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:
university of washington
ubicomp lab
university of washington
Computer Science &
Engineering
Electrical Engineering
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
2
Location

Well studied topic (3,000+ PhD theses??)

Application dependent

Research areas

Technology

Algorithms and data analysis

Visualization

Evaluation
3
Location Tracking
4
Representing Location Information

Absolute


Relative


Geographic coordinates (Lat: 33.98333, Long: -86.22444)
1 block north of the main building
Symbolic

High-level description

Home, bedroom, work
5
No one size fits all!

Accurate

Low-cost

Easy-to-deploy

Ubiquitous

Application needs determine technology
6
Consider for example…

Motion capture

Car navigation system

Finding a lost object

Weather information

Printing a document
7
Others aspects of location information

Indoor vs. outdoor

Absolute vs. relative

Representation of uncertainty

Privacy model
8
Lots of technologies!
GPS
WiFi Beacons
VHF Omni Ranging
Ultrasound
Ad hoc signal strength
Floor pressure
Laser range-finding
Stereo camera
Array microphone
Ultrasonic time of flight
Infrared proximity
E-911
Physical contact
9
Some outdoor applications
E-911
Bus view
Car Navigation
Child tracking
10
Some indoor applications
Elder care
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Outline

Defining location

Methods for determining location




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
13
Proximity

Simplest positioning technique

Closeness to a reference point

Based on loudness, physical contact, etc
14
Lateration

Measure distance between device and
reference points

3 reference points needed for 2D and 4
for 3D
15
Hyperbolic Lateration

Time difference of arrival (TDOA)

Signal restricted to a hyperbola
16
Angulation

Angle of the signals

Directional antennas are usually needed
17
Determining Distance

Time of flight


Signal strength


Speed of light or sound
Known drop off characteristics 1/r^2-1/r^6
Problems: Multipath
18
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
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starbucks
00:0F:C8:00:15:13
15
newark wifi
00:06:25:98:7A:0C
23
20
Fingerprinting

Easier than modeling

Requires a dense site survey

Usually better for symbolic localization

Spatial differentiability

Temporal stability
21
Reporting Error

Precision vs. Accuracy
22
Reporting Error

Cumulative distribution function (CDF)

Absolute location tracking systems
CDF of Localization error
1
0.9
Percentage
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Error (m)

Accuracy value and/or confusion matrix

Symbolic systems
23
Location Systems

Distinguished by their underlying signaling
system

IR, RF, Ultrasonic, Vision, Audio, etc
24
GPS

Use 24 satellites

TDOA

Hyperbolic lateration

Civilian GPS

L1 (1575 MHZ)

10 meter acc.
25
Active Badge

IR-based

Proximity
26
Active Bat

Ultrasonic

Time of flight of ultrasonic pings

3cm resolution
27
Cricket

Similar to Active Bat

Decentralized compared to Active Bat
28
Cricket vs Active Bat

Privacy preserving

Scaling

Client costs
Active Bat
Cricket
29
Ubisense

Ultra-wideband (UWB) 6-8 GHz

Time difference of arrival (TDOA) and
Angle of arrival (AOA)

15-30 cm
30
RADAR

WiFi-based localization

Reduce need for new infrastructure

Fingerprinting
31
Place Lab

“Beacons in the wild”

WiFi, Bluetooth, GSM, etc

Community authored databases

API for a variety of platforms

RightSPOT (MSR) – FM towers
32
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)
33
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)
34
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)
35

Beacon-based localization
36

Wifi localization (ex. Ekahau)
37

GSM localization
Tower IDs and signals change
Coverage?
over time!
38
PowerLine Positioning

Indoor localization using standard household
power lines
39
Signal Detection

A tag detects these signals radiating from the
electrical wiring at a given location
40
Signal Map
1st Floor
2nd Floor
41
2
d ( x, y )  ( ( xi  yi ) 2 )
i 1
Example
42
2
d ( x, y )  ( ( xi  yi ) 2 )
i 1
Passive location tracking

No need to carry a tag or device


Hard to determine the identity of the person
Requires more infrastructure (potentially)
43
2
d ( x, y )  ( ( xi  yi ) 2 )
i 1
Active Floor

Instrument floor with load sensors

Footsteps and gait detection
44
2
d ( x, y )  ( ( xi  yi ) 2 )
i 1
Motion Detectors

Low-cost

Low-resolution
45
2
d ( x, y )  ( ( xi  yi ) 2 )
i 1
Computer Vision

Leverage existing infrastructure

Requires significant communication and
computational resources

CCTV
46
2
d ( x, y )  ( ( xi  yi ) 2 )
i 1
Other systems?

Inertial sensing

HVACs

Ambient RF

etc.
47
Considerations

Location type

Resolution/Accuracy

Infrastructure requirements

Data storage (local or central)

System type (active, passive)

Signaling system
48