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Location in Pervasive Computing
Shwetak N. PatelUniversity of Washington
More info: shwetak.com
Special thanks to Alex Varshavsky and Gaetano Borriello for their contribution to this content
design:use:build:
ubicomp labuniversity of washington
university of washington
Computer Science & Engineering
Electrical Engineering
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Location
A form of contextual information
Person’s physical position
Location of a device Device is a proxy of a person’s location
Used to help derive activity information
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Location
Well studied topic (3,000+ PhD theses??)
Application dependent
Research areas Technology
Algorithms and data analysis
Visualization
Evaluation
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Location Tracking
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Representing Location Information
Absolute Geographic coordinates (Lat: 33.98333, Long: -86.22444)
Relative 1 block north of the main building
Symbolic High-level description
Home, bedroom, work
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No one size fits all!
Accurate
Low-cost
Easy-to-deploy
Ubiquitous
Application needs determine technology
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Consider for example…
Motion capture
Car navigation system
Finding a lost object
Weather information
Printing a document
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Others aspects of location information
Indoor vs. outdoor
Absolute vs. relative
Representation of uncertainty
Privacy model
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Lots of technologies!
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Ultrasonic time of flight
E-911
Stereo camera
Ad hoc signal strength
GPS
Physical contact
WiFi Beacons
Infrared proximity
Laser range-findingVHF Omni Ranging
Array microphone
Floor pressureUltrasound
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Some outdoor applications
Car Navigation Child tracking
Bus view
E-911
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Some indoor applications
Elder care
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Outline
Defining location
Methods for determining location Ex. Triangulation, trilateration, etc.
Systems Challenges and Design Decisions Considerations
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Approaches for determining location
Localization algorithms Proximity Lateration Hyperbolic Lateration Angulation Fingerprinting
Distance estimates Time of Flight Signal Strength Attenuation
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Proximity
Simplest positioning technique
Closeness to a reference point
Based on loudness, physical contact, etc
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Lateration
Measure distance between device and
reference points
3 reference points needed for 2D and 4
for 3D
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Hyperbolic Lateration
Time difference of arrival (TDOA)
Signal restricted to a hyperbola
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Angulation
Angle of the signals
Directional antennas are usually needed
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Determining Distance
Time of flight Speed of light or sound
Signal strength Known drop off characteristics 1/r^2-1/r^6
Problems: Multipath
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Fingerprinting
Mapping solution
Address problems with multipath
Better than modeling complex RF
propagation pattern
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Fingerprinting
SSID (Name) BSSID (MAC address) Signal Strength (RSSI)
linksys 00:0F:66:2A:61:00 18
starbucks 00:0F:C8:00:15:13 15
newark wifi 00:06:25:98:7A:0C 23
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Fingerprinting
Easier than modeling
Requires a dense site survey
Usually better for symbolic localization
Spatial differentiability
Temporal stability
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Reporting Error
Precision vs. Accuracy
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Reporting Error
Cumulative distribution function (CDF) Absolute location tracking systems
Accuracy value and/or confusion matrix Symbolic systems
CDF of Localization error
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Error (m)
Pe
rce
nta
ge
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Location Systems
Distinguished by their underlying signaling
system IR, RF, Ultrasonic, Vision, Audio, etc
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GPS
Use 24 satellites
TDOA
Hyperbolic lateration
Civilian GPS L1 (1575 MHZ)
10 meter acc.
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Active Badge
IR-based
Proximity
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Active Bat
Ultrasonic
Time of flight of ultrasonic pings
3cm resolution
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Cricket
Similar to Active Bat
Decentralized compared to Active Bat
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Cricket vs Active Bat
Privacy preserving
Scaling
Client costs
Active Bat Cricket
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Ubisense
Ultra-wideband (UWB) 6-8 GHz
Time difference of arrival (TDOA) and Angle
of arrival (AOA)
15-30 cm
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RADAR WiFi-based localization
Reduce need for new infrastructure
Fingerprinting
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Place Lab “Beacons in the wild”
WiFi, Bluetooth, GSM, etc
Community authored databases
API for a variety of platforms
RightSPOT (MSR) – FM towers
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ROSUM
Digital TV signals
Much stronger signals, well-placed cell towers, coverage over large range
Requires TV signal receiver in each device
Trilateration, 10-20m (worse where there are fewer transmitters)
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Comparing Approaches
Many types of solutions (both research and commercial)
Install custom beacons in the environment Ultra-wideband (Ubisense), Ultrasonic (MIT Cricket, Active
Bat), Bluetooth
Use existing infrastructure GSM (Intel, Toronto), WiFi (RADAR, Ekahau, Place Lab), FM
(MSR)
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Limitations
Beacon-based solutions Requires the deployment of many devices
(typically at least one per room)
Maintenance
Using existing infrastructure WiFi and GSM
Not always dense near some residential areas
Little control over infrastructure (especially GSM)
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Beacon-based localization
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Wifi localization (ex. Ekahau)
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GSM localizationTower IDs and signals change over time!Coverage?
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PowerLine Positioning
Indoor localization using standard household
power lines
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Signal Detection
A tag detects these signals radiating from the
electrical wiring at a given location
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Signal Map
1st Floor 2nd Floor
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Example
))((),( 22
1i
ii yxyxd
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Passive location tracking
No need to carry a tag or device Hard to determine the identity of the person
Requires more infrastructure (potentially)
))((),( 22
1i
ii yxyxd
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Active Floor Instrument floor with load sensors
Footsteps and gait detection
))((),( 22
1i
ii yxyxd
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Motion Detectors Low-cost
Low-resolution
))((),( 22
1i
ii yxyxd
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Computer Vision Leverage existing infrastructure
Requires significant communication and
computational resources
CCTV
))((),( 22
1i
ii yxyxd
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Other systems? Inertial sensing
HVACs
Ambient RF
etc.
))((),( 22
1i
ii yxyxd
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Considerations
Location type
Resolution/Accuracy
Infrastructure requirements
Data storage (local or central)
System type (active, passive)
Signaling system
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