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Plume Tracking in Sensor Networks. Glenn Nofsinger PhD Thesis Defense August 22, 2006. Outline. Motivation and Problem Statement Other Work Theoretical Background 2-Step Algorithm Experiments Results and Conclusions. Motivation.

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Plume Tracking in Sensor Networks

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Plume tracking in sensor networks

Plume Tracking in Sensor Networks

Glenn Nofsinger

PhD Thesis Defense

August 22, 2006



  • Motivation and Problem Statement

  • Other Work

  • Theoretical Background

  • 2-Step Algorithm

  • Experiments

  • Results and Conclusions



  • Current monitoring lacks information sharing and high sampling density

  • Method needed for estimating highly unpredictable events: chemical, biological, radioactive agents

  • Many current sensors for such agents are binary

Problem statement 1

Problem Statement (1)

  • Gedanken-experiment: city with fixed, binary sensors of harmful agent

  • At an unexpected time a series of sensors activated, cause of release unknown

  • Where was the release?

  • How many release sources?

  • How are observations correlated?

Problem statement 11

Problem Statement (1)


Problem statement 12

Problem Statement (1)


Problem statement 13

Problem Statement (1)


Problem statement 14

Problem Statement (1)


Problem statement 15

Problem Statement (1)


  • What is the best estimate of the true source locations given these observations?

Problem statement 16

Problem Statement (1)


  • True initial state: two source locations

  • Thesis work estimates this truth state

Problem statement 2

Problem Statement (2)

  • This problem is hard!

  • Having an unknown number of sources and only binary detections at a large number of nodes is a new type of problem

Problem statement 3

Problem Statement (3)

  • Problem Summary:

    • Use a sensor network capable of only binary detection to estimate source locations

    • Evaluate performance of this estimation

      • As a function of wind

      • As a function of sensor density

Other work

Other work

Mobile scout robots

Swarm robots







Static sensor networks

with high density

cheap fixed sensors


environmental techniques

with high resolution sensors,

low sensor density

(Our approach)


Graphical conventions

Graphical Conventions:

Theoretical Background (1)

  • Source

  • Sensor

  • Sensor with detection

  • Track

    • A collection of sensors with detections believed to originate from the same event

    • Each track has different color


Plot conventions

Plot Conventions:

Theoretical Background (2)

  • Agent concentration for some area, A

  • Likelihood map given sensor observations

Theoretical background 3

Theoretical Background (3)

  • Fick’s Law for diffusion and linear wind

  • First order approximation to process

  • Standard Gaussian solution

Advection-diffusion Model

Theoretical background 4

Theoretical Background (4)

Plume Model

  • Solution of differential equations for advection-diffusion lead to a superposition of Gaussians

  • Peclet number measures relative strengths of diffusion to wind. A typical Peclet number is 10. This ratio determines plume width in our model

Theoretical background 5

Theoretical Background (5)

  • Assume a spatially uniform wind over the matrix A

  • Concentration state matrix A is designed to simulate an area of size 25mi x 25mi

  • Decorrelation length scale in wind data indicates the distances over which spatially uniform assumption holds

  • Typical values are on the order of 50-200 miles, therefore to first order we can assume spatially uniform wind

Wind Model

Classic analytical approaches

Classic Analytical Approaches

Theoretical Background (6)

  • Unique response per source location

  • Relative differences of Tmax unique

  • 3 sensors for 2D location

  • Can solve for (X0,Y0)

Analytical approach

Analytical Approach

Theoretical Background (7)

  • Can solve differential equations for advection-diffusion

  • Solution of the source (X0,Y0) based on measurements of C(t)

  • Method breaks down:

    • No continuous time series available

    • Very noisy, possibly binary data

Theoretical background 8





Theoretical Background (8)


Radii of location for rising and falling edges of agent detection – one for each edge is possible in binary sensor

Analytical approach no longer useful, need statistical methods.

Leads to Bayesian formulation



Typical Sensor Response Curve

Bayesian estimation

Bayesian Estimation

Theoretical Background (9)

  • Goal to obtain good estimate of target state Xt based on measurement history Zt

  • p(x) – a priori probability distribution function of state x (plume concentration) – assumed uniform

  • p(z|x) – the likelihood function of z given x

  • p(x|z) – the a posteriori distribution of x given measurement z, also called the current belief

Bayesian formulation

Relationship between a posteriori distribution, a priori distribution, and the likelihood function

Bayesian Formulation

Theoretical Background (10)

  • Our state estimate

  • True state

  • Want our state estimate to be as close to true state as possible

  • Given observation set, what is:


MMSE –minimum-mean-squared error. It is the mean posterior density. Equal weight to obs.

