As simple as it may seem, using your I-Phone to tell your
exact location is extremely complicated. Paul A Zandbergen in the Department of
Geography at University of New Mexico recently published a study to find out
how this proses works and just how accurate the I-phone 3G is.
Most Phones use Assisted GPS, or A-GPS, were many of the
functions of a full GPS receiver are performed by a remote GPS location server.
The major problems of A-GPS are that it cannot find location in high density
urban areas, or indoors. Wi-Fi and cellular positioning are used to supplement
this problem. But even with the wide array of different technologies used for
this, including ultrasound, and infrared, no one has been able to achieve a
seamless indoor-outdoor transition. With the mapping of AP’s, it is possible to
have a seamless indoor-outdoor transition. This method will only work in dense urban
areas, where there is an abundance of Wi-Fi networks. It works by mapping where
you are in proximity to the nearest Wi-Fi network, but it relies on Wi-Fi
networks overlapping with each other. A similar method is used for cellular identification,
but instead of Wi-Fi networks, it uses cell towers. Unfortunately due to the
less density and farther spread of cell towers, Cellular Positioning is much
less effective. Even though it is less effective, Cellular Positioning is still
researched and invested in heavily, because it is a backup for when Wi-Fi and
GPS are unavailable, or for if a device does not have Wi-Fi or GPS
capabilities. In the case of the I-Phone 3G, a hybrid was created giving the appearance
of a seamless transition indoors or out, and in both rural and urban areas.
This hybrid uses both A-GPS, Wi-Fi location, and Cellular Positioning. The
hybrid tests first the more accurate A-GPS, but if not able to find its
position it will move to Wi-Fi location software, and if that proves
ineffective it will move to the broadest form and check for the Cellular
Positioning.
Because the I-Phone transitions between the three software,
and doesn’t use them simultaneously there were some challenges in comparative
analysis. The study was done by testing the A-GPS at outdoor sites with ideal
conditions of visibility. The Wi-Fi and Cellular Positioning sites were inside,
and were no A-GPS was available. By switching the Wi-Fi off, the Cellular
Positioning would turn on, and vice versa. There was also a third party
application that recorded the locations, and it was all transferred to GIS
software to be analyzed. It was found that A-GPS had an average median error of
8m. Wi-Fi was much less accurate with an average median error of 74m. Cellular
Positioning found itself least accurate by far with an average median of error
at 600m.
Zandbergen, P. A. (2009). Accuracy of iphone locations: A comparison of
assisted gps, wifi and cellular positioning. Transactions in GIS, 13,
5–25.
http://engineeringblog.yelp.com/2012/08/gps-vs-wifi-the-battle-for-location-accuracy-using-yelp-check-ins.html
ReplyDeleteThis study from Yelp Engineers is interesting comparison of the location service accuracy of Smartphones comparing the difference in performance between the different devices.
The iPhone location software for WiFi is based on the Skyhook system. Check out this video to see how Skyhook works. It is a pretty cool solution, showing that good system design can improve technology with out a hardware solution, ie indoor GPS. The skyhook system works by treating all WiFi base stations as triangulation devices. Then based on network strength determine how far away the receiver is. The tricky part is making a self learning system, because WiFi base station can move. For example if you move from one apartment to another. The Skyhook system will still think your WiFi base station is at your old place. Skyhook made a deal with Apple and now uses your device to gather location information on the WiFi base stations locations making a self learning system. Once your WiFi base station is pinged by someone iPhone and it is shown to be in the wrong spot, that information is sent to Skyhook updating their location database. Pretty cool.
ReplyDeleteWhen this paper was written (2009) the Skyhook system was still in infancy that's why the huge error, but today the system preform much better with error improvement to about the same as GPS. It is hard to believe that the iPhone just came out before this work was done.
This is the Video Anwar is talking about http://www.skyhookwireless.com/howitworks/
ReplyDeleteIf this is interesting to you, in Spring 2012 Darick Pennel did a poster about accuracy of GPS units from high end survey quality down to the iPhone. His poster is available on the blog. http://southwesterngis.blogspot.com/p/projects.html
ReplyDeleteI was wondering how Pandora keeps playing on my Ipod touch as I walk around campus and it alternates between Apogee and Pirate networks; now I do: WiFi overlapping. I wonder at what point the device makes the decision to swap from one network to the other. When one signal becomes stronger than the other, perhaps?
ReplyDeleteAlso, not that it's a big deal or anything, but you had a few spelling/grammar errors throughout the posting. If you are concerned about some rando reading this article down the road as reflection of your writing you might want to correct the errors.
Relevant and interesting all the same.
Also good to know how inaccurate the WiFi and Cellular Positioning techniques are. If I'm ever on the run I just need to find I really good hiding place in a very densely populated area.
ReplyDelete