A comparison of Assisted GPS, WiFi and cellular positioning. Paul Zandbergen Department of Geography

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1 Accuracy of iphone locations: A comparison of Assisted GPS, WiFi and cellular positioning Paul Zandbergen Department of Geography University of New Mexico

2 How good is it?

3 iphone inside the SDCC

4 Outline A GPS, WiFi and cellular positioning Hybrid positioning on the iphone Experimental design Results and Conclusions

5 Assisted GPS Technology of choice for positioning on cell phones to meet FCC requirements Similar to regular GPS, but some processing functions are performed by remote sensing Rapid time to first fix Like GPS, poor performance indoors and urban environments

6 WiFi Positioning Employs signals from existing i WiFi access points these are mapped in a calibration phase During positioning ii i phase, observed signal strengths for unique MAC addresses are matched to calibration database Well established system for controlled indoor environment feasibility for outdoor metropolitan scale was demonstrated by the PlaceLab project Works wherever WiFi APs are available (and have been mapped) indoors and outdoors

7 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, Dell, e, MapQuest, AOL, O,etc. Has mapped over 100 million WiFi APs Coverage includes most urban areas in US and Canada, expanding across Europe and Asia

8 How Does WiFi Positioning Work? Source: Skyhook Wireless, 2008

9 Performance Specifications Source: Skyhook Wireless, 2008

10 Coverage Examples

11 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) Accuracy is a functions of cell tower density and varies from several hundred meters to a few km

12 iphone

13 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 positioning Many unique/novel/obscure applications

15 iphone Positioning Modes A GPS WiFi Cellular

16 Experimental Design A GPS Run iphone in A GPS mode and Garmin 60Cx in autonomous mode Idealconditions conditions, excellentsatellitevisibility visibility, stabletripod 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 dusing 6 inch color orthos

17 Sample A GPS Results

18 A GPS Results Site ID Horizontal Error (m) Vertical Error (m) Garmin iphone Garmin iphone Median RMSE Median RMSE Median RMSE Median RMSE # # # # # # # # # # Average

19 WiFi and Cellular Results WiFi Positions Cellular Positions Number of observations Number of valid position fixes Percent valid fixes 87.7 % 98.5 % Horizontal error (m) Minimum Maximum 562 2,731 Mdi Median th percentile RMSE # Observations with error <20 m 3 0 # Observations with error <50 m 15 1 # Observations with error <100 m 41 3

20 Cumulative Distribution Function

21 Spatial pattern in WiFi errors

22 WiFi location estimates appear to snap to the road network

23 Conclusions A GPS Average median error of ~8 m Not as accurate as dedicated GPS receiver WiFi positioning Median error of ~74 m Not as accurate as specifications Erratic pattern in direction of error Cellular positioning Median error of 600 m Similar to other studies

24 Implications Hybrid positioning: Automatic switching is convenient and clever, but positioning mode is notalways clear touser WiFi and cellular: Clearly not sufficiently accurate for many applications, like detailed navigation (walking, driving) Skyhook s WiFi positioning systems: Availability and positional accuracy lower than advertised

25 Future Research A GPS: Performance under adverse conditions Comparisonbetweencellular networks WiFi and cellular: Replicate in other study areas WiFi: Performance of as a function of AP density Variability by neighborhood type, indoors/outdoors Effects of calibration effort Hybrid: Reliability of switching positioning modes Effects on applications

widespread adoption of hybrid positioning in other

26 What s Next? competition from Google G1/Android, Palm Pré, etc. iphone 3GS has a magnetic compass, Google Streetview, and multi model model network routing built in More widespread adoption of hybrid positioning in other devices (laptops, cameras, etc.) Competitor to Skyhook will likely emerge (Google, Sony, Nokia) ArcGIS Mobile for the iphone

27 Contact Information

MOBILE LOCATION-BASED SERVICES

MOBILE LOCATION-BASED SERVICES INDEX Index Table of Contents Table of Contents... i List of Figures... vi Executive summary...1 1 Introduction to location-based services...3 1.1 Definition of mobile location-based