在线英语听力室_免费在线英语听力学习网站

 　　When the first shots were fired at JFK's murder case,police couldn't immediately locate the gunman based on the sound alone. Today, the technology exists for them to do it with their smart phones,less than a second after the first shot. Here's how, most bullets travel at super sonic speeds, generating a shock wave along their path. To track that path, researchers built a small bluetooth sensor¹ for smart phones. The sensor uses four mikes 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 smart phone screen with in second of the gun blast. Researchers tested the system with a AK-47 and were able to calculate the shooter's bearing with less than 7 degrees of error, and get decent estimate of its range. They presented the method at the conference on the information processing and sensor networks in Philadelphia. 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 the fast as a speeding bullet.