The Cricket Compass for Context-Aware Mobile Applications - PowerPoint PPT Presentation

The Cricket Compass for Context-Aware Mobile Applications

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The Cricket Compass for Context-Aware Mobile Applications Nissanka Priyantha, Allen Miu, Hari Balakrishnan, Seth Teller MIT Laboratory for Computer Science – PowerPoint PPT presentation

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Learn more at: http://nms.lcs.mit.edu

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Tags: pervasive | applications | aware | compass | computing | context | cricket | mobile

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Title: The Cricket Compass for Context-Aware Mobile Applications

• Nissanka Priyantha, Allen Miu,
• Hari Balakrishnan, Seth Teller
• MIT Laboratory for Computer Science
• http//nms.lcs.mit.edu/

• Original Version mobicom00
• Location information room, floor, building, etc.
• New extensions The Cricket Compass
• Position information
• (x, y, z) coordinates within a space
• Orientation information
• direction at which device faces

• Compact, integrated, self-contained
• Should not rely on motion to determine heading
  (as in GPS navigation systems)
• Robust under a variety of indoor conditions
• Low infrastructure cost easy to deploy
• Enough accuracy for mobile applications
• (5o accuracy)

• Need to measure
• a) (d2 - d1)
• z/d

• Consider a typical situation
• Let L 5cm, d 2m, z 1m, ? 10º
• (d2 d1) 0.6cm

• Impossible to measure d1, d2 with such precision
• Comparable with the wavelength of ultrasound ( ?
  0.87cm)

• Observation The differential distance (d2-d1) is
  reflected as a phase difference between the
  signals received at two sensors

• Phase difference is periodic ? ambiguous
  solutions
• We dont know the sign of the phase difference to
  differentiate between positive and negative
  angles
• Cannot place two sensors less than 0.5? apart
• Sensors are not tiny enough.
• Placing sensors close together produces
  inaccurate measurements

• Estimate 2 phase differences to find unique
  solution for (d2-d1)
• Can do this when L12 and L23 are relatively-prime
  multiples of l/2
• Accuracy increases!

• Accurate to 3? for ? 30?, 5? for ? 40?
• Error increases at larger angles

• Improves accuracy
• Disambiguates
• ? in -?, ?

• The Cricket Compass provides accurate position
  and orientation information for indoor mobile
  applications
• Orientation information is useful
• Novel techniques for precise position and phase
  difference estimation to obtain orientation
  information
• Prototype implementation with multiple ultrasonic
  sensors

• Beacon placement
• At least one beacon within range
• Avoid degenerate configuration (not in a circle)
• Ultrasonic reflections
• Use filtering algorithms to discard bad samples
• Configuring beacon coordinates
• Auto-configuration, auto-calibration

• Magnetic based sensors (magnetic compass,
  magnetic motion trackers)
• suffers from ferromagnetic interference commonly
  found indoors
• Inertial sensors (accelerometers, gyroscopes)
• used in sensor fusion to achieve high accuracy
• require motion to determine heading
• suffer from cumulative errors
• Other systems require
• Extensive wiring expensive hard to deploy
• Multiple active transmitters worn by the user
  obtrusive, inconvenient, not scalable

• Orientation information provides a geometric
  primitive that is general and useful among a
  variety of direction-aware applications, e.g.
• In-building navigation
• Point and Shoot User Interfaces
• Line-of-sight systems are limited
• awkward to use, not robust
• do not support navigation

• Direction-aware applications could be implemented
  using TV remotes!
• But orientation information is useful
• Application-specific semantics are possible
• Convenient for navigation applications
• Eliminates the need for a line of sight to target

• Observation The differential distance (d2-d1) is
  reflected as a phase difference between the
  signals received at two sensors

• Observation The differential distance (d2-d1) is
  reflected as a phase difference between the
  signals received at two sensors

• Observation The differential distance is
  reflected as a phase difference between the
  signals received at two receivers

• We know ?t, ?t lt L/v
• Let L ?
• Observed time difference is ?t
• Possible time differences are ?t and ?t

• Navigational information
• Space
• address, room number
• Position
• coordinate, with respect to a given origin in a
  space
• Orientation
• angle, with respect to a given fixed point in a
  space
• Low cost, low power
• Completely wireless
• Deployable in existing buildings
• Scalable
• Autonomous
• Mobile device determines its own location

• We know ?t lt L/v
• Let L ?/2