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 lab

university of washingtonuniversity 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 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Error (m)

Perc

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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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