A High-Accuracy, Low-Cost Localization System for Wireless Sensor Networks Radu Stoleru, Tian He, John A. Stankovic, David Luebke University of Virginia

A High-Accuracy, Low-Cost Localization System for Wireless Sensor Networks

Radu Stoleru, Tian He, John A. Stankovic, David Luebke

University of Virginia

Department of Computer Science

Overview

• Problem Statement

• State of Art

• Spotlight: THE System

• System Implementation

• Performance Evaluation

• Conclusions

UNIVERSITY of VIRGINIAUNIVERSITY of VIRGINIA ACM SenSys November 2-4, 2005.ACM SenSys November 2-4, 2005.

Problem Statement

• Localization:

– Find the position of sensor nodes

– Research Challenge:

• Current limitations:

– Sophisticated Hardware: timesynch with satellites (GPS)

– Short Range, Highly Directional (ultrasound)

– Reduced Cost Effectiveness: hefty price for a 1-time event

– Large Form Factor (any solution that requires additional hardware)


High Location Accuracy ≡ High Cost

How to solve this, practically?

State of Art

• Many algorithmic solutions, few evaluated in realistic environments

• Solutions that use ranging:– MIT Cricket (Priyantha et at., 2000)

– UCLA AHLoS (Savvides et al., 2000)

– Microsoft RADAR (Bahl et al., 2000)

– UWashington SpotON (Hightower et al., 2000)

– GPS

• Range-free solutions:– USC Centroid (Bulusu et al., 2000)

– MIT Amorphous Computing (Nagpal et al., 2003)

– Virginia APIT (He et al., 2003)

– Rutgers DV-Hop (Niculescu et al., 2003)


System Design


• Main Idea: – Generate well-controlled (spatial/temporal) events in the sensor field

– A node generates a timestamp when it detects an event

– Using the above, obtain spatial information (i.e., location) for a node

• Scenario:– Nodes deployed from UAV

(Spotlight device)

– Nodes self-organize, time-synchronize

– The UAV generates light events

– Sensor nodes detect the events and report the timestamps

– The Spotlight device computes the location of the sensor nodes

System Design


• Assumptions:– Line of Sight

– Position and orientation for the Spotlight device

• GPS/INS (Inertial Navigation System) – cm positioning, arcsec attitude

– Map of deployment area:• LIDAR (Light Detection

and Ranging) 10s centimeter accuracy

• SAR (Synthetic Aperture Radar) 10s centimeter accuracy.

– Powerful Spotlight device• >1000meters

System Design


Event DetectionFunction D(e)

Event DistributionFunction E(t)

Localization Function L(T)

e(t)

Ti = { ti1 ti2 ….}

Sensor Node i Spotlight Device

pi(x,y,z)

Generates Events

Timestamp Events

Report Timestamps

Compute Location

Report Location

Spotlight System Architecture

Event Distribution Function: the core of Spotlight We propose 3 Functions

System Design


• Event Distribution Functions:– Point Scan

– Area Cover

(0,0) (0,l)

αd

N2

3(b)3(a)

N1 Nj

Ey(tj)

Ex(ti)

0000 0001 0010 0011

0100 0101 0110 0111

1000 1001 1010 1011

1100 1101 1110 1111

t = 2

0000 0001 0010 0011

0100 0101 0110 0111

1000 1001 1010 1011

1100 1101 1110 1111

t = 0

0000 0001 0010 0011

0100 0101 0110 0111

1000 1001 1010 1011

1100 1101 1110 1111

t = 3

0000 0001 0010 0011

0100 0101 0110 0111

1000 1001 1010 1011

1100 1101 1110 1111

t = 1

– Line Scan

0000 0001 0010 0011

0100 0101 0110 0111

1000 1001 1010 1011

1100 1101 1110 1111

System Design


• Execution Cost assuming:– All nodes in a square area, with length D

– N events / unit time generated by the Spotlight device

– r is tolerable localization error

• Event Overhead / Unit Time Spotlight Device Power

NrD /)/( 22 NrD /)/2( NDlog r /

D2logrD/22D2D2Event Overhead

logrD21# Time Stamps

logrD21# Detections

Localization Time

Area CoverLine ScanPoint ScanCriterion

System Implementation


• Spotlight System

• Spotlight Device:

– Projector

– Laptop

• Mica2 motes

• Short range (10-20m)

