Imagine a platoon of soldiers carrying personal digital assistants that can display the location of enemy shooters in three dimensions and accurately identify the caliber and type of their weapons.
Engineers at Vanderbilt University’s Institute for Software Integrated Systems (ISIS) have developed a system that can give soldiers just such an edge by turning their combat helmets into “smart nodes” in a wireless sensor network.
ISIS developed the technology with the support of the Defense Advanced Research Projects Agency (DARPA). The university has patented the system’s key elements.
Like several other shooter location systems developed in recent years, the ISIS system relies on sound waves produced when a high-powered rifle is fired. These acoustic signals have distinctive characteristics that allow the systems to pick them out from among other loud noises and track them back to their source. Current systems, however, rely on centralized or stand-alone sensor arrays. This limits accuracy and restricts them to identifying shooters at line-of-sight locations.
The ISIS system combines information from a number of nodes to triangulate on shooter positions and improve the accuracy of its location identification process. It also uses a patented technique to filter out echoes that can throw off other acoustic detection systems, explains Akos Ledeczi, PhD’95, the senior research scientist at ISIS who heads up the development effort.
“When DARPA gave us the assignment of creating a shooter location system using nodes with very limited capabilities, they didn’t think we could solve the technical problems,” Ledeczi admits. “At first I didn’t think we could do it either, but we figured out how to make it work.”
Retired U.S. Army Lt. Col. Albert Sciarretta, who assesses new military technologies in urban environments for DARPA, is one of the experts impressed by the ISIS system: “Its strong points are that it isn’t limited to locating shots fired in direct line-of-sight, it can pick up multiple shooters at the same time, and it can identify the caliber and type of weapon being fired.”
When a high-powered rifle is fired, it produces two different kinds of sound waves. One is the “muzzle blast” that expands outward in a spherical wave from the muzzle. The second is a conical shock wave produced by the bullet as it travels at supersonic speed. Each node of the shooter location system contains an array of four sensitive microphones. If at least three of the microphones in a single node detect the muzzle blast, the information allows the nodes’ microprocessor to calculate the direction from which the sound came. If the same array also detects the arrival time and angle of the bullet shock wave, a simple calculation gives the shooter’s location.
“Because the microphones on the helmet are so close together, the precision is not very high,” Ledeczi says. “However, the nodes are continuously exchanging the times and angles of arrival for these acoustic signals, along with their own locations and orientations. When two or more nodes detect the shot, they can provide the bearing with better than one degree accuracy. The range is typically within a few meters even from as far as 300 meters. The more sensors that pick up the shot, the more accurate the localization.”
The ISIS system communicates its findings with the personal digital assistants that the soldiers carry. The PDAs are loaded with maps or overhead pictures of the area upon which the shooter locations are displayed.