MAP- maximum a posteriori, maximizes the posterior distribution

ML- maximum likelihood, considers information in measurement only


Theoretical Background (11)

Estimator example source localization

Estimator Example: Source Localization

Theoretical Background (12)

  • Each sensor measurement produces independent likelihood function

  • Cone shaped likelihood function

  • Localization based on sequential Bayesian estimation

  • Measurements combined, assuming independence of likelihood

Uniform state estimation

Uniform State Estimation

Theoretical Background (13)

Plume tracking in sensor networks


Theoretical Background (14)

  • The previous uniform estimator can be improved with advanced data association (DA) techniques such as multiple hypothesis tracking (MHT)

  • By maintaining multiple “tracks” observations partitioned into subsets which correspond to unique “targets” – in this case unique plume sources

Theoretical background 15

Theoretical Background (15)


  • MHT handles the combinatorial growth of possible track assignments via accurate pruning

  • Once tracks are built in the plume problem, assume 1 target per track, therefore focusing the custom estimation on one exclusive source



2 step algorithm 1

2-Step Algorithm (1)

  • 2-Step track-estimate algorithm

    • Step 1 is track building

    • Step 2 is state estimation of tracks

  • Custom Estimator based on tracks, ignoring observations not associated to a track

  • Able to work in two scenarios:

    • Sources distant, distributed sensor groups

    • Overlapping tracks, mixing sensor groups

End of background

End of Background

  • (20 minutes)

2 step algorithm 2

2-Step Algorithm (2)


Sensor “Hits” (x,y,t)

Step 1:

Track estimation

Output: N Tracks


Step 2:

State Estimation

For each track



2 step algorithm 3

Step 1:

Track Formation

1.1 Track Initialization –All new observations potentially create tracks. The terminal node on track is designated leader node

2-Step Algorithm (3)

2 step algorithm 4

2-Step Algorithm (4)

  • Step 1:

  • Track Formation

    • 1.2 Data Association – All sensors with new observations calculate a likelihood function based on wind history. Function evaluated at all leader nodes

2 step algorithm 5

2-Step Algorithm (5)

  • Step 1:

  • Track Formation

    • 1.3 Track extension – observations that were associated in step 1.2 become the new leader nodes.

2 step algorithm 6

2-Step Algorithm (6)

  • Step 1:

  • Track Formation

    • 1.4 Track termination – The track is terminated once simulation ends or no new associations within cutoff parameter. Track outputs sent to Step 2

2 step algorithm 7

Track A

Leader nodes

New Observation

Track B

Likelihood Function

2-Step Algorithm (7)

Detail of likelihood function for track association

2 step algorithm 8

2-Step Algorithm (8)

  • Step 2:

  • State Estimation

    • Each track sequence produces an individual likelihood map

    • In this case only 4 sensor observations used to form belief map

2 step algorithm 9

2-Step Algorithm (9)

  • Step 2:

  • State Estimation

    • Each track sequence produces an individual likelihood map.

    • Only subset of observations applied to belief

Plume tracking in sensor networks

2-Step Algorithm (10)

Track Assisted State Estimation

Gradual update of estimated source position, as sensor data is aggregated along the path ABCD.

Track assisted state estimation

Track Assisted State Estimation

2-Step Algorithm (11)

Final estimated likelihood map after integration of ABCD, and renormalization for easier viewing.

Final update of estimated source position, as sensor data is aggregated along the path ABCD.

End of 2 step algorithm

End of 2-Step Algorithm

  • (40 minutes)

Experiments 1

Experiments (1)

  • Experimental Setup

  • Originally intended on collecting data from a field of physical sensors, however this hardware component distracted from analytical purpose of thesis

  • Forward data generated based on real wind data, numerical approximation to diffusion, on a grid size m=n=250

  • All code implemented in LabVIEW graphical programming language, allows for easy future hardware integration

Experiments 2

Initialize, setup scenarios

And control batch runs

Main loop –

heavy computation

Result Outputs,

statistical calculations

Experiments (2)

  • LabVIEW simulation design

2-Step Alg.

Experiments 3

Experiments (3)

DiffuseNumerical implementation

Fick’s law for diffusion implemented numerically using standard 2D centered difference scheme

Concentration of Agent assumed=0 at boundaries, agent “floats off screen”

Same code used for forward diffusion and backward belief state propagation

Large batch study 1 wind study

Large Batch Study #1: Wind Study

Experiments (4)

  • Likelihood as a function of wind direction standard deviation

  • As wind variability increases tracks become critical and perform dramatically better, operating in regions of high wind shift

  • A dataset containing 40,000 samples of real wind data are used to generate samples of length 200 spanning 5 degrees to 90 degrees

Experiments 5

Experiments (5)

  • Wind Data Example, heavy processing needed

Data Imported from web

YYYY MM DD hh mm DIR SPD GDR GSP GTIME 2004 12 31 23 00 116 7.5 999 99.0 9999

2004 12 31 23 10 115 6.7 999 99.0 9999

2004 12 31 23 20 134 7.2 999 99.0 9999

2004 12 31 23 30 136 8.2 999 99.0 9999

Large batch study 2 sensor density study

Large Batch Study #2: Sensor Density Study

Experiments (6)

  • Increase number of sensors from N=50, 100, 150, 200, 250, 300 for a 250x250 grid. Random addition of new sensors to existing set.

