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A gunfire locator is a system that detects and conveys the location of gunfire or other weapon fire using acoustic, optical, potentially other types of sensors, as well as a combination of such sensors. These systems are used by law enforcement, security, military 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 either co-located or geographically dispersed
- A processing unit
- A user-interface that displays gunfire alerts.
Systems used in urban settings integrate a geographic information system so the display includes a map and address location of each incident.
Determination of the origin of gunfire by sound was conceived prior to World War I where it was first used operationally.
In the early 1990s, the areas of East Palo Alto and eastern Menlo Park, California, were besieged with crime related to drug traffic. During 1992 there were 42 homicides in East Palo Alto, making it the per capita 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 U.S. Geological Survey, approached the Menlo Park police department to ask if they would be interested in applying seismological techniques to locate gun shots. 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 point 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 1 wired and 4 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 muzzle blast 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 “snap” or “crack” that occurs as a projectile moves through the air at supersonic speeds
Optical flashes can be detected using optical and/or infrared sensing techniques; note that 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 only capable of determining 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. 640X480 pixels) is constantly evaluated.
The projectile generally must travel within 50 to 100 meters of a sensor in order for the sensor to hear a supersonic “snap”. The combination of a muzzle blast and a supersonic snap 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.
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; this has been termed a ‘spatial filter’ in the first patent issued to ShotSpotter, Inc. 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 IR contributions. Most flash suppressors are designed to minimize the visible signature of the gunfire. Flash suppressors break up the expanding gases into focused cones therby 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 silenced gunfire.
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, only sound 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 systems "listen" either for 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. Therefore 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.
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 that they have 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 structure. Just like acoustic based systems, electro-optical systems can generally be degraded by specialized suppression devices that minimize their sound or optical signatures.
Optical and electro-optical systems have seen success in military environments where immediacy of response is critical and because they generally do not need careful location registration as is generally the case for more permanently installed "Civil" crime fighting systems. Just as acoustic systems require more than one microphone to locate gunshots, most electro-optical systems require more than one sensor when covering 360 degrees. Acoustic and optical sensors can be co-located and their data can be fused thereby enabling the gunshot location processing to have a more exact discharge time that can be used to calculate the distance of the discharge to the sensors with the greatest possible precision. Optical systems are (essentially) not limited to the number of individual shots being fired or the number of different shooters simultaneously shooting, which allows optical-based sensing to easily declare and locate shooters conducting ambushes that employ multiple shooters, shooting from multiple locations during the same time period.
The combination of the both approaches (acoustic and Infrared) assists in overcoming each system's own limitations while improving the overall capability to eliminate false declarations of gunshots and/or ambiguous declaration locations. Even when these combined systems are employed, shots fired from far enough away will not be detected because the amount of gunshot signal (both acoustic and Infrared) eventually fades into the background signals. For acoustic systems that require the supersonic shock wave for location determination, the bullet must still be traveling at supersonic speed when it passes the sensor, and it must pass the sensor within the lateral span of the shock wave. For infrared sensing of the flash upon weapon's discharge, the bullet path is not determined. By combining these two approaches the capability under various conditions anticipated in a combat scenario is improved.
Both Optical and acoustic sensors have been used from vehicles while on the move in urban and rural environments. These sensors have been tested on airborne and waterborne platforms as well.
Electro-optical detection systems currently tested (2011) can process the incoming shot signatures at very fast speeds which provides an excellent method to not only discriminate between weapon firings and other non-gunshot events, but the systems can identify categories, characteristics, and sometimes specific weapon types automatically.
Discriminating gunfire 
Many techniques can be used to discriminate gunfire (also referred to as “classifying gunfire”) from similar noises such as cars backfiring. As discussed previously, the SPL and corresponding acoustic propagation characteristics of high SPL impulsive sounds gave rise to the ‘spatial filter’ technique patented and used by ShotSpotter in its Gunshot Location System. This is just one of several methods used to distinguish between gunfire and other impulsive sounds. Analysis of the spectral content of the sound, its envelope, and other heuristics are also commonly used methods to distinguish and correctly classify impulsive sounds as gunfire.
