This study evaluates the accuracy of the iPhone 3G location services, being the first to feature a hybrid of Assisted Global Positioning System, cellular positioning, and WiFi. To begin, the article gives an overview of the history of the Global Positioning System (GPS), the leading system in transmitting information to the Location Based Services (LBS). GPS devices have historically been affordable and reliable to all users. However, after smart phones began incorporating Assisted Global Positioning Systems (A-GPS) into their design, users became dependent on this feature.
The A-GPS feature of iPhones uses remote servers such as satellites to operate. When the A-GPS system is turned on, the iPhone does not need to decode complex messages from satellites that would otherwise compromise the effectiveness of the GPS. However, despite the perks of using an A-GPS, the A-GPS and standard GPS are less effective in largely populated, urban areas or indoors because of limited satellite exposure. High-sensitivity geographic information systems (HSGPS) were attempting to correct this issue when this article was written. But no matter what the system being used by the iPhone device, WiFi positioning is needed to determine the specific position or access point (AP) of the device. There are several APs in urban areas, making the coverage and performance of WiFi positioning much stronger. There are many WiFi systems out there, but the way they all work remains the same: first, an application needs to be installed to a WiFi-enabled device. The iPhone 3G already has this application installed in the device’s firmware. Once this is complete, the application records surrounding WiFi signals and sends them back a remote location server. The server then compares the information to information already recorded in its database and the estimated location is then sent back to the iPhone. There is varying effectiveness from system to system. Cellular networks have jumped on the opportunity to improve wireless connectivity. This competition amongst networks has allowed wireless networks to stretch worldwide. Each network has a series of its own towers, and the device’s location is determined in order to transfer its signal to the nearest tower. This process is known as cell identification (cell ID) or cellular positioning. The accuracy of this process, much like LBS, depends heavily on the density of the device’s location.
With a hybrid of all three of these systems, the iPhone caught a lot of attention. To study the effectiveness of this system, this study adopted the following method:
“A-GPS locations were collected at outdoor sites under ideal conditions, i.e. excellent satellite visibility. WiFi and cellular positions were collected at indoor sites where A-GPS position fixes were not available. Switching between these two positioning modes was accomplished by turning the iPhone’s WiFi receiver on and off – when no A-GPS or WiFi is available, the iPhone’s location service defaults to cellular positioning. A third-party application was used to record the locations as waypoints and these were transferred to GIS software for processing.”
With ten field tests, all 20 minutes in length, the study resulted yielded that the iPhone 3G is much less accurate than singular GPS devices that only have one function: to determine location. The iPhone 3G resulted in an average median error of 8 meters in each of the ten 20-minute field tests. However, when WiFi abilities were enabled on the iPhone 3G, it resulted in an average median error of 74 meters after 58 field tests. When cellular positioning services were enabled in the iPhone 3G, this resulted in the least accurate results: an average median error of 600 meters for 64 field tests. In order to improve results, this study suggests that “future research in this area should try to extend the recent progress made in controlled indoor environments to larger indoor/outdoor environments.”
Zandbergen, P. A. (2009). Accuracy of iPhone locations: A comparison of assisted GPS, WiFi and cellular positioning. Transactions in GIS, 13(s1), 5-25.