GunSpot – A gunshot detection application for mobile phones

by

Simon Kleinsmith

A mini-thesis submitted in partial fulfillment of the requirements for the degree of

Bachelor of Science (Honours) in Computer Science

University of the Western Cape 2010

Supervisor: Mr Mehrdad Ghaziasgar

Co-supervisor: Mr James Connan

September 2010

Table of Contents

1. Chapter 1

1.1. The user’s view of the problem

1.2. The problem domain

1.3. Expectations of the software solution

2. Chapter 2

2.1. The designer’s interpretation of the problem

2.2. Requirements for the system

2.3. Existing solutions

3. Chapter 3

3.1. User Interface Specification

4. Chapter 4

4.1. Object oriented analysis

5. Chapter 5

5.1. Object oriented design

Introduction

A gunshot is the process where a firearm has been discharged. This involves a bullet passing

through the barrel of a gun by means of an explosion of the chemicals reactants in the bullet

itself. The sound of a gunshot is produced by two factors. One is the blast that the muzzle emits

when the explosive charge is triggered. Secondly, if the bullet travels at supersonic speed, it

produces a shock wave which travels in the opposite direction as the bullet. The characteristic

of the sound is dependant on the caliber of the gun, the length of the barrel, and the chemical

properties of the propellant found within the bullet.

The detection of gunshots offers many contributions to the public. It can speed up the dispatch

of both emergency services as well as the police. It can be used to generate statistical data

which can help identify areas where gunshots occur frequently. This can provide an indication

of where the presence of police should be increased.

The software being developed, GunSpot is a mobile phone application that detects the

occurrence of gunshots in the nearby vicinity. After the detection of a gunshot the location of

the phone is determined via GPS and the coordinates are sent back to a mapping server which

stores and graphically presents the location of all the gunshots.

Chapter 1

USERS REQUIREMENTS DOCUMENT (URD)

This chapter describes the needs of the user. It describes the problem domain pertaining to

gunshot detection, provides a description of the problem and what the user requires the software

to do. This chapter generally describes the problem to be solved as well as the functionality of

the software.

1.1. The user’s view of the problem

The user requires a portable system that can used to correctly identify gunshots in the nearby

surrounding area of where he/she is currently situated. The application should capture sound

from the immediate surrounding environment using the phone’s microphone, process it in real-

time and provide an indication of whether a gunshot has occurred. The application should then

allow users to view the locations of the gunshots on a map.

1.2. The problem domain

As the system constantly captures sound from the microphone, noise as well as the location

where a firearm is discharged can distort the sound being captured. The location of the cell

phone in relation to the user’s body also poses a problem as thick clothes and items in the user’s

pockets can dampen the sound. Should the cellular phone be located indoors, it can greatly

hamper the ability for the software to function correctly as the sound will be distorted and

dampened and the Global Positioning System (GPS) will not be able to determine the location of

the phone fast enough, if at all. The aforementioned factors affect the ability for the system to

correctly identify a gunshot.

1.3. Expectations of the software solution

The system is expected to be able to accurately detect the occurrence of a gunshot, alert the user

of this occurrence and map the location on a mapping system. This is achieved by performing a

series of steps.

1.3.1. Capture sound

The software has to continually capture sound from the environment using the phone’s

integrated microphone.

1.3.2. Apply the detection algorithm

Should a spike in the amplitude of the sound be detected, the gunshot detection algorithm

is applied in order to determine whether it is a gunshot or not.

1.3.3. Determine the location

The location of the phone is determined by either using the phone’s integrated GPS or by

using the base stations of the cellular network the user is subscribed to, to identify its

position. The phone should then send the location to the map server.

1.3.4. Map the location

The map software should display all the locations of gunshots stored on the server. The

map should have various controls so that the user can zoom in and out, see the

approximate location clearly and control the direction in which he/she is moving.

1.4. Not expected from the software solution

The software is not required to:

determine which type of firearm was discharged,

localize the occurrence of a gunshot,

determine whether a person was injured by the occurrence of a gunshot, and

dispatch emergency services to the location.

