SST Expert Testimony Common Questions and Answers This document is a collection of questions that have commonly been asked about the ShotSpotter system during court testimony and deposition. If possible, you should review these questions with your client attorney to determine which questions are relevant to the case and should asked during direct examination. Please bear in mind that these are general questions and answers about the ShotSpotter system and not about a specific shooting incident. Incident specific information should be obtained from the ShotSpotter Investigator Portal or from SST Customer Support at: support@shotspotter.com What is ShotSpotter and how does it work? The ShotSpotter system is an acoustic gunshot detection and location system that detects gunfire using an array of GPS enabled microphone sensors to geo locate detected gunfire using a mathematical processes called multilateration. When a weapon is fired, the sound of the muzzle blast (bang) travels out in all directions at the speed of sound. That sound arrives at each of the system's sensors at slightly different times. The system then measures the differences in the arrival times(tdoa) and uses the GPS positions of the detecting sensors and the speed of sound to calculate the latitude and longitude of the gunshot(s). The results of those calculations are reported to the ShotSpotter Incident Review Center where they are verified then published to the customer. Describe the parts of the ShotSpotter system The ShotSpotter system is comprised of three primary elements: The first element are the sensors. The ShotSpotter sensors detect the gunshot pulses and are installed in a geographic area that is designated by the customer. Each sensor consists of one or more microphones, a processor chip, a GPS receiver, memory, and a radio network link. The second element is the Location Server which is the software application that matches up the gunshot pulses detected by the individual sensors then calculates the geographic location that those pulses originated from. The Location Server also requires a database. The Location Server and the database run on servers that are hosted at a remote facility. 1
The third element is the user interface called the ShotSpotter Alert Console. The Alert Console is a software application that is installed on the dispatch operator s PC. The Alert Console receives ShotSpotter incident information as soon as it is published by the ShotSpotter Incident Review Center. It displays the date, time, and location of the incident and provides the ability to play back audio clips of the shooting event to the operator. There is a second user interface called the Investigator Portal that does not receive real time alerts but provides access to historical incident data. What is Multilateration? From Wikipedia: Multilateration (MLAT) is a navigation technique based on the measurement of the difference in distance to two stations at known locations that broadcast signals at known times. Unlike measurements of absolute distance or angle, measuring the difference in distance between two stations(sensors) results in an infinite number of locations that satisfy the measurement. When these possible locations are plotted, they form a hyperbolic curve. To locate the exact location along that curve, multilateration relies on multiple measurements: a second measurement taken to a different pair of stations(sensors) will produce a second curve, which intersects with the first. When the two curves are compared, a small number of possible locations are revealed, producing a "fix". We know the precise locations of every sensor in the array as they all have a GPS receiver built into them. We also know that sound travels at a constant speed of 767 mph (adjusted by temperature readings). Then we compare the different times that each detecting sensor heard the gunshot, use those differences to calculate hyperbolic curves, plot those curves onto a map, and then determine the latitude and longitude of where those hyperbolic curves intersect. This lat/lon is the gunshot location. When was the ShotSpotter system installed in XXX city? Information obtained from DB incident table How many ShotSpotter sensors are installed in XXX city? Check and update before taking the stand. How many cities have a ShotSpotter system installed? Check and update before taking the stand. ~95 cities worldwide as of the end of 2014. How long has SST been in operation? Since 1995. SST s first system was installed in Redwood City, CA in 1996. 2
Where is the ShotSpotter Service Operations Center? the ShotSpotter Service Operations Center is located at SST headquarters in Newark, CA and is where all ShotSpotter incidents are reviewed by human operators for classification accuracy. How accurate is the system? SST guarantees that the system will detect at least 80% of all detectable outdoor gunfire that occurs within the coverage area and geo locate it to within 25 meters of the actual shooter location. We verify this with live fire testing of newly installed customer systems. In practice, we regularly see the system locate gunfire to within 10 feet. How accurate are the clocks and shot timing? All of our sensors as well as our servers are synchronized to GPS satellite time which is in turn, synchronized to the atomic clock at the National Institute of Standards and Technology (NIST) in Boulder, CO whose clocks are accurate to 1/1,000,000,000 of a second. Because of the atomic clock and GPS synchronization, we are able to time gunshots down to 1/1000th