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Location in Pervasive Computing

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  1. 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: ubicomp lab build: university of washington university of washington Computer Science & Engineering Electrical Engineering

  2. 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

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

  4. Location Tracking

  5. 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

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

  7. Consider for example… • Motion capture • Car navigation system • Finding a lost object • Weather information • Printing a document

  8. Others aspects of location information • Indoor vs. outdoor • Absolute vs. relative • Representation of uncertainty • Privacy model

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

  10. Some outdoor applications E-911 Bus view Car Navigation Child tracking

  11. Some indoor applications Elder care

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

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

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

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

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

  17. Angulation • Angle of the signals • Directional antennas are usually needed

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

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

  20. Fingerprinting

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

  22. Reporting Error • Precision vs. Accuracy

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

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

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

  26. Active Badge • IR-based • Proximity

  27. Active Bat • Ultrasonic • Time of flight of ultrasonic pings • 3cm resolution

  28. Cricket • Similar to Active Bat • Decentralized compared to Active Bat

  29. Cricket vs Active Bat • Privacy preserving • Scaling • Client costs Active Bat Cricket

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

  31. RADAR • WiFi-based localization • Reduce need for new infrastructure • Fingerprinting

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

  33. 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)

  34. 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)

  35. 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)

  36. Beacon-based localization

  37. Wifi localization (ex. Ekahau)

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

  39. PowerLine Positioning • Indoor localization using standard household power lines

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

  41. Signal Map 1st Floor 2nd Floor

  42. Example

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

  44. Active Floor • Instrument floor with load sensors • Footsteps and gait detection

  45. Motion Detectors • Low-cost • Low-resolution

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

  47. Other systems? • Inertial sensing • HVACs • Ambient RF • etc.

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