A gunfire locator or gunshot detection system is a system that detects and conveys the location of gunfire or other weapon fire using acoustic, vibration, optical, or potentially other types of sensors, as well as a combination of such sensors. These systems are used by law enforcement, security, military, government offices, schools and businesses to identify the source and, in some cases, the direction of gunfire and/or the type of weapon fired. Most systems possess three main components:
An array of microphones or sensors (accelerometers, infrared detectors, etc) either co-located or geographically dispersed; A processing unit; A user-interface that displays gunfire alerts.
In general categories, there are environmental packaged systems for primarily outdoor use (both military and civilian/urban) which are high cost and then also lower cost consumer/industrial packaged systems for primarily indoor use. Systems used in urban settings integrate a geographic information system so the display includes a map and address location of each incident. Some indoor gunfire detection systems utilize detailed floor plans with detector location overlay to show shooter locations on an app or web based interface.
History
Determination of the origin of gunfire by sound was conceived before World War I where it was first used operationally (see: Artillery sound ranging).
In 1990, a unique algorithm was used as a starting point : Metravib defence, working with Délégation Générale pour l’Armement (DGA) – the French defence procurement agency – studied the acoustic signature of submarines. The DGA & Section Technique de l’Armée de Terre (STAT), the French Army’s engineering section subsequently commissioned Metravib D. to find a solution for shot detection, a way to assist soldiers and peacekeepers who come under fire from snipers without knowing precisely where the shots were coming from.
In the early 1990s, the areas of East Palo Alto and eastern Menlo Park, California, were besieged with crime. During 1992 there were 42 homicides in East Palo Alto, which resulted in East Palo Alto becoming the murder capital of the United States. The Menlo Park police department was often called upon to investigate when residents reported gunshots; however there was no way to determine their source from scattered 911 calls.
In late 1992, John C. Lahr, a PhD seismologist at the nearby United States Geological Survey, approached the Menlo Park police department to ask if they would be interested in applying seismological techniques to locate gunshots. Others had also approached the Menlo Park police department suggesting ways to help the police by means of gunshot location systems. The police chief arranged a meeting with local inventors and entrepreneurs who had expressed an interest in the problem. At that time there were no solutions to tracking gunshots, only a desire to do so. One key attendee was Robert Showen, a Stanford Research Institute employee and expert in acoustics.
Lahr decided to go ahead with his plans to demonstrate the feasibility of locating the gunshots, relying on his background in the earthquake location techniques and monitoring in Alaska. A network consisting of one wired and four radio-telemetered microphones was established, with his home in eastern Menlo Park becoming the command center. Lahr modified the software typically used for locating earthquakes and recorded the data at a higher sample rate than is used for regional seismology. After gunshots were heard, Lahr would determine their location while his wife monitored the police radio for independent confirmation of their source.
Using this system, Lahr was able to demonstrate to the police and others that this technique was highly effective, as the system was able to locate gunshots occurring within the array to within a few tens of meters. Although additional techniques from the seismic world were known that could better automate the system and increase its reliability, those improvements were outside the scope of this feasibility study.
Gunfire characteristics There are three primary attributes that characterize gunfire and hence enable the detection and location of gunfire and similar weapon discharges:
An optical flash that occurs when an explosive charge is ignited to propel a projectile from the chamber of the weapon A typical muzzle blast generates an impulse sound wave with a sound pressure level (SPL) that ranges from 120 dB to 160 dB A shock wave that occurs as a projectile moves through the air at supersonic speed. Note, this does not apply to subsonic ammunition, whose bullet projectiles do not exceed 1120 feet per second (i.e. the speed of sound in air).
Optical flashes can be detected using optical and/or infrared sensing techniques; however there must be a line of sight from the sensor to the weapon, otherwise the flash will not be seen. Indirect flashes that bounce off nearby structures such as walls, trees, and rocks assist in exposing concealed or limited line-of-sight detections between the weapon and the sensor. Because only optical flashes are detected, such systems are typically capable of determining only the bearing of a discharge relative to sensor unless multiple systems triangulate the shot range. Multiple gunshots, fired from multiple locations at nearly the same time, are easily discriminated as separate gunshots because the sensors generally utilize a focal plane array consisting of many sensitive pixels. Each pixel in the entire focal plane (e.g. 640×480 pixels) is constantly evaluated.
