1. 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

2. 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

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: , Long: )
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. 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

10. Some outdoor applications
Bus view
Car Navigation
Child tracking

11. Some indoor applications
Elder care

12. Outline Defining location Methods for determining location Systems
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 Signal strength
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. 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

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” Community authored databases
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 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)

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 nd 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