  • Source fixed

  • Same wind series for each trial

  • Compared performance of belief maps generated by sensor network using tracks Vs. No Tracks

Results and conclusions

Results and Conclusions

  • Maximum likelihood, ML(M), in each belief map compared to likelihood value at true source M(i,j)

Likelihood performance metrics

Belief M(i,j)

Source A(i,j)


Results and conclusions1

Results and Conclusions

  • Performance Metric Definition, For a Single Source:

  • [M(i,j) / ML(M) ] = P(M), the performance of M

  • For P(M)=1, sensor occurs at the position (i,j) within M of maximum likelihood. 1 is considered a perfect score, while 0 is considered the lowest score

  • This is the metric used in wind study and sensor node density study

Typical m for same data

Typical M for same data

Results and Conclusions (2)

2-Step predictor


Observation set

MMSE predictor

Results and conclusions key result

Results and Conclusions – KEY RESULT

  • Wind Experimental Results Summary

Results and conclusions key result1

Results and Conclusions – KEY RESULT

  • Sensor Node Density Results Summary

Density conclusion

Density Conclusion

Results and Conclusions

  • Identical network with tracks can achieve sharper maps with lower densities of sensors

  • Major advantage of using tracks is the ability to establish number of unique sources

  • Theoretical information content of a sensor network grows as log(N), therefore diminishing returns as N gets large. Both estimators approach this limit but at different rates

Results and conclusions2

Results and Conclusions

  • Summary of Wind performance zones

Best performance


Mean wind Speed scaled into 4 groups

Standard deviation of direction divided into 4 groups

This produced 16 total wind categories


Intermediate performance





Mean wind speed





Worst performance

Zone: low wind

Speed, with

Frequent shifts




Wind direction Std. deviation

Results and conclusions3

Results and Conclusions

  • The 2-Step tracking based algorithm allows provides enhanced performance compared to uniform estimator

    • Sensor density – on average the tracker based maps received a likelihood metric better by a factor of 2

    • High Wind variability – in conditions of high wind direction variability, the tracking based estimator performs much better than uniform estimator. Maintaining tracks and therefore estimates up to 30 degrees Std. deviation higher.

Future plans

Future Plans

  • Application of sensor network physical process tracking to extreme remote environments

  • The computationally intensive data association portion of the 2-Step algorithm method could be exported to existing MHT/PQS infrastructures and improved (pruning, track maintenance, hypothesis management).



Sensor density study n 50 sensors

Sensor Density Study N=50 Sensors

Results and Conclusions (3)

P(M)=1 E-4

N 100


Results and Conclusions (4)

P(M)=1 E-4

N 200


Results and Conclusions (5)

P(M)=1.3 E-4

N 300


Results and Conclusions (6)


N 400 sensors

N=400 Sensors

Results and Conclusions (7)


Results belief map from uniform predictor

Results - Belief Map From Uniform Predictor

Results belief map track 1

Results - Belief Map, Track 1

Results belief map track2

Results - Belief Map Track2

Future plans1

Future Plans

  • Application of sensor network physical process tracking to extreme remote environments

  • The computationally intensive data association portion of the 2-Step algorithm method could be exported to existing MHT/PQS infrastructures and improved (pruning, track maintenance, hypothesis management).



My papers

My papers

  • SPIE 2004

  • MILCOM 2005

  • SPIE 2006

Backup slides

Backup Slides

Source separation problem

Source Separation Problem

To what extent can

we differentiate two sources

as a function of sensor


In this example, two sources in constant wind can superimpose to create a 3rd peak

The goal of this sensor network is to correctly identify exactly 2 sources, not 3

Belief map without tracks

Belief Map Without Tracks

Inverse belief map of sensor network

Inverse Belief Map of Sensor Network

We want to construct a belief map after each trial, and look at

the value of the cell where the actual source was released.

Once we introduce tracking, we get sharper regions with higher

Values per cell. This allows us to compare the predicted map

with ground truth on any selected trial.

Forward simulation

Likelihood Map M

Belief no tracks

Belief No Tracks

Belief no tracks1

Belief No Tracks

Track formation

3 sources


300 sensors

Constant Wind

Track Formation

Example likelihood belief map

Example Likelihood (Belief) Map

The inverse scale here is E-5, which is likelihood that

The source was released from that particular cell.

Typical values for a single cell are between 10E-3 and 10E-5

Plume tracking in sensor networks

Known release event : A

Forward Probability, P(B|A)

No Wind

Constant Wind

Variable Wind

Inverse Probability, P(A|B)

Known detection event : B

No Wind

Constant Wind

Variable Wind

Bayes Rule:

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