Accuracy of iPhone locations: A comparison of Assisted GPS, WiFi and cellular positioning

Paul Zandbergen Department of Geography University of New Mexico How good is it? iPhone inside the SDCC Outline

•A‐GPS, WiFi and cellular positioning • Hybrid positioning on the iPhone • Experimental design •Results and Conclusions 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 WiFi Positioning

• ElEmploys siilgnals from exiiisting WiFi access points – these are mapped in a calibration phase • DiDuring posiiition ing phase, obbdserved siilgnal 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 Skyhook Wireless

• Currently operates the only widely available metropolitan scale WiFi • Private company since 2003, many patents for technology development •Widely adopted by technology partners, e.g. Apple, Dee,ll, MapQuest, AOO,L, etc. •Has mapped over 100 million WiFi APs • Coverage includes most urban areas in US and Canada, expanding across Europe and Asia How Does WiFi Positioning Work?

Source: Skyhook Wireless, 2008 Performance Specifications

Source: Skyhook Wireless, 2008 Coverage Examples 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 iPhone 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 ppgositioning •Many unique/novel/obscure applications

iPhone Positioning Modes

A‐GPS WiFi Cellular Experimental Design

•A‐GPS –Run iPhone in A‐GPS mode and Garmin 60Cx in autonomous mode – 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 dddetermined using 6‐inch color orthos Sample A‐GPS Results A‐GPS Results

Site ID Horizontal Error (m) Vertical Error (m) Garmin 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 #3 060.6 070.7 595.9 737.3 131.3 131.3 525.2 818.1 #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 #10 101.0 101.0 696.9 838.3 090.9 141.4 12.1 13.6 Average 1.4 1.6 7.7 9.0 3.0 3.1 8.0 10.6 WiFi and Cellular Results WiFi Positions Cellular Positions Number of observations 65 65 Number of valid position fixes 57 64 Percent valid fixes 87.7 % 98.5 % Horizontal error (m) Minimum 16 30 Maximum 562 2,731 MdiMedian 74 599 68th percentile 88 827 RMSE 128 962 # Observations with error <20 m30 # Observations with error <50 m 15 1 # Observations with error <100 m413 Cumulative Distribution Function Spatial pattern in WiFi errors WiFi location estimates appear to “snap” to the road network 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 Implications

• Hybrid positioning: –Automatic switching is convenient and clever, but positioning mode is not always clear to user •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 Future Research

• A‐GPS: – Performance under adverse conditions – 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 – Effects of calibration effort •Hybrid: – Reliability of switching positioning modes – Effects on applications What’ s Next?

competition from G1/Android, Palm Pré, etc.

iPhone 3GS has a magnetic compass, Google Streetview, and multi‐model network routing built‐in Competitor to Skyhook will likely More widespread adoption of hybrid emerge (Goog le, , Nki)Nokia) positioning in other devices (laptops, cameras, etc.) ArcGIS Mobile for the iPhone Contact Information

[email protected] www.ppgaulzandbergen.com