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Sensor Network-Based Countersniper System

Akos Ledeczi

Senior Research Scientist

Institute for Software Integrated Systems

Vanderbilt University

G. Simon, M. Maroti, A. Ledeczi, G. Balogh,B. Kusy, A. Nadas, G. Pap, J. Sallai, K. Frampton

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Overview

• Ad-hoc wireless network of cheap acoustic sensors is used to accurately locate enemy shooters in urban terrain

• Performance:– Average 3D accuracy: ~1 meter

– Latency: <2 seconds

– Multipath elimination

– Multiple simultaneous shot resolution

• Challenges:– Severely resource constrained nodes

– Very limited communication bandwidth

– Significant multipath effects in urban environment

• Funded by DARPA through the IXO NEST program

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ersity• Detect TOA of acoustic shockwave and muzzle blast

• MICA2 mote

• Proprietary acoustic sensor board:–3 acoustic channels (only a single channel is used in final system)–High-speed AD converters–FPGA for signal processing: shockwave and muzzle blast detection on board

• Timestamp of shockwave and/or muzzle blast sent to mote

• Motes send TOA data to base station

• Base station fuses data, estimates shooter position and displays result

• Middleware services:–Time synchronization–Message routing–Remote control

Technical Approach

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Software Architecture

SensorboardTime Sync

Muzzle Blast&

ShockwaveDetector

RemoteControl

SensorboardInterface

SensorboardConfig/Monitor

StackMonitor

DataRecorder

DownloadManager

Acoustic Event

Encoder

TimeSync

MessageRouting

UserInterface

MessageCenter

SensorFusion

PlotterLogger

SensorLocation

RemoteController

I2C UART

SENSORBOARD MICA2 MOTE BASE STATION

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ZC coder τ2 = n/a Mm2 = 0

τ1

L1

τ3Mm1

L2

L3

Mm3

T2T1 T3

ADC ZC CoderShock wave

detector

Muzzle blast

detector

Board Clock

I2C

In

terf

ace

time

ZC Filter

Detection

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Flooding Time Synchronization Protocol (FTSP)

• Sender-receiver multi-hop time synchronization• Integrated leader election, global time is synchronized to the

local time of the leader• End-to-end accuracy: average 1.6 μs per hop, maximum 6.1 μs

per hop (experiment included simulated root failure)• Constant network load: 1 msg per 30 seconds per mote• Start up time: network diameter times 60 seconds• Uses the Time Stamping module• Topology change tolerant: motes can move at speeds less

than 1 hop per 30 seconds.• Available from the TinyOS CVS.

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Directed Flood-Routing Framework (DFRF)

app id “rank” packet 1 packet 2 packet nmsg format:

3 bytes

Engine

sent

rece

ived

aged

getR

ank

acce

pt

rece

ive

send

OS / Radio stack

unre

gist

er

regi

ster

PolicyPolicyUserUserApplication(s)

• Flood Routing Engine:–Ad-hoc routing–Automatic aggregation–Implicit acknowledgments–Table/cache management–Very low overhead

• Flooding Policy:–Defines the meaning of “rank”–Controls the flooding and

retransmission• Application:

–Can change the packet on the way–Can drop the packet on the way

Data packet:– Fixed size length– Must contain unique part

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RITS: Routing Integrated Time Synch

• Combination of Time Synchronization and Message Routing

• No extra messages• Stealth operation• Uses the Time Stamping

module that has 1.4 μs average precision per hop

• No clock skew estimation• Precision depends on the

hop count of the route and on the total routing time

• Plug-in replacement for the Directed Flood Routing Framework (DFRF)

node1time

node2time

node3time

roottime

Tevent

Δt1

Δt2

Δt1 + Δt2 + Δt3

Troot

Tevent = Troot - Δt1 - Δt2 - Δt3

Δt3

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RITS Experimental Evaluation