The projectile generally must travel within 50 to 100 meters of a sensor in order for the sensor to hear the shockwave. The combination of a muzzle blast and a shockwave provides additional information that can be used along with the physics of acoustics and sound propagation to determine the range of a discharge to the sensor, especially if the round or type of projectile is known. Assault rifles are more commonly used in battle scenarios where it is important for potential targets to be immediately alerted to the position of enemy fire. A system that can hear minute differences in the arrival time of the muzzle blast and also hear a projectile's shockwave "snap" can calculate the origin of the discharge. Multiple gunshots, fired from multiple locations at nearly the same time, such as those found in an ambush, can provide ambiguous signals resulting in location ambiguities.
Gunfire acoustics must be distinguished reliably from noises that can sound similar, such as firework explosions and cars backfiring.
Urban areas typically exhibit diurnal noise patterns where background noise is higher during the daytime and lower at night, where the noise floor directly correlates to urban activity (e.g., automobile traffic, airplane traffic, construction, and so on). During the day, when the noise floor is higher, a typical handgun muzzle blast may propagate as much as a mile. During the night, when the noise floor is lower, a typical handgun muzzle blast may propagate as much as 2 miles. Therefore, a co-located array of microphones or a distributed array of acoustic sensors that hear a muzzle blast at different times can contribute to calculating the location of the origin of the discharge provided that each microphone/sensor can specify to within a millisecond when it detected the impulse. Using this information, it is possible to discriminate between gunfire and normal community noises by placing acoustic sensors at wide distances so that only extremely loud sounds (i.e., gunfire) can reach several sensors.
Infrared detection systems have a similar advantage at night because the sensor does not have to contend with any solar contributions to the background signal. At night, the signature of the gunshot will not be partially hidden within the background of solar infrared contributions. Most flash suppressors are designed to minimize the visible signature of the gunfire. Flash suppressors break up the expanding gases into focused cones, thereby minimizing the blossoming effect of the exploding gasses. These focused cones contain more of the signature in a smaller volume. The added signal strength helps to increase detection range.
Because both the optical flash and muzzle blast are muffled by flash suppressors and muzzle blast suppressors (also known as "silencers"), the efficacy of gunshot detection systems may be reduced for suppressed weapons. The FBI estimates that 1% or fewer of crimes that involve gunfire are committed with suppressed guns.
Design
Sensing method Gunshot location systems generally require one or more sensing modalities to detect either the fact that a weapon has been fired or to detect the projectile fired by the weapon. To date, sound, vibration and visual or infrared light have successfully been used as sensing technologies. Both applications can be implemented to detect gunfire under static and dynamic conditions. Most police-related systems can be permanently mounted, mapped and correlated as the sensors remain in place for long periods. Military and SWAT actions, on the other hand, operate in more dynamic environments requiring a fast setup time or a capability to operate while the sensors are on move.
Acoustic
Acoustic systems "listen" for either the bullet bow shockwave (the sound either of the projectile or bullet as it passes through the air), the sound of the muzzle blast of the weapon when it fires the projectile, or a combination of both.
Due to their ability to sense at great distances, to sense in a non line-of-sight manner, and the relatively low bandwidth required for transmitting sensor telemetry data, systems deployed for law enforcement, public safety and homeland security in the United States have primarily been based on acoustic techniques.
Acoustic-only based systems typically generate their alerts a few seconds slower than optical sensing systems because they rely on the propagation of sound waves. For example, the sound reaching a sensor 1 mile from its origin will take almost 5 seconds. A few seconds to accommodate pickup from distant sensors and to discern the number of rounds fired, often an indicator of incident severity, are both tolerable and a drastic improvement for typical police dispatching scenarios when compared against the several minutes that elapse from when an actual discharge occurs to the cumulative time of several minutes that pass when a person decides to place a 9-1-1 call and that information is captured, processed, and dispatched to patrol officers.
Because such systems have arrays of highly sensitive microphones that are continuously active, there have been concerns over privacy with this broad ability to record conversations without the knowledge of those being recorded (this is "collateral eavesdropping", because capturing conversations is only an inadvertent capability of the system's design, and law enforcement agencies have stated that the recording happens only after shots have been detected.)Gunfire locator used to record conversations (KBCW CW San Francisco news report, posted to YouTube on May 23, 2014)
Optical Optical or electro-optical systems detect either the physical phenomenon of the muzzle flash of a bullet being fired or the heat caused by the friction of the bullet as it moves through the air. Such systems require a line of sight to the area where the weapon is being fired or the projectile while it is in motion. Although a general line of sight to the shot event is required, detections are sometimes available as the infrared flash event bounces off surrounding structures. Just like acoustic-based systems, electro-optical systems can generally be degraded by specialized suppression devices that minimize their sound or optical signatures.