Courtesy of U.S. Department of Justice Office of Justice Programs National Institute of Justice
Jeremy Travis, Director
Gunshot detection systems
What are they? Acoustic gunshot detection systems are designed to pick up the sound of a muzzle blast from a gun and, within seconds of the shot being fired, pinpoint or triangulate within
some margin of error the gunshot’s location, before alerting the police about the shot being fired.
Manufacturers of gunshot detection systems expect the technology to increase the ability of the police to get to the scene of random gunfire quickly, increase the number of people arrested
for firing weapons, and reduce the detrimental effects (injuries, fear, disinvestment) of shots being fired in urban settings. Community advocates of gunshot detection systems believe the
technology can deter would-be shooters and improve the quality of life in their neighborhoods.
Alliant Techsystems Inc.’s
SECURES. The gunshot detection system installed in Oakcliff was developed and demonstrated by Alliant Techsystems Inc. (ATI) and subsequently marketed as SECURES (System for the Effective Control of Urban Environment Security). SECURES identifies the location and time of gunfire in a specified target area through a series of small units (or acoustic sensor modules) mounted on utility poles. These battery-powered “pole units,” which are about the size of a video cassette, are composed of an acoustic sensing element, gunshot identification electronics, and a transmitter. Eighty-six pole units were erected in the 1-square-mile Oakcliff target area to provide adequate system coverage for the 2-month study period.18
The pole units are designed to acoustically identify gunshots and transmit that information to a police dispatch center through a network of transmitters and receivers connected to the local phone system. The gunshot location and time are transmitted to a personal computer in the dispatch center in less than 2 seconds, and the gunfire information is displayed on a computerized map, enabling dispatchers to relay the information to officers on the street.
The SECURES prototype alerts police dispatchers to the location of the first pole unit to detect a shot. ATI claims, however, that subsequent enhancements to the system “triangulate” gunfire alerts such that real-time information from responding pole units pinpoint the precise location from which the shot was fired. ATI claims this type of “triangulation” procedure can pinpoint 99 percent of gunshots within a 65-foot radius of the firing spot, 88 percent of gunshots within 30 feet, 63 percent of gunshots within 20 feet, and 35 percent of gunshots within 10 feet.19
Trilon Technology’s ShotSpotter.
The ShotSpotter gunshot detection technology installed in Redwood City was designed and demonstrated by Trilon Technology. The ShotSpotter system, installed in the 1-square-mile Redwood Village area since early 1996, consists of eight acoustic sensors, a central computer located in the Redwood City Police Department’s dispatch center, and gunshot detection and location identification software.
The acoustic sensors include microphones, acoustic sensing elements, and gunshot identification electronics. The sensors installed in Redwood Village resemble birdhouses and heating vents and are enclosed in weatherproof containers approximately 1 cubic foot in size. The acoustic sensors detect muzzle blasts from gunfire or other explosions and then transmit the sound of the gunfire via telephone line to a central computer located in a police department dispatch center.
Parameter settings in the ShotSpotter software determine the system’s level of sensitivity: if the thresholds are set quite high, background noise is less often identified as gunfire. Conversely, if the thresholds are set quite low, more background noise can be detected as gunfire, increasing the potential that extraneous noises will be incorrectly identified as gunfire.20 Once the sensors detect a sound and transmit the information to the central computer, the ShotSpotter software discriminates against most other community sounds (such as car backfires, jackhammers, thunder, and barking dogs) and pinpoints the location of the gunfire or explosions. Gunshot events are displayed on a computer map in the police dispatch center within approximately 15 seconds of the noise being made. The computer map distinguishes property boundaries, including front or side yards, curbsides, or street corners.
Determination of the precise location of gunfire events is conducted through a series of iterations of triangulation algorithms. The system can generate an overview map that presents the locations of historical shootings to discern patterns in space or time. The ShotSpotter computer can be placed in a dispatch center with stand-alone or integrated outputs, or it can be placed
at a remote site.
The ShotSpotter system stores all waveforms for every detected gunfire event and 6 seconds of audio from each detecting acoustic sensor (2.3 megabytes each). As such, a significant amount of system memory is required when numerous gunfire events occur simultaneously or when many noises are relayed to the system in quick succession (during New Year’s Eve or Fourth of July, for example). Once the ShotSpotter system detects a shot and reports the location on the computer screen, dispatchers can playback the 6-second snippet of sound from any sensor to assist them in determining what they believe to be the true source of the sound: firecracker string, multiple gunshots, shotgun blast, or car backfire. The ability to playback the sound of the apparent gunfire alert is unique to ShotSpotter and offers police an opportunity to determine whether they think the sound is in fact gunfire.
What do the police think of gunshot detection systems?
Patrol officers from both the Dallas and Redwood City police departments were surveyed about their perceptions of the impact of gunshot detection systems on their work routine, their confidence in the technology to report gunfire incidents, and their perceptions of the ability of the technology to improve police effectiveness in handling such incidents. All officers assigned to areas in which the technology was deployed and they could be dispatched to gunshot incidents received questionnaires. In Dallas, 58 percent (124 of 212) of patrol officers completed the questionnaires, while in Redwood City, 66 percent (27 of 41) of patrol officers returned the questionnaires.
As happens with many technological and strategic innovations introduced into police departments,34 patrol officers from both Dallas and Redwood City reported some frustrations with the gunshot detection systems. They generally lacked confidence in the ability of the systems to identify and locate gunfire occurrences. They also worried about false alerts, and they expressed concern about the time spent responding to gunfire alerts and the low likelihood of catching or arresting the shooter. Officers in both cities feel they are more likely to talk with citizens when
responding to citizen-generated calls than to gunshot detection system calls, and they make more problem-solving progress on citizen alerts than technology alerts. Generally, officers feel citizen calls about gunfire give them a focal point in responding to the call.
Officers can ask the citizen about what he or she heard, and they can glean details about the context of the shot fired. By contrast, officers explain that the gunshot detection systems provide
no details about the apparent shot, leaving them without any guidance to pursue an investigation.
The study of the use of gunshot detection technology in local law enforcement led the research team to four broad conclusions:
● Gunshot detection systems are likely to reveal a rather high citizen under-reporting rate of random gunfire problems (23 percent of incidents are reported).
● The technology is likely to increase the workloads of police officers, particularly if departments dispatch a patrol unit to every gunfire incident detected by a technological system.
● Gunshot detection systems are not likely to lead to more arrests of people firing weapons in urban settings because it is highly unlikely that offenders will stay at a gunshot location long enough for the police to arrive.
● Finally, gunshot detection systems seem to offer the most potential as a problem-solving tool and would fit nicely within the emerging problem-oriented policing paradigm. The technology can
help police identify random gunfire hot spots and develop strategies to address the problem.