Understanding Gunshot Detection Technology

High-tech police tool coming to more U.S. cities

Even the best police gumshoe can go in circles chasing down a reported gunshot. Was the sound echoing from another direction? Was it just a firecracker? Sorting it out takes precious minutes - especially if a victim lies wounded, or a criminal is slipping away.

Enter gunshot detection technology.

Sixteen cities across the country have installed ShotSpotter, a system of rooftop listening devices that triangulates the origin of gunshots and pinpoints, in seconds, the location on a map. This week, Boston introduces a plan to spend $1.5 million on the system.

The company, ShotSpotter Inc., touts the system's ability to gather forensics, including when shots were fired, how many, from what angle, and, in some drive-bys, the direction that the car was moving.

But the system is not dead-on accurate, meaning police must be circumspect about how they use the new trove of data, warn civil liberty advocates. Data from ShotSpotter has not yet been challenged in court, and both the company and defense attorneys predict an eventual showdown.

"As long as these kinds of systems have a margin of error, it seems to me there are always going to be questions about whether or not it's appropriate for police to move forward on an investigation absent some independent corroboration of the report," says Ed Yohnka, communications director with the Illinois chapter of the American Civil Liberties Union (ACLU).

ShotSpotter has notched some success stories:

* Washington, D.C., police say the system helped them capture a suspect fleeing from a gunshot homicide.

* Redwood City, Calif., has reportedly cut celebratory gunfire dramatically, and Oakland, Calif., police say they caught a man firing off a gun in his backyard.

* ShotSpotter Inc. says its system saved the life of a gunshot victim in 2004 in "an East Coast city." Nobody called 911, but the sensors alerted police.

Less headline-grabbing are the cases seen in Minneapolis since installing ShotSpotter last month. Police have netted three felons, two semiautomatic guns, and recovered one stolen car. It also provided additional information in three shooting cases.

"It's just a better compass. It still takes good cops, persistent investigation, and good police skills," says Lt. Gregory Reinhardt, spokesman for the Minneapolis police department. "It's just pointing us in a better direction."

However, Lt. Reinhardt admits that none of the arrested felons and confiscated items were necessarily involved in the original shooting. In one case, police arrived to find a car speeding off. Police pursued, then apprehended a suspect - a convicted felon - who tried to flee. In the car was a loaded semiautomatic pistol. In two other cases, police arrived to find people loitering. On each occasion they took names and found a person wanted on a warrant.

"It's sort of hard to fathom that the purpose of the thing is to put police in a place where they can pick up people who are wanted on other warrants," says Mr. Yohnka.

And it's debatable, say lawyers, how much suspicion should fall on those in the vicinity of a ShotSpotter report, given that the system is only specified to be accurate 80 percent of the time within 25 meters (82 feet). An independent study in 1999 found ShotSpotter to be accurate 80 percent of the time within 25 feet.

Boston police commissioner Edward Davis says ShotSpotter records would probably not be enough to obtain a search warrant, but might meet that bar in combination with other information.

He's concerned with making sure officers arrive at gunshot scenes with enough resources to keep them safe. Some watchdogs, however, worry about fearful police arriving in force and possibly overreacting, particularly in cases of a false report.

ShotSpotter is programmed to screen out loud noises like car backfires, nail guns, and thunder. Dispatchers can also listen to audio of each event to weed out some reports. Minneapolis says the system is now 95 percent accurate, but still can get tripped up by helicopters. Yet gun silencers can defeat the technology.

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