Gunshot Location System
Encyclopedia
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:
Systems used in urban settings integrate a geographic information system
so the display includes a map and address location of each incident.
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 10's 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.
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. Because only optical flashes are detected, such systems are typically only capable of determining the bearing of a discharge relative to sensor.
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.
Gunfire 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 correlated 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. [Showen and Dunham 1997].
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.
(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 slower than optical sensing systems because they rely on the propagation of sound waves at 1,125 feet/second or 768 mile/hour (in dry air at 68 °F). Therefore the sound reaching a sensor 1 mile from its origin will take almost 5 seconds. Additional delays may be imposed on the conveyance of alerts for systems that distinguish between single rounds and multiple rounds of gunfire. While seconds do matter, especially when gunfire is involved, 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.
s.
Optical and electro-optical systems have seen success in military environments where immediacy of response is critical. To date, no such system has delivered 360-degree sensing capability, and therefore multiple optical sensors with relatively narrow fields of view must be used. Acoustic and optical sensors can be co-located and their data can be fused 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.
As optical flash detectors need a line of sight to the discharge, they cannot be used in urban areas for public safety use where such lines of sight are impossible.
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 brain is coded in terms of variation in the sequence of all-or-none (spike) events, or temporal patterns, transmitted between nerve cells. Identifying the nonlinear input/output properties of neurons involved in forming memories for new patterns, and developing mathematical models of those nonlinear properties provide a revolutionary pathway to neural-based classifications of sounds, which 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, standard triangulation methods can then be used to locate the source of the gunshot once it has been recognized as a gunshot.
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.
In addition to using gunshot location systems to convey incident alerts, they also can relay their alert data video surveillance systems in real-time enabling them 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.
Gunshot location systems have been used domestically in urban areas since the mid 1990’s 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 GLS 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. In the end, it did not account for a single conviction. However, it was effective in reducing random gunfire. Surveys conducted for the DOJ
showed it was most effective as a "perception" of action.
A pilot 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.
- 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
Geographic Information System
A geographic information system, geographical information science, or geospatial information studies is a system designed to capture, store, manipulate, analyze, manage, and present all types of geographically referenced data...
so the display includes a map and address location of each incident.
History
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
SRI International
SRI International , founded as Stanford Research Institute, is one of the world's largest contract research institutes. Based in Menlo Park, California, the trustees of Stanford University established it in 1946 as a center of innovation to support economic development in the region. It was later...
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 10's 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. Because only optical flashes are detected, such systems are typically only capable of determining the bearing of a discharge relative to sensor.
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.
Gunfire 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 correlated 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. [Showen and Dunham 1997].
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.Acoustic
Systems that use acoustic-only techniques are defined as those systems with sensing modalities targeted at acoustic phenomena. Such systems "listen" either for the bullet bow shockwaveBullet bow shockwave
A bullet bow shockwave is a physical and audible wave created in the air when a bullet travels at supersonic speeds; meaning faster than the speed of sound....
(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 slower than optical sensing systems because they rely on the propagation of sound waves at 1,125 feet/second or 768 mile/hour (in dry air at 68 °F). Therefore the sound reaching a sensor 1 mile from its origin will take almost 5 seconds. Additional delays may be imposed on the conveyance of alerts for systems that distinguish between single rounds and multiple rounds of gunfire. While seconds do matter, especially when gunfire is involved, 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
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 clear line of sight to the weapon being fired or the projectile while it is in motion. Such systems can generally be defeated by specialized Flash suppressorFlash suppressor
A flash suppressor, also known as a flash guard, flash eliminator, flash hider, or flash cone, is a device attached to the muzzle of a rifle or other gun that reduces the visible signature of the burning gases that exit the muzzle. This reduces the chances that the shooter will be blinded in dark...
s.
Optical and electro-optical systems have seen success in military environments where immediacy of response is critical. To date, no such system has delivered 360-degree sensing capability, and therefore multiple optical sensors with relatively narrow fields of view must be used. Acoustic and optical sensors can be co-located and their data can be fused 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.
As optical flash detectors need a line of sight to the discharge, they cannot be used in urban areas for public safety use where such lines of sight are impossible.
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 but 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
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
. Information in the brain is coded in terms of variation in the sequence of all-or-none (spike) events, or temporal patterns, transmitted between nerve cells. Identifying the nonlinear input/output properties of neurons involved in forming memories for new patterns, and developing mathematical models of those nonlinear properties provide a revolutionary pathway to neural-based classifications of sounds, which 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, standard triangulation methods can then be used to locate the source of the gunshot once it has been recognized as a gunshot.
Architectures
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.
Applications
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 video surveillance systems in real-time enabling them 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.
Public safety
In public safety and law enforcement, gunshot location systems are often used in high crime areas for rapid 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 and improving officer safety, 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 1990’s 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 GLS during the 2003-2004 Ohio highway sniper attacks
Ohio highway sniper attacks
The Ohio highway sniper attacks were a series of 24 sniper attacks along Interstate 270 and other nearby highways in the central part of the U.S. state of Ohio against traffic and homes. The shootings began in May 2003 and continued for several months...
, 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. In the end, it did not account for a single conviction. However, it was effective in reducing random gunfire. Surveys conducted for the DOJ
United States Department of Justice
The United States Department of Justice , is the United States federal executive department responsible for the enforcement of the law and administration of justice, equivalent to the justice or interior ministries of other countries.The Department is led by the Attorney General, who is nominated...
showed it was most effective as a "perception" of action.
A pilot system installed in Washington, DC has been successfully relied upon to locate gunfire in the area of coverage. The Washington, DC Police Department
Metropolitan Police Department of the District of Columbia
The Metropolitan Police Department, also known as the DC Police, DCPD, MPD, and MPDC is the municipal police force in Washington, D.C...
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.