Last month, the Nassau County Police Department, on New York’s Long Island, became the latest in a series of communities and public safety offices to implement the ShotSpotter Gunshot Location System (GLS), a product that uses wireless audio sensors to detect if guns were fired and, if so, calculate the location of those shots. Nassau County police officials deployed the system in Uniondale and Roosevelt, two communities plagued by gun violence.
The system is already in use in about 30 municipalities across the United States. On Dec. 31, 2007, the East Palo Alto Police Department in California completed its own installation of the ShotSpotter. Ten minutes after the system was switched on, it detected gunshots and determined the address of the gunfire, enabling the police officers to quickly find and seize two weapons suspected of being involved. That was the beginning of what has been a successful use of technology to track gunfire in one area of the city, says Captain Carl Estelle. In the past eight months, the system has helped the department make multiple arrests, confiscate guns and nab suspects, Estelle says.
ShotSpotter GLS allows government agencies to not only pinpoint the source of a gunshot or explosion, but to use sensor data and auditory recordings for crime-scene investigations (such as searching for spent shells and tire skid marks) and as courtroom evidence. The system’s sensors use an IEEE 802.11 protocol to transmit data via signals broadcast at frequency of 900 MHz to 5 GHz.
ShotSpotter Inc. first invented the system about 12 years ago, says Gregg Rowland, the company’s senior VP, and spent about six years in further development before marketing it commercially. In 2003 the FBI contacted ShotSpotter for assistance with a Columbus, Ohio, criminal case in which a sniper was shooting at traffic on Interstate 270 around the city.
ShotSpotter provided a server, as well as software to interpret data from sensors, which the company installed in the surrounding area. The system reported the location of subsequent gunshots, helping law enforcement officials to find, apprehend and prosecute the shooter. Since that time, Rowland says, the demand for ShotSpotter GLS has been steady, and a number of major metropolitan areas—including Washington, D.C., Boston, Baton Rouge, La., Philadelphia, Chicago, St. Louis and Glendale, Calif.—now use the system. In addition to the 30 existing systems in operation, he says, the company is installing about one new system each month.
First a ShotSpotter engineer visits the location in which the deployment will take place and uses a spectrum analyzer to sweep the area for RF transmissions, determining which frequency has the least interference. The system sensors are then set to transmit at that frequency.
Once installed, ShotSpotter GLS detects gunfire using audio sensors that are installed typically on rooftops throughout an area where police are monitoring gunfire activity. Cities install about 18 to 20 sensors, which measure about 8 inches across, and Rowland likened them in appearance to an air-cleaner on a carburetor. The quantity of sensors depends on environmental factors such as the height of buildings in the area or other obstacles, Rowland says. Because of security concerns, the company does not release pictures of the sensors.
Each audio sensor comes with a digital recorder. If the sensor detects a suspicious sound, it transmits its unique ID number and a recording of that sound to the ShotSpotter Location server. The server analyzes the sound with what is called its “classifier” software, matching it against a database of various gunshot sounds. If the system finds that the newly recorded sound matches one in its database, it instructs all sensors in the vicinity to send a recording of all sounds that followed the immediate gunshot in question. Because there are multiple sensors, the system uses triangulation to determine the exact location of the gunshot and sends that data to the necessary parties, which could include 911 dispatchers, police in their vehicles, or even pilots in aircraft in the vicinity.
The system also includes the ShotSpotter Public Safety Console (PSC) computer that displays a digital map of the area being monitored and also allows the dispatchers to see the location of a gunshot, portrayed as a red dot on a map of the area in question. If there are multiple gunshots and the shooter is moving, the ShotSpotter PSC software will show a series of red dots. Some communities are also using ShotSpotter PSC Mobile software, which allows laptops in police vehicles to also display the same data.
Estelle says East Palo Alto uses ShotSpotter not only to determine where the gun was fired, but also to see what direction the shooter is traveling based on the chronological order of red dots (gunshots) on the screen.
Microsoft SQL Server software runs on the ShotSpotter GLS server and functions as the repository for all of the data stored by the system. The server is installed on the customer’s premises and is maintained by its IT department. “We integrate the system to their IT backbone, and we train them to use it,” Rowland says.
When ShotSpotter began its earliest deployments, the greatest challenge for engineers was installing the sensors—such as getting permissions from property owners to install equipment on their building. Since that time, Rowland says, however, that designing, installing and troubleshooting a unique wireless network for each location has become fairly simple. “We have done this so many times now, it is just a process and we are good at it,” says Rowland.
The system also tracks weather, with either a temperature sensor in each audio sensor or a connection to an Internet-based weather server. Weather conditions are then factored into the sound classification that is done by the ShotSpotter server. For example, says Rowland, sound is more easily pinpointed in cooler weather.
Currently East Palo Alto has coverage of one-third of its approximately 2-square-mile area. The county dispatch office uses the system to send officers to an appropriate area based on the information displayed on the PSC system. Eventually, says Estelle, the department would like to equip each in police vehicle with a computer on which the Mobile PSC software is installed, so that officers could see the same display that 911 dispatchers see, as soon as the ShotSpotter detects a gunshot.
In the meantime, he says, the system allows the department to respond to gunshots much faster and more accurately than they could in the past. Previously, Estelle says, officers responded to gunshots based on calls from neighbors and had to determine the location of the gunfire according to witness accounts. Recently Estelle himself heard gunfire, and within 15 to 20 seconds, he saw the dispatch indicating the ShotSpotter-provided address where the weapon had been fired. “That’s why we can be there [at the scene of the gunfire] now literally within one or two minutes.”