April 30, 2009 Imagine being able to pinpoint an enemy shooter in difficult terrain with such deadly accuracy that you can see whether they are kneeling or standing and not only what kind of weapon they are firing but the caliber too. Well, engineers at Vanderbilt University's Institute for Software Integrated Systems (ISIS) have developed such a system by turning soldiers' combat helmets into "smart nodes" in a wireless sensor network.

The ISIS location system

ISIS has developed the technology with the support of the Defense Advanced Research Project Agency and Vanderbilt University has patented the system's key elements. The shooter surveillance system carried by the soldier – or it could be a police officer, for that matter – consists of a PDA that shows the enemy location in three-dimension and accurately identifies the caliber and type of weapon being used. The combat helmet of the soldier has four “smart nodes” attached, which work as a wireless sensor network to pinpoint the signal.

There have been several other sniper location systems developed in recent years and like most of these, the ISIS system relies on the sound waves that are produced when a high-powered rifle is fired.

Where the ISIS system differs, though, is that it doesn’t use centralized or stand-alone sensor arrays to pick out the distinctive characteristics of the ballistic sound waves. According to Akos Ledeczi, the senior research scientist at ISIS who heads up the development effort, the ISIS system combines information from a number of nodes to triangulate on shooter positions and improve the accuracy.

In combination with this, it uses a patented technique to filter out the echoes that can throw off other acoustic detection systems, says ISIS. This not only means the system can locate shots fired in a direct line of sight, but it can also pick up multiple shooters at the same time and the caliber and type of weapon each they are using.

More detail on how it works

The sound waves of a high-powered rifle can take two forms. The muzzle blast expands out in a spherical wave form, while the conical shock wave is produced by the bullet traveling at supersonic speeds. Each node of the ISIS shooter location system uses an array of four sensitive microphones. All that is required is for three of the microphones in a single node to detect the muzzle blast. This is where the node's microprocessor steps in to diagnose the sound wave. In doing this, it can determine the direction of the sound, the angle of the bullet shockwave and the arrival time – all which then provide the shooter's location.

With the nodes so close together on the wearers helmet, precision is an issue. However, the nodes are continuously exchanging the times and angles of arrival for the acoustic signals, along with their own locations and orientations. If two or more nodes detect the shot, they can provide the bearing within one degree of accuracy to a range within a few meters at 300 meters.

The more sensors that detect the shot the more accurate is its ability to locate the shooter. These findings are communicated to the PDAs which are loaded with maps and overhead pictures of the shooter's area. To overcome the inaccuracy of standard GPS keeping track of the mobile nodes and spotty satellite coverage in urban areas, the ISIS team recently added an inexpensive radio chip to track the relative position of nodes using high-precision radio interferometry. A patent has been applied for by the university.

The wrap

The ISIS shooter system uses wireless nodes invented at UC Berkeley and produced by Crossbow Technology Inc of San Jose, Calif. The entire node for the ISIS system weighs slightly more than the four AA batteries that power it and costs about $1,000 to construct. This is much cheaper, say ISIS, than other commercial shooter location systems that range in price from USD$10,000 to USD$50,000. For more information visit Vanderbilt University.

David Greig