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Detecting and accurately locating snipers has been an elusive goal of the armed forces and law enforcement agencies for a long time. Most successful sniper-detecting systems are based on acoustic measurements. We develop an acoustic system that works well even in complex urban environments. Funded through the Network Embedded Systems Technology program of the US Defense Advanced Research Projects Agency's Information Exploitation Office, the PinPtr system uses a wireless network of many low-cost sensors to determine both a shooter's location and the bullet's trajectory by measuring both the muzzle blast and the shock wave. The PinPtr sensor-fusion algorithm, which runs on a base station, performs a search on a hyper-surface defined by a consistency function. This function provides the number of sensor measurements that are consistent with hypothetical shooter positions and shot times. The algorithm automatically classifies measurements and eliminates those that result from multipath effects or are otherwise erroneous. A fast search algorithm finds the global maximum of the surface, which corresponds to the shooter position.