Another method of classifying gunfire uses "temporal pattern recognition ", as referred by its developer, that employs artificial neural networks that are trained and then listen for a sound signature in acoustic events. Like other acoustic sensing systems they are fundamentally based on the physics of acoustics, but they analyze the physical acoustic data using a neural network. Information in the network is coded in terms of variation in the sequence of all-or-none (spike) events, or temporal patterns, transmitted between artificial "neurons". Identifying the nonlinear input/output properties of neurons involved in forming memories for new patterns and developing mathematical models of those nonlinear properties enable the identification of specific types of sounds. These neural networks can then be trained as "recognizers" of a target sound, like a gunshot, even in the presence of high noise.
Regardless of the methods used to isolate gunfire from other impulsive sounds or infrared sensing, standard triangulation methods can then be used to locate the source of the gunshot once it has been recognized as a gunshot.
Optical discriminating had previously consisted of methods to eliminate solar glint as a false alarm. Spatial, spectral, and creative temporal filters were used to minimize false alarms caused by solar glint. Earlier sensors could not operate at speeds fast enough to allow for the incorporation of matched temporal filters that now eliminate solar glint as a false alarm contributor.
Different system architectures have different capabilities and are used for specific applications. In general there are 2 architectures: stand-alone systems with local microphone arrays, and distributed sensor arrays (“wide-area acoustic surveillance”). The former are generally used for immediate detection and alerting of a nearby shooter in the vicinity of the system; such uses are typically used to help protect soldiers, military vehicles and craft, and also to protect small open-space areas (e.g., parking lot, park). The latter are used for protecting large areas such as cities, municipalities, critical infrastructure, transportation hubs, and military operating bases.
Most stand-alone systems have been designed for military use where the goal is to immediately alert human targets so they may take evasive and/or neutralization action. Such systems generally consist of a small array of microphones separated by a precise small distance. Each microphone hears the sounds of gunfire at minute differences in time allowing the system to calculate the range and bearing of the origin of the gunfire relative to the system. Military systems generally rely on both the muzzle blast and projectile shockwave “snap” sounds to validate their classification of gunfire and to calculate the range to the origin.
Distributed sensor arrays have a distinct advantage over stand-alone systems in that they can successfully classify gunfire with and without hearing a projectile “snap” sound, even amid heavy background noise and echoes. Such systems are the accepted norm for urban public safety as they allow law enforcement agencies to hear gunfire discharges across a broad urban landscape of many square miles. In addition to urban cityscapes, the distributed array approach is intended for area protection applications, such as critical infrastructure, transportation hubs, and campuses.
Using common data networking methods, alerts of the discharges can be conveyed to dispatch centers, commanders, and field-based personnel allowing them to make an immediate assessment of severity and initiate appropriate and decisive force response. Some systems have the capability of capturing and conveying audio clips of the discharges with the alert information that provides additional invaluable information regarding the situation and its severity. Similarly for the protection of critical infrastructure; where the information is clearly and unambiguously conveyed in real-time to regional crisis command and control centers, enabling security personnel to cut through often inaccurate and delayed reports so they may react immediately to thwart attacks and minimize subsequent activity.
Gunshot location systems are used by public safety agencies as well as military/defense agencies. In public safety, they are usually referred to as "gunshot location systems," and have primarily been used in dispatch centers for rapid reaction to gunfire incidents. In military/defense, they are variously known as counter-sniper systems, weapons detection and location systems, or other similar terms. Uses include alerting potential human targets to take evasive action, to direct force response to neutralize threats, including automated weapon cuing.
In addition to using gunshot location systems to convey incident alerts, they also can relay their alert data to video surveillance systems in real-time enabling them to automatically slew cameras to the scene of an incident. Real-time incident location data makes the video surveillance smart; once cameras have slewed to the scene the information can be viewed to assess the situation and further plan necessary response, and the combined audio and video information can be tagged and stored for subsequent use as forensic evidence.
