Nancy Sinatra once mused that her boots were made for walking. In these days of global positioning, going walkabout is not as random an event as it might once have been, but there are still occasions when the all-seeing GPS device can't pick up a satellite. In such cases, having a back-up could mean the difference between getting out of the deep, dark underground cave in one piece or being lost in its tunnels forever. Researchers from North Carolina State University and Carnegie Mellon University have combined technology that is used to measure speed and distance with portable radar equipment to help keep track of a user's location.

Given the number of satellites orbiting our little planet and the huge number of portable devices that regularly receive location information from them, one would assume that there's nowhere to escape GPS technology. But there is.

"There are situations where GPS is unavailable, such as when you’re in a building, underground or in places where a satellite connection can be blocked by tall buildings or other objects," said Dr. Dan Stancil, co-author of the paper entitled A Low-Power Shoe-Embedded Radar for Aiding Pedestrian Inertial Navigation. "So what do you do without satellites?"

Microelectromechanical systems equipped with accelerometers and gyroscopes known as inertial measurement units (IMUs) can help to track pedestrian movement and speed, and can be used to inform a user of the distance traveled. Working in conjunction with GPS, this technology could be used to guide users back to a point where the GPS signal was lost.

However, any minor miscalculations could lead users even further into those dangerous and dark recesses in underground caverns. For example, it's possible that the IMU could record movement of around 4 inches per second (0.1m/s) even though a user is actually standing still. After a few minutes, the system will show you about 60 feet (18m) away from your actual position. Not ideal, and possibly quite dangerous.

To address the accumulate errors, the research team has developed a portable embedded terrain-relative velocity radar system prototype using modular, commercial off-the-shelf components that attaches to a shoe or boot.

The radar is connected to a small navigation computer that measures the distance between the heel and the ground at regular intervals. If it doesn't change, then the system knows that the user isn't actually moving and resets the IMU's inertial velocity reading to zero.

The prototype is quite bulky, so any future development will likely look at reducing the size of the unit and lowering its power needs.

The research paper was co-authored by Dr. Chenming Zhou, James Downey, and Dr. Tamal Mukherjee from Carnegie Mellon University and Dr. Daniel Stancil from North Carolina's Department of Electrical and Computer Engineering. It has been published in the October issue of IEEE Transactions On Microwave Theory And Techniques