Why this topic

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• Presented by Chaitanya K. SambharaPaper by
  Rahul Gupta and Samir R. Das
  - Univ of Cincinnati SUNY
  Stony Brook

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Why this topic ?

• Networks of small, densely distributed wireless
  sensor nodes are capable of solving a variety of
  collaborative problems.
  
• Monitoring
• Surveillance

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Introduction

• Rapid advances in miniaturization in computing
  and sensor technologies and advent of low-power
  short-range radios recently have given rise to
  strong interest in smart sensor networks.
• The idea is to be bring together sensor nodes
  with on-board processing capability and radio
  interface into a large network to enable them to
  process higher level sensing tasks in a
  co-operative fashion.

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Introduction

• The system must leverage data processing and
  decision making ability inside the network as
  much as possible, instead of shipping the data to
  a central controller to make decisions.

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Aim

• Sensors are distributed, likely randomly, in a
  geographic area to be monitored.
  
• The goal is to track and predict the movement of
  an appropriate target and alert the sensors which
  are close to the predicted path of the target.

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What is the target ?

• The target can be a moving vehicle, for example,
  or can be a phenomenon such as an approaching
  fire.
  
• It is assumed that each individual sensor node is
  equipped with appropriate sensory device(s) to be
  able to detect the target as well as to estimate
  its distance based on the sensed data.

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How it works

• The sensors that are triggered by the target
  collaborate to localize the target in the
  physical space to predict its course.
  
• Then the sensor nodes that lie close to the
  predicted course of the target are alerted. This
  alert is meant to serve as a trigger for these
  nodes to activate additional on-board sensors.

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Example

• These additional sensors may be of a different
  modality (e.g., alerts coming from heat sensors
  activating vibration sensors) that are ordinarily
  turned off or not sampled to conserve power.
• Then the sensor nodes that lie close to the
  predicted course of the target are alerted. This
  alert is meant to serve as a trigger for these
  nodes to activate additional on-board sensors.

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Example

• The alert can also serve as a trigger to actuate
  certain on-board devices, depending on the
  capability of the nodes and the application.
  
• The goal is to develop techniques for the above
  moving target tracking problem and report
  experience in testing them in a live low-cost
  sensor network testbed using Berkeley motes

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Assumptions

• Sensor nodes are scattered randomly in a
  geographical region. Each node is aware of its
  location.
  
• Absolute location information is not needed. It
  is sufficient for the nodes to know their
  location with respect to a common reference point.

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Assumptions for location lnformation

• In their experiment the sensor nodes are
  stationary and we have directly encoded the
  location information into the sensor nodes to
  eliminate the possibility of any localization
  error.
• Hence there is no emphasis on any particular
  localization technique.

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Assumptions

• The sensors must be capable of estimating the
  distance of the target to be tracked from the
  sensor readings.
  
• It is assumed that the sensor has already learned
  the sensor reading to distance mapping.

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Tracking a target

• Tracking a target involves three distinct steps
• Detecting the presence of the target.
• Determining the direction of motion of the
  
  target.
  
• Alerting appropriate nodes in the network.

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Detection

• Each node periodically (every 1 sec in our
  experiments) polls its sensor module to detect
  the presence of any target to be tracked.
  
• Sensor reading above a particular threshold
  indicates the presence of a target in the
  vicinity. As soon as this threshold is crossed, a
  TargetDetected message is broadcast by the node.

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Detection

• Each TargetDetected message contains the location
  of the originating node and its distance from the
  target, as determined from the sensor reading.
  
• When this message is received by a neighboring
  node, it stores the coordinates of the originator
  and the targets distance from the originator in
  a table.
• Table entries expire after a timeout unless
  refreshed.

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Tracking

• A minimum of three nodes sensing the target are
  needed to apply the commonly used triangulation
  method.
  
• When a node that has already detected the target
  hears two additional TargetDetected messages from
  two different neighbors, it computes a location
  estimate via triangulation

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Tracking

• Here we limit this computation only to the nodes
  that themselves have detected the target, and
  hears from two other neighbors that also detected
  the target.
• This This limits the estimation to be done only
  in nodes within a close vicinity of the target,
  thus localizing the computation.

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Tracking

• In order to estimate the trajectory of the
  target, its location must be estimated at a
  minimum of two instants of time.
  
• A straight line through these two points defines
  the trajectory in the direction of the latest
  location estimate.
  
• Three or more estimates, however, work
  significantly better.

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Alerting

• After estimating the trajectory, the network must
  alert nodes that lie near the trajectory.
  
• Specifically, within a perpendicular distance d
  from it by sending them a Warning message so that
  they are aware of the approaching target and can
  take appropriate actions.

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Alerting

• Any node that is able to estimate the trajectory
  by using three location estimates broadcasts a
  Warning message.
  
• The message contains the location of the sender
  and parameters describing the equation of the
  straight line trajectory. Any node receiving the
  Warning message rebroadcasts it, if it is located
  within a distance d from the trajectory.

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Alerting

• The node receiving a Warning message computes,
  through itself, a line perpendicular to the
  trajectory.
  
• The line divides the geographic area into two
  regions R1 and R2, R2 being towards the direction
  of motion. A node forwards the Warning message

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Alerting Conditions

• 1. It lies within a distance d from the
  trajectory,
  
• 2. The Warning message is received from a node
  in region R1.

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Alerting
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Why such conditions needed

• Such conditions are needed as the warning message
  propagation is suppressed outside this region.
  
• Without this assumption, simply larger regions
  need to be flooded with warning messages.
  
• Because multiple nodes may originate Warning
  messages for the same detected target.
  
• This is because any node detecting an target
  independently attempts to carry out location and
  trajectory estimations

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Experimental Evaluation

• The moving target is a light source.
• The experimental platform is a 60 inch 60 inch
  square area with 16 motes placed at random
  locations in a dark room.
  
• A threshold of 15 inches is used for the distance
  of the light source beyond this distance the
  sensor reading is assumed too low to be reliable.

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Characteristics of the Photo Sensor

• Different sensors generate different readings for
  the same distance of the light source.
  

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Tracking moving target

• WO denotes the node that originates the Warning
  message.
  
• WR denotes the node that receives a Warning
  message and lies within distance d of the
  estimated trajectory,but does not forward the
  message because it lies in the direction opposite
  to the direction of motion.
• WF denotes the node that receives a Warning
  message, lies within distance d of the estimated
  trajectory and also forwards the message
  

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Tracking moving target
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Tracking moving target an interesting
experimental scenario.
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Concerns

• To decrease the variability in sensor readings,
  one needs to calibrate each sensor individually.
  
• This will be difficult to do for a very large
  number of sensors
  
• A large number of location samples has a strong
  potential to reduce errors in estimating the
  trajectory.

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Conclusions

• The sensor nodes detect and track the moving
  target in a collaborative fashion and alert the
  nodes near the predicted path of the target.
  
• There are several factors that influence the
  accuracy of target tracking using low-cost sensor
  nodes such as the motes, and the potential
  problems a designer can face.

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Conclusions

• The accuracy is greatly influenced by the number
  of location estimation samples the designer can
  work with.
  
• You cannot ensure that the light source always
  emits light with the same power.
  
• Experimental research using distributed networks
  of smart sensors is in its infancy and more work
  needs to be done.

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• Suggestions/ Question time