• Versatile, it generates:

– Point Scan

– Line Scan

– Area Cover

• Purpose: Investigate Capabilities

System Implementation


• Spotlight System

• Spotlight Device:

– Telescope Mount

– Diode Laser

– Laptop

• XSM motes

• Long range (>1000m)

• It generates:

– Point Scan

– Line Scan (recently)

• Demo: later today

Performance Evaluation

• Performance metric:

– Localization Error

– Localization Time

– Localization Range

– Bias in Location Estimation


• Parameters Investigated

– EDF (Point & Line Scan, Area Cover)

– Event Size

– Scanning Speed

– Laser Power

– Distance: Spotlight Device, Nodes


0

2

4

6

8

10

12

14

7.0 10.5 14.0 17.5 21.0 24.5

Event Size [cm]

Lo

catio

n e

rro

r [c

m]

87cm/sec

58cm/sec

43cm/sec

35cm/sec

Localization Error vs. Event Size

• Spotlight - Point Scan EDF

0

20

40

60

80

100

120

7.0 10.5 14.0 17.5 21.0 24.5

Event Size [cm]L

oca

liza

tion

Du

ratio

n [s

ec] 87cm/sec

58cm/sec

43cm/sec

35cm/sec

Localization Duration vs. Event Size




• Spotlight - Line Scan EDF

0123456789

10

1.7 3.5 7.0 10.5 14.0 17.5

Event Size [cm]

Lo

catio

n e

rro

r [c

m] 87cm/sec 58cm/sec

43cm/sec 35cm/sec


0

5

10

15

20

1.7 3.5 7.0 10.5 14.0 17.5

Event Size [cm]

Lo

caliz

atio

n D

ura

tion

[se

c]

87cm/sec 58cm/sec

43cm/sec 35cm/sec



Position Estimation Bias


-6

-5

-4

-3

-2

-1

0

1

2

3

1.7 3.5 7.0 10.5 14.0 17.5

Event Size [cm]

Ho

rizo

nta

l Bia

s [c

m]

87cm/sec 58cm/sec

43cm/sec 35cm/sec

Position Estimation w/o Bias (ideal)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.7 3.5 7.0 10.5 14.0 17.5

Event Size [cm]L

oca

liza

tion

Err

or

w/o

Bia

s [c

m]

87cm/sec 58cm/sec

43cm/sec 35cm/sec




• Spotlight - Area Scan EDF

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

6 8 10 12

Number of Bits

Lo

catio

n e

rro

r [c

m]

20ms/event 40ms/event 60ms/event

80ms/event 100ms/event


012345

6789

10

6 8 10 12

Number of BitsL

oca

liza

tion

Du

ratio

n [s

ec] 20ms/event 40ms/event 60ms/event

80ms/event 100ms/event







0

5

10

15

20

25

6 12 18

Event Size [cm]

Lo

caliz

atio

n E

rro

r [c

m] 0.7m/sec

1.4m/sec

3m/sec

0

5

10

15

20

25

30

35

40

0 25 50 75 100 125 150

Event Size [cm]L

oca

liza

tion

Tim

e [m

in] 3m/sec

6m/sec

9m/sec


Localization Range


950

1000

1050

1100

0 50 100 150 200

Distance [m]

Inte

nsi

ty [A

DC

co

un

t] 35mW

7mW

Detectable Event Sizes


0

200

400

600

800

1000

1200

0 50 100 150 200

Event Size [cm]

Inte

nsi

ty [A

DC

co

un

t]

35mW

7mW

Future Work


• Distributed Spotlight System

• Localization Overhead Reduction

• Dynamic Event Distribution Function

• Stealthiness

• Deployment in Unknown Terrain

• Self-Calibration

• Event Distribution Detection and Reporting

Conclusions


• Contributions: designed, implemented, evaluated the Spotlight Localization System

• Main ideas:

–

– Well-controlled (spatio-temporal) events in the network. Infer spatial information from temporal knowledge

• Long Effective Range (1000s meters)

• High Accuracy (10s centimeter)

• No hardware changes to existing motes

• Reusable localization system (amortized cost)

Complexity


Thank you!

Documents

A High-Accuracy, Low-Cost Localization System for Wireless Sensor Networks Radu Stoleru, Tian He, John A. Stankovic, David Luebke University of Virginia