reckoning, a navigation method that calculates current position based on the direction and speed a user has moved away from a known starting point.

Dubbed MINT, short for Micro-Inertial Navigation Technology, the system is comprised of a computer and three sensor units embedded in a pair of boots. In the heel of each boot sits an inertial measurement unit (IMU). The ball area of one of the boots is fitted with a tiny radar device that measures the distance each IMU travels with each footstep.

“We’re building reference as users move around,” Kelly says. “We can draw their path overlaid on a map or floor plan of a building or a mine.”

MINT isn’t needed where GPS is available. The problem is that GPS only works where a receiver can pick up a signal from satellites. But people often find themselves in places where reception drops to zero—inside buildings and urban canyons, underground, underwater and below dense tree canopies.

The average person’s walking speed equates to about 3,600 steps an hour, which means MINT performs that many reference position updates as a user walks around. It also means that the system’s location accuracy improves as time passes. Kelly says it can now provide a wearer’s position accurate to better than 33 feet within 30 minutes of walking around.

For soldiers and firefighters in buildings
Funding for the project comes from the U.S. Defense Advanced Research Projects Agency, and the military version of the MINT unit is more accurate. Since the system’s positioning data can be transmitted from a wearer to a supervisor or command center, Its possible military applications include coordinating soldiers’ movements while searching caves and clearing buildings. In a post on its website, Darpa says it is looking for accuracy “on the order of 1 meter [3.3 ft.] over 10 hours…”

“The combination of micro scale navigation aiding sensors will provide navigation accuracy that a traditional IMU – equipped with only accelerometers and gyroscopes – cannot accomplish,” the agency says.

Though MINT has not yet gone through the process to put it on the market, Kelly says components for a unit useful to firefighters or miners currently costs $500. The military-grade unit’s pieces cost $5,000. He estimates that three engineers could have a commercial prototype ready in a year.

“There’s an intense interest from the firefighting community in what we’re doing,” he says. “A supervisor could track where firefighters are and monitor them, or the system could let a firefighter retrace his path to get out of a building.”

With more programming work, an even smarter system could tell a firefighter a faster and safer way to get out. If multiple MINTs were linked up to a squad along with other basic sensors, the network could provide all-around situational awareness: where all the firefighters in a burning building are scattered, the condition of the building where others are, the ambient temperatures around each unit, among other important pieces of information. “We’re discovering that there are so many things that you can do,” Kelly says.

Underground intelligence
For possible application in the mining industry, MINT went through a successful National Institute of Occupational Safety and Health (NIOSH) demonstration in an underground mine more than a year ago.

But mines are hostile places—for people and communication signals. Numerous agencies and researchers are studying how to get wireless signals, like those transmitted by a MINT unit, out of underground tunnels if an emergency occurs. They have so far found that the tunnels themselves can act as waveguides for cell phone radio frequencies. Other radio frequencies offer better options in the event of an emergency: at lower frequencies, radio signals can propagate through conductive materials like cables, pipes, power lines and wires. At the lowest frequencies, NIOSH says, radio signals can penetrate earth.

The agency says a 2006 law mandated that coalmines install wireless communications and electronic tracking systems underground, which positioning systems could use to broadcast the whereabouts of stranded miners.

“These mines have invested in their communication infrastructure,” Kelly says. “If you have a way to locate yourself—using MINT or some other location-positioning method—then the communications structure will already be there.”

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