iPhones and Location

Apparently I am at a house just off Dutton Road, a few miles south-west of Bangor, Maine. Or at least my iPhone thinks so.



I happen to know that I'm in Cambridge, Massachusetts just off of the Charles River. Looking out my window, I can see Kendall Square, some MIT buildings and a bit of the Boston skyline. So why doesn't my iPhone agree?

I don't know exactly, but I have what might be an interesting guess. The iPhone uses several methods to figure out it's location. GPS, while the most accurate, can take a few minutes to get an good fix and generally won't work indoors. Another option is to get a fix given the cellular towers in the area. Because the positions of the cell towers are known and signal power can be measured, a very quick rough estimate can be made. However, the accuracy is down to 200 meters at best so another option is to see what WiFi access points are within range. Using a similar algorithm, a more accurate fix can be had.

But how does the iPhone know where a particular cell tower or WiFi network is? Enter SkyHook, a Boston-based software-only location service that Apple uses to take this information and return a location in under a second. Skyhook catalogs cell towers and WiFi networks from around the world and make the data available via query. Given a constellation of WiFi networks and cell towers, a fairly quick and somewhat accurate fix can be made.

Or so I thought.

Enter another key piece of information that you may have already picked up from the picture above: About a month ago or so, I got a micro-cell from AT&T. As it's name implies, a micro-cell is a $149 low-power cellular radio transceiver that can be deployed where needed. It uses an Internet connection to communicate with AT&T and send calls back and forth. I use one because not too surprisingly, AT&T service is flaky and a micro-cell happens to solve my dropped-call problems.

My micro-cell has a GPS in it so AT&T knows where it is for 911 emergency reasons but Skyhook doesn't. Add to that the fact that many people have micro-cells and I am willing to bet that at least one or more of them share the same ID. As long as the micro-cells aren't too close together, this isn't a problem. But it is for Skyhook because they drive around cataloging networks for use in their service. In my case, I'm guessing that my micro-cell is new enough not to be in Skyhook's database and that another micro-cell with the same ID as mine was cataloged off of Dutton Road a few miles south-west of Bangor, Maine. I think that is why my iPhone is very misguided at the moment.

That's my guess, at least. I also installed iOS 4, the latest version of the iPhone operating system, and that may have contributed to a change in the way location is reported but I think the above explanation is more feasible. Does anyone else have a better explanation?

Tags

iPhone Location GPS WiFi Skyhook micro-cell

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