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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

Plume Tracking in Sensor Networks

Glenn Nofsinger

PhD Thesis Defense

August 22, 2006

outline
Outline
  • Motivation and Problem Statement
  • Other Work
  • Theoretical Background
  • 2-Step Algorithm
  • Experiments
  • Results and Conclusions
motivation
Motivation
  • 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 15
Problem Statement (1)

t=4

  • What is the best estimate of the true source locations given these observations?
problem statement 16
Problem Statement (1)

t=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

II

I

Model

Complexity

III

IV

Static sensor networks

with high density

cheap fixed sensors

Traditional

environmental techniques

with high resolution sensors,

low sensor density

(Our approach)

Mobility

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

C(x,y,t)

A

B

t

Theoretical Background (8)

Sensors

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

A

B

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 functionBayesian 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:
estimators
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

Estimators

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)

slide29
MHT

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

  • 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

observations

Tracks

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)

Input:

Sensor “Hits” (x,y,t)

Step 1:

Track estimation

Output: N Tracks

M(Track1)

Step 2:

State Estimation

For each track

M(Track2)

M(TrackN)

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
slide41

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.

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

http://www.ndbc.noaa.gov/

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)

ML(M)

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

ZT

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

zone

Mean wind Speed scaled into 4 groups

Standard deviation of direction divided into 4 groups

This produced 16 total wind categories

high

Intermediate performance

3

4

1

2

Mean wind speed

5

ZONE 1

ZONE 2

ZONE 3

Worst performance

Zone: low wind

Speed, with

Frequent shifts

16

low

high

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
N=100

Results and Conclusions (4)

P(M)=1 E-4

n 200
N=200

Results and Conclusions (5)

P(M)=1.3 E-4

n 300
N=300

Results and Conclusions (6)

P(M)=1E-4

n 400 sensors
N=400 Sensors

Results and Conclusions (7)

P(M)=9E-5

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
source separation problem
Source Separation Problem

To what extent can

we differentiate two sources

as a function of sensor

density?

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

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

track formation
3 sources

M=N=250

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

slide79

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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