of a second. How many sensors are required to detect/locate a shooting incident? The system typically requires four sensors to detect and locate a single gunshot event. Three sensors to triangulate and a fourth for confirmation and error correction. Whereas a multiple gunshot event only requires three sensors as the repetition of shots provide enough confirmation and error correction data. In some cases, a shooting event can be located by two sensors using both the hyperbolic curve solution from a single pair of sensors and the azimuth solution generated by the multiple microphones installed on each sensor. What factors can affect the accuracy of a shot/incident location? The accuracy of any given location can be affected by the number of sensors that participate in detecting an incident and the location of the participating sensors relative to the location of the shooter. Location accuracy can also be affected by the environment. Any element of the environment that might prevent the sound of a gunshot pulse from either reaching enough sensors to detect it, or any element that muffles or modifies the sound of the gunshot so that it no longer meets a sensor s programmed criteria to be called impulsive. Large buildings or other obstructions between the shooter and a sensor can prevent the sound of some or all of the shots from being detected by some sensors. Construction noise, 3
traffic noise, heavy wind and rain can muffle the sound of gunshots, the topography of the area can also affect location accuracy if there are intervening hills or mountains. Also, location accuracy is known to be significantly degraded if the shooting occurs close to the outside edge of the jurisdictional area as a shooting event is no longer fully surrounded by the sensor array. On the other hand, with a greater number of detecting sensors, locations tend to be more accurate as the varying locations of the sensors allow for a greater opportunity for the sound to be detected. Does the ability to detect or locate a gunshot diminish if the shot occurs outside the coverage area? Yes. A shooting that occurs outside of the coverage area may be too far away for enough sensors to detect it. Or if the detecting sensors are all aligned in one direction relative to the shooting, the location may be subject to errors. SST does not guarantee the accuracy of either the detection of the location of any shooting event that occurs outside of the coverage area. How can you tell if the ShotSpotter system was working properly at the time of the shooting? The first, and most obvious, answer is that that the system detected, located, and alerted to a given shooting incident. If a given shooting incident was not automatically detected, or it was not accurately located, it does not necessarily mean that ShotSpotter was not working properly. There may have been any number of environmental factors (see above) that can cause ShotSpotter to not detect or accurately locate a shooting. Please consult with the SST Customer Support team to determine any causes for failure to detect or properly locate. How can you tell if a sensor is working properly? First, every sensor communicates its operational status to the server every 60 seconds. SST monitors all of their customer systems on a constant basis for sensors that no longer communicate with the server. These are replaced in as timely a fashion as possible. Second, SST also monitors for sensors that fail to provide audio for detected incidents, these are also replaced as quickly as possible. Furthermore, SST monitors for sensors that fail to provide GPS coordinates. Again these sensors are replaced. How do broken/inoperative sensors affect the system? ShotSpotter is designed to use multiple, redundant sensors that operate independently from one another. This redundancy tends to reduce the negative effects of a broken sensor in 4
regards to both detection and location. Typically, depending on the locations of the affected sensors, the system could lose up to 30% of its operational sensors and still adequately detect and locate 80% of detectable, outdoor gunfire. Again, as long as the sound of gunfire can reach at least three sensors, the system can accurately detect and locate it. Because the sound of gunfire typically travels 0.5 to 1.5 miles many sensors up to a mile or more away from the incident may be used by the location software to calculate a location. Do the sensors require any adjustment or calibration? No. Because the three primary pieces of information that the sensor gathers are the time of each gunshot pulse, the exact location of the sensor, and digitally recorded audio. Each sensor automatically receives both GPS time and location information from its onboard GPS receiver and records audio directly to memory. The status of all three elements are then constantly monitored. Does SST keep a record of broken/inoperative sensors? Yes. Since SST constantly monitors each sensors health, that information is saved to the system database and stored permanently. How are ShotSpotter sensors installed? Each customer determines the overall geographic area it wants to have ShotSpotter coverage. The SST project manager then determines where to install the sensors to achieve the best coverage within the area designated by the customer. Permissions to install sensors are obtained from the owner of each desired site, then the sensors are installed. Typically, 12 14 sensors are installed per square mile of coverage area. Ideally, sensors are installed ~20 30ft above the ground. Sensors can be installed on poles, towers, on rooftops, or on the corners of buildings. ShotSpotter sensors are never installed in trees. Does SST release to the public the locations of installed sensors or what the sensors look like? No. First, the locations of installed ShotSpotter sensors are highly sensitive law enforcement information, and are confidential and proprietary. SST, Inc. is contractually obligated to refrain from disclosing such information. Furthermore, if the location of all sensors were to become known to the public, criminals would have the capability to disable the gunshot location and detection functionality of the system, or otherwise seriously compromise the law enforcement utility of the system. Such information should be produced, if at all, only subject to the entry of an appropriate 5
