Location in Pervasive Computing Shwetak N. Patel University of Washington More info: shwetak.com Special thanks to Alex Varshavsky and Gaetano Borriello.

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

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

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

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0.1

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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Error (m)

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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

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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)

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Active Floor Instrument floor with load sensors

Footsteps and gait detection

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Motion Detectors Low-cost

Low-resolution

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Computer Vision Leverage existing infrastructure

Requires significant communication and

computational resources

CCTV

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Other systems? Inertial sensing

HVACs

Ambient RF

etc.

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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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