Chapter 2

REQUIREMENTS ANALYSIS DOCUMENT

This chapter focuses on the system and software requirements needed to implement the user

requirements. The requirements outlined in Chapter 1 are used as a starting point and interpreted

from the view of a designer.

2.1. Designer’s interpretation of the problem

One important factor of the system is the battery life of the mobile phone. Due to the system

constantly capturing sound from the phone’s microphone, the battery will drain much faster than

when it does under normal use of the phone. The sensitivity of the phone’s microphone is also a

factor as it determines the range in which a gunshot can be detected as well as the probability of

detection.

2.2. Requirements for the system

The system requires a mobile phone which is capable of running the software. The phone should

incorporate a processor which is powerful enough to run the software, an integrated microphone

that boasts sufficient sensitivity, and ideally a GPS to allow the location to be determined more

accurately. A server is required in order for the system to keep track of the location of the

gunshots.

2.3. Existing solutions

To my knowledge there is no system that implements gunshot detection on a mobile phone.

There is, however, a system known as ShotSpotter which implements gunshot detection and

localization. It achieves this by deploying acoustic sensors throughout the area which gunshots

are to be detected.

Chapter 3

User Interface Specification

This chapter documents how the user interfaces look, its functions and how the user interacts

with the application. The user interface comprises two parts: The mobile application, GunSpot;

and the map web application that runs on the server. Both of their interfaces and functions are

documented below.

3.1. The mobile application

GunSpot requires very little or no user intervention at all as the application is intended to run as a

service rather than to interact with the user continuously. The only user interface that the system

requires is to turn the application on and off as well as to provide the user with a notification of

the occurrence of a gunshot. The notification will be displayed for a period of time, e.g. 5

seconds. Images illustrating these two scenarios can be found below.

Figure 3.1.1 -A menu allowing the software to be turned on or off.

Figure 3.1.2 - An example of a notification presented to the user

3.2. The map interface

The images below shows the map interface as it is presented to the user. It consists of the map in

two different scenarios. Figure 3.3.1 shows the map in its default view without the user clicking

on a marker and triggering the associated changes. Figure 3.3.2 shows the map after the user has

clicked on a location marker. The map interface contains navigation and map type controls,

information windows, and location markers. Each of this will be explained in detail.

Figure 3.2.1

Figure 3.2.2

3.2.1. Location marker

This icon is used to depict the location of where the gunshot was detected. The user is able to

click on the icon in order to zoom in on the location, change the map type from Road to Hybrid

and also display an information window containing an approximate address of the location as

well as the GPS coordinates.

3.2.2. Navigation: Pan control

This control will move the map in the direction selected by the user. In addition to using the pan

controls, the user is able to click and drag the map in order to move to a different viewable area.

3.2.3. Navigation: Zoom control

This control will either zoom in or out of the map, depending on whether the user clicks on the

plus sign (zoom in) or minus sign (zoom out). The user can either drag the scroll manually to

zoom in (upward) or out (downward). In addition to aforementioned methods the user can also

double left-click on a point on the map to zoom in or double right-click to zoom out.

3.2.4. Map type control

Using this menu bar, the user can select to change the map type. Each of the images below shows

the same location except that they are using a different map type.

3.2.4.1 Road

This view shows the default road map view. It displays road names, area names and various

transport infrastructure types.

Figure 3.3.4.1

3.2.4.2 Satellite

This view shows the location in satellite view. No additional information is shown on the map.

Figure 3.3.4.2

3.2.4.3 Hybrid

This view shows a combination of the satellite and road map views. The location is shown by

means of satellite images, but with the addition of road names, area names, and transport

infrastructure types.

Figure 3.3.4.3

3.2.4.4 Terrain

This view displays the location based on information of the terrain.

