Accuracy of iPhone locations:
Accuracy
of iPhone locations:
A comparison of Assisted GPS, WiFi and cellular positioning
Paul Zandbergen
Department of Geography
University of New Mexico
University of New Mexico
How good is it?
How good is it?
iPhone inside the SDCC
iPhone inside the SDCC
Outline
•
•
•
•
A‐GPS, WiFi and cellular positioning
Hybrid positioning on the iPhone
Hybrid positioning on the iPhone
Experimental design
Results and Conclusions
Assisted GPS
Assisted‐GPS
• Technology of choice for positioning on cell p
phones to meet FCC requirements
q
• Similar to regular GPS, but some processing functions are performed by remote sensing
functions are performed by remote sensing
• Rapid time‐to‐first‐fix
• Like GPS, poor performance indoors and urban environments
urban environments
WiFi Positioning
WiFi Positioning
• EEmploys signals from existing WiFi access points –
l
i l f
i i WiFi
i
these are mapped in a calibration phase
• During positioning phase, observed signal strengths D i
ii i
h
b
d i l
h
for unique MAC addresses are matched to calibration database
calibration database
• Well established system for controlled indoor environment feasibility for outdoor metropolitan environment ‐
feasibility for outdoor metropolitan
scale was demonstrated by the PlaceLab project
• Works wherever WiFi APs are available (and have Works wherever WiFi APs are available (and have
been mapped) – indoors and outdoors
Skyhook Wireless
Skyhook Wireless
• Currently operates the only widely available metropolitan scale WiFi positioning system
• Private company since 2003, many patents for technology development
• Widely adopted by technology partners, e.g. Apple, e , apQuest, O , etc.
Dell, MapQuest, AOL, etc.
• Has mapped over 100 million WiFi APs
• Coverage includes most urban areas in US and Coverage includes most urban areas in US and
Canada, expanding across Europe and Asia
How Does WiFi Positioning Work?
How Does WiFi Positioning Work?
Source: Skyhook Wireless, 2008
Performance Specifications
Performance Specifications
Source: Skyhook Wireless, 2008
Coverage Examples
Coverage Examples
Cellular Positioning
Cellular Positioning
• Well established method, widely used prior to A‐GPS
• Relies on knowledge of cell tower locations
• Basic cell‐ID method is most common but more complex algorithms exist (e.g. triangulation)
complex algorithms exist (e.g. triangulation)
• Accuracy is a functions of cell tower density and varies from several hundred meters to a few km
varies from several hundred meters to a few km
iPhone
iPhone 3G Positioning
iPhone 3G Positioning
• First smart‐phone with hybrid positioning
– A‐GPS, WiFi and cellular
– Switches automatically between them
– Provides measure of accuracy
• ~3,000 applications in App store that employ p
positioning
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• Many unique/novel/obscure applications
iPhone Positioning Modes
iPhone Positioning Modes
A‐GPS
A
GPS
WiFi
Cellular
Experimental Design
Experimental Design
• A‐GPS
–
–
–
–
Run iPhone in A‐GPS mode and Garmin 60Cx in autonomous mode
Ideal conditions excellent satellite visibility stable tripod
Ideal conditions, excellent satellite visibility, stable tripod
Log positions every 5 seconds for 20 minutes
Reference locations: 10 fist order bench marks
• WiFi and cellular
–
–
–
–
Run iPhone in WiFi and cellular mode
Indoor locations where A‐GPS cannot get a fix
Random set of 65 commercial/institutional buildings
Reference location determined using 6‐inch color orthos
f
l
d
d
h l
h
Sample A GPS Results
Sample A‐GPS Results
A GPS Results
A‐GPS Results
Site ID
Horizontal Error (m)
Garmin
Vertical Error (m)
iPhone
Garmin
iPhone
Median
RMSE
Median
RMSE
Median
RMSE
Median
RMSE
#1
1.1
1.1
5.2
6.2
1.3
1.4
4.4
5.6
#2
0.8
1.1
10.1
12.4
3.3
3.1
6.4
9.6
#3
06
0.6
07
0.7
59
5.9
73
7.3
13
1.3
13
1.3
52
5.2
81
8.1
#4
2.5
2.6
8.1
9.0
8.8
9.0
9.7
11.7
#5
0.4
0.5
7.7
7.6
2.5
2.7
8.7
11.1
#6
1.0
1.6
12.6
15.5
4.5
4.3
10.1
17.3
#7
2.1
2.2
5.2
6.1
1.6
2.0
10.6
11.6
#8
3.4
3.4
11.2
11.4
4.5
4.4
7.4
10.0
#9
0.9
1.7
4.3
5.8
1.4
1.3
4.9
7.5
#10
10
1.0
10
1.0
69
6.9
83
8.3
09
0.9
14
1.4
12 1
12.1
13 6
13.6
Average
1.4
1.6
7.7
9.0
3.0
3.1
8.0
10.6
WiFi and Cellular Results
WiFi and Cellular Results
WiFi Positions Cellular Positions
Number of observations
65
65
Number of valid position fixes
57
64
87.7 %
98.5 %
Minimum
16
30
Maximum
562
2,731
M di
Median
74
599
68th percentile
88
827
128
962
# Observations with error <20 m
3
0
## Observations with error <50 m
Observations with error 50 m
15
5
1
# Observations with error <100 m
41
3
Percent valid fixes
Horizontal error (m)
RMSE
Cumulative Distribution Function
Cumulative Distribution Function
Spatial pattern in WiFi errors
WiFi location estimates appear to pp
“snap” to the road network
Conclusions
• A‐GPS
– Average median error of ~8 m
– Not as accurate as dedicated GPS receiver
Not as accurate as dedicated GPS receiver
• WiFi positioning
– Median error of Median error of ~74
74 m
m
– Not as accurate as specifications
– Erratic pattern in direction of error
• Cellular positioning
– Median error of 600 m
– Similar to other studies
Implications
• Hybrid positioning:
– Automatic switching is convenient and clever, but positioning mode is not always clear to user
not always clear to user
• WiFi and cellular:
– Clearly not sufficiently accurate for many applications, like detailed y
y
y pp
,
navigation (walking, driving)
• Skyhook’s WiFi positioning systems:
– Availability and positional accuracy lower than advertised
Future Research
Future Research
• A‐GPS:
A GPS
– Performance under adverse conditions
– Comparison between cellular networks
Comparison between cellular networks
• WiFi and cellular:
– Replicate in other study areas
• WiFi:
– Performance of as a function of AP density
– Variability by neighborhood type, indoors/outdoors
Variability by neighborhood type indoors/outdoors
– Effects of calibration effort
• Hybrid:
y
– Reliability of switching positioning modes
– Effects on applications
What’ss Next?
What
Next?
competition from Google G1/Android, Palm Pré etc
Palm Pré, etc.
iPhone 3GS has a magnetic compass, Google Streetview, and multi‐model
and multi
model network network
routing built‐in
More widespread adoption of hybrid More
widespread adoption of hybrid
positioning in other devices (laptops, cameras, etc.)
Competitor to Skyhook will likely emerge (Google, Sony, Nokia)
(G
l S
N ki )
ArcGIS Mobile for the iPhone
Contact Information
Contact Information
[email protected]
www.paulzandbergen.com
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