protective order limiting the materials to the specified case only, restricting access to counsel only, and requiring that any such materials filed with the court or received into evidence be filed or received only under seal or protective order. Please also be aware that many of these sensors are typically installed on homes and other private property where the owners have provided access under non disclosure agreements. This, not only for contractual reasons, but also some private citizens are fearful of personal retribution or property damage should such information be revealed. Entry of this information into the public domain puts those homeowners at risk. So for the security of the system, we do not release the locations of installed sensors or images of the hardware. Can the ShotSpotter system determine the caliber of a weapon? Typically no. While technically possible, there are far too many variables for the system, as installed, to calculate: Weapon model/type, barrel length, bullet diameter, bullet weight, powder load, powder type, the acoustic environment at the time of the shooting all contribute. It is possible to determine weapon caliber if enough data of sufficiently high quality can be obtained. But for ShotSpotter to do so would require far more expensive equipment and significantly more computing power than is currently deployed. Can ShotSpotter tell if there are multiple shooters? Not automatically. A ShotSpotter analyst can determine if there were multiple weapons being fired during a shooting event by analyzing three things. The gunshot locations, the audio measurements of the individual shots and groups of shots, and the timing of the shots. How long/where is gunshot audio kept? There are two types of recorded gunshot audio. Every sensor has what we call a spool file. The spool file is a continuous loop containing everything that the sensor has heard for at least the last 72 hours. This loop is constantly overwriting its own tail so that once 72 hours have passed, any given sound recording is overwritten and cannot be recovered. Incident audio clips are 4 8 second audio clips that are automatically clipped from the spool file and downloaded from each sensor that participates in a detection. These clips are permanently stored on the system. Incident audio clips are what the Incident Review Center listens to when deciding to publish an incident to the customer, what the dispatch operator gets to listen to, and are used in forensic analysis. 6
What is post processing? In real time, the system records the audio of all gunfire it detects and stores some of it for use later. One such use is forensic post processing: the audio is re run through the ShotSpotter Browser software. This software applies the same location algorithms as the live system to each individual shot and allows us to geo locate them one by one. Post processing allows a forensic analyst to account for gunshot pulses that are present in an audio clip, but were not automatically detected by the system. As well, the software also allows an analyst to account and correct for erroneously detected pulses. Does the Flex/PSC operator have the ability to create or alter the details of an incident? It is not possible for the Flex/PSC operator (dispatcher) to create an incident, or alter the times, addresses or location of an alerted incident. The Flex/PSC operator can only change the classification of the incident and add comments to the record. What is the Law Enforcement purpose of the ShotSpotter system? ShotSpotter detects and locates gunfire then alerts Law Enforcement of shots fired within seconds. This allows the Police and EMS to arrive on the scene minutes faster than if they were answering to a 911 call. And by providing an accurate location of shooting, it allows officers to respond directly to the scene rather than having to first search for the shooting location. As well as knowing where to immediately begin collecting evidence. Why not just rely on people to notify 911 of shots fired? Typically, 911 notifications begin to arrive 3 7 minutes after the shots are fired. And the description is typically: "I heard shots in my neighborhood a few minutes ago" or "Someone is shooting behind/near my house!". When officers respond to such calls, more often than not, they end up driving around the neighborhood searching for evidence or witnesses to the shooting. This wastes time. The ShotSpotter system provides an accurate location of the shooting within seconds of it occurring allowing officers to respond directly to that location. Also, in neighborhoods where gunfire is a common occurrence, residents may not call 911 at all out of fear, don't think it will do any good, or just because they are "used to it". 7