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.
It seems like the average median error would go down when the three systems are allowed to work together. Would it be possible to use the same data and software from the GPS systems in the iPhone rather than try and create a balance between three different systems? And if that is already in place, would it just be easier to create an app with external storage that requires access to the internet to increase accuracy?
ReplyDeleteI am surprised that these phones have such a large error in estimating their position. Does this mean that the iPhone3G is less likely to be accurate based on the type of positioning program they use or is the actual location (indoors versus outdoors) more of an influence?
ReplyDeleteI am surprised that these phones have such a large error in estimating their position. Does this mean that the iPhone3G is less likely to be accurate based on the type of positioning program they use or is the actual location (indoors versus outdoors) more of an influence?
ReplyDelete