Understanding Gunshot Detection Technology

Jan. 22, 2007
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.

Regardless of accuracy rates, the high-tech nature of ShotSpotter may dazzle juries. "It's the gee-whiz effect: It seems so scientific, it must be true, right?" says Jack King, a spokesman for the National Association of Criminal Defense Lawyers. If he was defending a client, he would move to keep gunshot acoustic evidence out. "If a district attorney wanted it in, I would make him put on some $1,000-an-hour experts to convince the court that it's scientifically reliable."

Data security will be one of the first questions. The entire system uses encryption, from sensor to server to dispatcher, says James Beldock, president of ShotSpotter. The server stores a record of each gunshot report that includes the time, the sensor readings, and calibration data.

"You can take data out of the system, and with graph paper and a little physics, you can come to the same answer," says Mr. Beldock. "It's going to go to court eventually, and I'm looking forward to it."

In the future, Boston and Minneapolis hope to pair ShotSpotter with surveillance cameras already in place in both cities. In a demonstration Saturday in Boston, a camera was able to automatically swivel and focus on the location of a fired gun within seconds.

"A technology that is installed for one purpose which is legitimate, could, down the road, be used for other purposes," says Carol Rose, head of the Massachusetts ACLU chapter. She says that gunshot detection systems are not inherently problematic - and may be useful - if used as advertised. "[But] the city council needs to take steps to make sure that listening devices used to triangulate gunshots aren't used to listen to private conversations."

The company says the sensors don't pick up voices.

The system would cover 5.6 square miles of Boston, in places where gun violence is highest, say city officials.

"[Over four years,] $375,000 a year is nothing to ensure public safety," says Ron Consalvo, a Boston city councilor who pushed for the system. "It does take more police on the street, and we're doing that.... But it also takes the police department doing everything they can to seize the latest technology to do their job better."

(c) Copyright 2007. The Christian Science Monitor