Infrared based detection systems can detect ordnance blast signatures. They can detect large caliber weapons such as mortars, artillery, Rocket Propelled munitions, machine guns as well as small arms. These systems can also detect bomb impact explosions thereby locating the impacts of indirect fire weapons like artillery and mortars. The detector can be used as an automated shot correction sensor for close arms support.
Public safety 
In public safety and law enforcement, gunshot location systems are often used in high crime areas for rapid alerts and awareness into the communications and dispatch center where the alerts are used to direct first responders to the scene of the gunfire, thus increasing arrests rates, improving officer safety, securing witnesses and evidence, and enhancing investigations, as well as in the long run deterring gun crimes, shootings and especially "celebratory gunfire" (the practice of shooting weapons in the air for fun). Gunshot location systems based upon wide-area acoustic surveillance coupled with persistent incident data storage transcends dispatch-only uses because reporting of urban gunfire (via calls to 9-1-1) can be as low as 20%, which means that law enforcement agencies and their crime analysts have incomplete data regarding true activity levels and patterns. With a wide-area acoustic surveillance based approach combined with a persistent repository of gunfire activity (i.e., a database), agencies have closer to 100% activity data that can be analyzed for patterns and trends to drive directed patrols and intelligence-led policing. Additional benefits include aiding investigators to find more forensic evidence to solve crimes and provide to prosecutors to strengthen court cases resulting in a higher conviction rate. With the accuracy of a gunshot location system and the ability to geo-reference to a specific street address, versus a dearth of information that typically is the case when citizens report gunfire incidents to 9-1-1, agencies can also infer shooters by comparing with known criminal locations, including those on parole and probation; investigators can also at times infer intended victims and hence predict and prevent reprisals.
Gunshot location systems have been used domestically in urban areas since the mid 1990s by a growing list of cities and municipalities that are embracing gunshot location systems as a mission-essential tool in their arsenal for fighting violent crime. Federal and homeland security agencies too have embraced gunshot location systems and their benefits; notably the FBI successfully used a ShotSpotter gunshot location system during the 2003-2004 Ohio highway sniper attacks, in conjunction with the Franklin County Sheriff.
The technology was tested in Redwood Village in April 1996. Through 2007, the manufacturer touted the device as having benefits, but local officials were split as to its effectiveness. It is effective in reducing random gunfire. Surveys conducted for the DOJ showed it was most effective as a "perception" of action.
A ShotSpotter system installed in Washington, DC has been successfully relied upon to locate gunfire in the area of coverage. The Washington, DC Police Department reported in 2008 that it had helped locate 62 victims of violent crime and aided in 9 arrests. In addition to assaults, the system detected a large amount of "random" gunfire, all totaling 50 gunshots a week in 2007. Based on the system's success, the police department decided to expand the program to cover nearly a quarter of the city.
As of 2012, detection systems were deployed to a number of cities, including Boston, Chicago, San Francisco, Oakland, Milwaukee, Minneapolis, Omaha, Kansas City, Washington DC, New Bedford, Massachusetts, Springfield, Massachusetts, Wilmington, North Carolina, Los Angeles, San Francisco, Bellwood, Illinois, and some in the United Kingdom and Brazil. Integration with cameras that point in the direction of gunfire when detected is also implemented.
Military and defense 
Determination of the origin of gunfire by sound was conceived prior to World War I where it was first used operationally. Early sound based systems were used primarily for large weapons. Weapons detection and location systems and counter-sniper systems have been deployed by the US Department of Defense as well as by the militaries of other countries.
See also 
- US application/patent 8134889, Showen, Robert L. (Los Altos, CA, US) Calhoun, Robert B. (Oberlin, OH, US) Dunham, Jason W. (San Francisco, CA, US), "Systems and methods for augmenting gunshot location using echo processing features", published 2012-03-13, issued 2012-03-13
- http://www.shotspotter.com/company/fast-facts Customer Data, ShotSpotter Gunshot Location Systems
- Klein, Allison (2008-07-05). "District Adding Gunfire Sensors". The Washington Post. Washington Post. Retrieved 2010-02-10.