When the first shots were fired at JFK's motorcade, police couldn't immediately locate the gunman based on sound alone. Today, the technology exists for them to it with their smartphones, less than a second after the first shot.
Here's how. Most bullets travel at supersonic speeds, generating a shock wave along their path. To track that path, researchers built a small bluetooth sensor for smartphones. The sensor uses four mics to measure the shock wave's angle, and its time of arrival. Then each phone networks with nearby phones to triangulate the sniper's location, mapping it on the smartphone screen within a second of the gun blast. Researchers tested the system with an AK-47, and were able to calculate the shooter's bearings with less than seven degrees of error, and get a decent estimate of his range. They presented the method at the Conference on Information Processing in Sensor Networks in Philadelphia. [János Sallai et al., Acoustic Shockwave-Based Bearing Estimation]
Since the system requires at least two phones to work, researchers say it would be ideal for a security force fanned out around a likely target—allowing them to respond to threats almost as fast as a speeding bullet.
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