Figure 3.3.4.4

Chapter 4

Object Oriented Analysis

This content of this chapter provides a high level view of the application and the objects it

contains. The chapter is focused providing an object oriented analysis, which is an object-

oriented view of the problem. Each object is defined and documented in the data dictionary. The

relationship between the objects is shown in the UML diagram below, along with the names,

attributes and methods of each of the classes.

Figure 5.1 – A class diagram of the objects in the GunSpot system.

4.1. Data dictionary

In the table below each of the objects are found along with a description of their function.

Object Description

ProcessAudio

This function of this class is to capture the

audio from the phone’s microphone and

process the sound. Processing includes

breaking the sound into its constituent samples,

applying the gunshot detection algorithm. This

class instantiates the LocationListener in order

to determine the location of the phone on

successfully detecting a gunshot.

LocationListener

The function of this class is to determine the

location of the phone and instantiate the

SendLocation class in order to send the

coordinates to the server for storage and

mapping.

SendLocation

This class sends the coordinates of where the

phone is located to the server for storage and

processing by the mapping application.

ReceiveLocation

This class resides on the server and receives

the coordinates sent from the server and writes

it to an XML file containing all the locations

where a gunshot has been detected.

GunSpotMap

This class resides on the server and displays

the map, markers and controls for the user. It

also converts the coordinates sent from the

phone into a more user readable version and

converts derives an approximate address from

the coordinates.

Chapter 5

Object Oriented Design

This chapter provides an overview of the algorithms that are implemented in the objects of the

system as well as a description of the methods in each class.

5.1. The class that is currently under development is the algorithm which forms the core of the

GunSpot application. The methodology which the algorithm follows is described below.

Figure 5.1

While(true) {

Capture sound from microphone;

Break the sound into samples;

if(spike in sound) {

process the samples;

compare the samples to the stored values;

if(certain number of samples falls within range)

A gunshot has been detected;

Notify the user of the occurrence;

Determine the location of the phone;

Send the location to the server;

}

}

5.2. The object below is that of the mapping application. The algorithm which is

implemented is provided below.

Figure 5.2

Initialize the map;

foreach(location) {

read latitude and longitude;

convert the coordinates to an address;

create the marker;

create information windows for each marker;

display the marker on the map;

}

5.3. The object below is located on the server and its function is to receive the coordinates

sent from the phone. The pseudo code of its functioning is provided below.

Figure 5.3

Receive location;

Open file stream;

If(XML file doesn’t exist) {

Create file;

}

else {

Add marker to the file;

}

Close file stream;

5.4. The object below is part of the GunSpot application on the phone and is instantiated

when the location is to be sent to the server.

Figure 5.4

Create HTTP POST object linking to the server;

Create the list containing the name-value pairs;

Add the values for the determined location to the list;

Send the values to the server;

5.5. The object below is an abstract class that is called once the location of the phone’s

determined location has changed.

Figure 5.5

5.6. The data dictionary below provides a description of the purpose of the methods found in each

of the objects.

Class name Method name Description

ProcessAudio

verify

This method verifies

whether the sound captured

is a gunshot.

transformSamples

This method transforms the

samples from byte values

into integers.

readBounds

This method reads the

upper and lower bounds

from the text file packaged

with the application.

GunSpotMap

createMarker

This method creates the

marker and associated

information window on the

map for a location.

loadMarkers

This method reads all the

locations from the XML

file stored on the server

and passes the values to the

createMarker method.

reverseGeocode This method converts the

coordinates into an address.

convertCoordinate

This method converts the

coordinates from decimal

values to the more readable

degree-minute-second

notation.

LocationListener

onLocationChanged

This method is called when

the location on the phone

changes.

onProviderDisabled

This method is called when

the GPS or network

provider is disabled.

onProviderEnabled

This method is called when

the GPS or network

provider is enabled.

ReceiveLocation

addMarkerToFile

This method adds the

location received from the

phone to the XML file.

createFile This method creates the

XML file if it doesn’t exist.

SendLocation postData

This method sends the

location to the server in the

form of two name-value

pairs.