Location in Pervasive Computing Shwetak N. Patel University 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 washington Computer Science & Engineering Electrical Engineering university of washington

A form of contextual information Person’s physical position 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

Location Well studied topic (3,000+ PhD theses??) Application dependent Research areas Technology Algorithms and data analysis Visualization Evaluation

Location Tracking

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

No one size fits all! Accurate Low-cost Easy-to-deploy Ubiquitous Application needs determine technology

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

Lots of technologies! GPS WiFi Beacons Ultrasound Floor pressure Ad hoc signal strength Laser range-finding VHF Omni Ranging Stereo camera E-911 Array microphone Ultrasonic time of flight Physical contact Infrared proximity

Some outdoor applications Bus view Car Navigation Child tracking

Some indoor applications Elder care

Outline Defining location Methods for determining location Systems Ex. Triangulation, trilateration, etc. Systems Challenges and Design Decisions Considerations

Approaches for determining location Localization algorithms Proximity Lateration Hyperbolic Lateration Angulation Fingerprinting Distance estimates Time of Flight Signal Strength Attenuation

Proximity Simplest positioning technique Closeness to a reference point Based on loudness, physical contact, etc

Lateration Measure distance between device and reference points 3 reference points needed for 2D and 4 for 3D

Hyperbolic Lateration Time difference of arrival (TDOA) Signal restricted to a hyperbola

Angulation Angle of the signals Directional antennas are usually needed

Determining Distance Time of flight Signal strength Speed of light or sound Signal strength Known drop off characteristics 1/r^2-1/r^6 Problems: Multipath

Fingerprinting Mapping solution Address problems with multipath Better than modeling complex RF propagation pattern

Signal Strength (RSSI) 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

Fingerprinting Easier than modeling Requires a dense site survey Usually better for symbolic localization Spatial differentiability Temporal stability

Reporting Error Precision vs. Accuracy

Reporting Error Cumulative distribution function (CDF) Absolute location tracking systems Accuracy value and/or confusion matrix Symbolic systems

Location Systems Distinguished by their underlying signaling system IR, RF, Ultrasonic, Vision, Audio, etc

GPS Use 24 satellites TDOA Hyperbolic lateration Civilian GPS L1 (1575 MHZ) 10 meter acc.

Active Badge IR-based Proximity

Active Bat Ultrasonic Time of flight of ultrasonic pings 3cm resolution

Cricket Similar to Active Bat Decentralized compared to Active Bat

Cricket vs Active Bat Privacy preserving Scaling Client costs Active Bat Cricket

Ubisense Ultra-wideband (UWB) 6-8 GHz Time difference of arrival (TDOA) and Angle of arrival (AOA) 15-30 cm

RADAR WiFi-based localization Reduce need for new infrastructure Fingerprinting

Place Lab “Beacons in the wild” Community authored databases WiFi, Bluetooth, GSM, etc Community authored databases API for a variety of platforms RightSPOT (MSR) – FM towers

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)

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)

Limitations Beacon-based solutions Using existing infrastructure 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)

Beacon-based localization

Wifi localization (ex. Ekahau)

Tower IDs and signals change over time! GSM localization Coverage?

PowerLine Positioning Indoor localization using standard household power lines

Signal Detection A tag detects these signals radiating from the electrical wiring at a given location

Signal Map 1st Floor 2nd Floor

Example

Passive location tracking No need to carry a tag or device Hard to determine the identity of the person Requires more infrastructure (potentially)

Active Floor Instrument floor with load sensors Footsteps and gait detection

Motion Detectors Low-cost Low-resolution

Computer Vision Leverage existing infrastructure Requires significant communication and computational resources CCTV

Other systems? Inertial sensing HVACs Ambient RF etc.

Considerations Location type Resolution/Accuracy Infrastructure requirements Data storage (local or central) System type (active, passive) Signaling system