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
• http//nms.lcs.mit.edu/

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Cricket Location System

• 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

Mobile device (x, y, z)
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You Are Here Great, now what?!
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Point-and-Use Application
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Orientation is important!
Orientation is a building block that supports a
wide variety of mobile applications
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Design Goals

• 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)

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The Cricket Compass Architecture
Beacons on ceiling
Y
X
RF Ultrasonic Pulse
Z
Cricket listener with RF and ultrasonic sensors
Mobile device
( x, y, z)
vt3 to solve for unknown speed of sound
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Definition of Orientation
B
Beacons on ceiling
Beacons on ceiling
Y
X
Z
Orientation relative to B
Mobile device
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Approach Use Differential Distance to Determine
Orientation
Beacon
Assume Device rests on horizontal plane Method
Use multiple ultrasonic sensors calculate
rotation using measured distances d1, d2, z
sin ? (d2 - d1) / sqrt (1 - z2/d2) where d
(d1d2)/2
d
z
d1
d2

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

S1
L
S2
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Problem Measuring (d2 d1) directly requires
very high precision!
Beacon

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

d

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

z
d1
d2
S1
L
S2
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Solution Differential Distance (d2-d1) from
Phase Difference (?)

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

Estimate phase difference between ultrasonic
waveforms to find (d2-d1)!
Beacon
f 2p (d2 d1)/l
d2
d1
t
S1
S2
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Problem Two Sensors Are Inadequate

• 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

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Solution Use Three Sensors!

• 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!

Beacon
d3
d1
d2
S3
S2
S1
t
L12 3l/2
L23 4l/2
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Cricket Compass v1 Prototype
RF module (xmit)
Ultrasonic transmitter
Ultrasound Sensor Bank 1.25 cm x 4.5 cm
RF antenna
Beacon
Sensor Module
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Angle Estimation Measurements

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

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Cricket Compass Hardware

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

Amplifiers, Wave shaping, and Selection Circuits
Microcontroller
RS 232 Driver
RF RX
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Conclusion

• 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

Orientation accurate to within 3-5 degrees
http//nms.lcs.mit.edu/cricket/
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Considerations

• 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

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Current Orientation Systems Are Not Adequate for
Indoor Use

• 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

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Point in the direction of the Service Not at the
Service

• 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

Orientation information is useful for
context-aware mobile applications!
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Is orientation necessary?

• 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

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System Model
Cricket
Service Discovery Database
Services, Other users
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System Model
Cricket
(x, y, z, ?)
Service Discovery Database
Services
pda_at_(x, y, z, ?)
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Differential Distance From Phase Difference

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

Ultrasound signal first hits sensor S1
Beacon
t
S1
S2
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Differential Distance From Phase Difference

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

The same signal then hits sensor S2
Beacon
d1
t
S1
S2
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Where am I?(Active map)
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Deployment
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Comparisons
Active Badge Bat RADAR Cricket
Tracking? Yes Yes Depends No
Deployment Central controller wired IR sensors Central controller wired RF /USsensors RF signal map great radios Beacon placement wireless
Spatial resolution Room ? (linear few cm) Room 30cm (linear 5cm)
Orientation No No No Yes 3-5 degree prec.
Scalability All devices transmit periodically All devices transmit periodically All devices must use same RF net Devices passive distributed scheduling
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Differential Distance From Phase Difference

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

Estimate phase difference between ultrasonic
waveforms to find (d2-d1)!
Beacon
f 2p v?t/ l 2p (d2 d1)/l
d2
d1
t
?t
R1
R2
?t lt L/v, where v is velocity of sound
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Ambiguous Solutions Example

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

Beacon
L/v
t
?t
?t
?t
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Requirements

• 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

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Ambiguous Solutions Example

• We know ?t lt L/v
• Let L ?/2

Beacon
t