New system locates origin of incoming fire
The time difference between the shock wave and the arrival of the blast — and clever algorithms — yield complete information about the direction, elevation, and range, all in less than a second and a half. The shooter’s direction is indicated as a clock position on a small console, the range and elevation are displayed on an LED screen, and all the information is spoken aloud by a recorded voice.
The company says that once the microphone array and the signal processing had been worked out, the principal technical challenge was eliminating false positives — sounds that might be misidentified as gunshots. Early on, slamming a Humvee door would be enough to trigger the system, Schmitt says. “It would look enough like a [bullet’s] shock wave that we would do the processing and report it falsely.” After some tinkering, BBN got Boomerang into shape. The trade‑off in reducing false positives was in reducing the system’s ability to figure the origin of bullets that do not come within thirty meters of a moving vehicle or fifty meters of a stationary one. Schmitt says that a soldier does not care about gunfire that misses by such a wide margin, “and Boomerang hasn’t registered a single false alarm in the past two years in Iraq and Afghanistan,” he claims.
At the same time, BBN began testing a bullet-detection system that can be mounted on helicopters. Such a system is much more complicated, in part because a helicopter can move much faster and in more directions. Bullets are so fast that Boomerang needn’t factor in the movement of a Humvee, even when traveling at 100 kilometers per hour. But a helicopter’s greater speed can’t be ignored, so the new system uses accelerometers to factor it in. A single set of sensors arranged on a mast strapped to the top of a vehicle serves all types of cars and trucks, but this won’t work on a helicopter, which can be shot at from below as well as above. The microphones have to be carefully positioned, and each model will require a custom configuration. Finally, the new system uses only the shock wave-and more sophisticated algorithms-not the muzzle blast, which cannot be reliably measured from a helicopter.
Last June and then again in August, a prototype was tested at Fort Rucker, in Alabama. Previous tests had collected data to be processed off-line back at BBN; these were the first to process data on board. Both BBN and the Department of Defense declined to comment on their outcomes, but Schmitt says that he believes a deployable version will be ready sometime in 2009.