• 50 Mica2 motes• 10 x 5 grid, neighbor to neighbor

comm is enforced in software• Five simulated shots separated by 10

msec• For each shot 13 motes send

simultaneous detection events to root– simulates a shot event– triggered by a radio message in

experiment• Root at the edge of the network• Experiment #1: normal routing:

– 1.5 hours long (2 tests/min)• 4.4 μs average error• 19.2 μs average maximum error• 74 μs peak maximum error

• Experiment #2: data is delayed by 5 seconds at each hop:

– 8 hours long (2 tests/min)• 28.5 μs average error• 107.6 μs average maximum error• 265 μs peak maximum error

average time synchronization error histogram

0%

5%

10%

15%

20%

25%

30%

0 2 3 5 6 8 10 11 13

synchronization error (microseconds)

perc

en

tag

e

maximum time synchronization error histogram

0%

5%

10%

15%

20%

25%

30%

0 7 15 22 30 37 44 52 59

synchronization error (microseconds)

perc

en

tag

e

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time

t2

t1

t4

t3

d1

f(x,y)

?

d3

d4

d2

t2 – d2/vt3 – d3/v t1 – d1/vt4 – d4/v

Shot #1 @ (x1,y1,T1)

Shot #2 @ (x2,y2,T2)

Echo #1 @ (x3,y3,T1)

f(x,y) = [max number of ticks in window] = 3Shot time estimate T

3 0 1

sliding window

Sensor Fusion

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Sensor fusion cont’d.

• Advantages:–Groups together consistent sensor readings–Only uses correct detections for localization: high accuracy–Enables multiple simultaneous shot resolution

• Search algorithm:– Loop {

• Multiresolution search locates maximum• If absolute time is close to a previously found peak, it is classified as an echo,

otherwise a shot• Contributing sensor readings are removed

– } Continue

• Remarks:–Size of sliding window is determined by the estimated detection error due to,

for example, sensor localization error–Only uses muzzleblast at this point. Shockwave is utilized after localization for

trajectory estimation.–Performance is remarkable: separates simultaneous shots, differentiates

between shooters in close proximity, can handle 10 shots per second or more (bottleneck is network bandwidth)

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Experiments at McKenna MOUT site at Ft. Benning

NORTH

B1Church

Sep 2003: Baseline system Apr 2004: Multishot resolution

60 motes covered a 100x40m area Network diameter: ~7 hops Used blanks and Short Range Training

Ammunition (SRTA) Hundreds of shots fired from ~40 different

locations Single shooter, operating in semiautomatic

and burst mode in 2003 Up to four shooters and up to 10 shots per

second in 2004 M-16, M-4, no sniper rifle Variety of shooter locations (bell tower, inside

buildings/windows, behind mailbox, behind car, …) chosen to absorb acoustic energy, have limited line of sight on sensor networks

Hand placed motes on surveyed points (sensor localization accuracy: ~ 0.3m)

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Results

Shooter Detection Error

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 3.75 4

error (meter)

perc

enta

ge

2D

3D

Based on 40 blank and SRTA shots from surveyed pointsAverage 2D error: 0.57mAverage 3D error: 0.98m

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2.5D Display, Single shot

Red circle:

Shooter position

White dot:

Sensor node

Small blue dot:

Sensor Node that detected current shot

Cyan circle:

Sensor Node whose data was used in localization

Yellow Area:

Uncertainty

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2.5D Display, Multiple Shots

Red circle: Shooter position

White dot: Sensor node

Small blue dot: Sensor Node that

detected current shot

Cyan circle: Sensor Node whose

data was used in localization

Yellow Area:

Uncertainty

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Future work

http://www.isis.vanderbilt.edu/projects/nest

• New sensor fusion utilizing both muzzle blast and shockwave:– Increased range and accuracy– Silenced weapons

• New sensor board:– Low-power DSP– More sophisticated detection: increased range– Power saving modes

• Sensor self localization:– < 0.5m 3D accuracy needed

• Scaling up:– Hierarchical network architecture– Distributed sensor fusion