Location, Location, Location with the iPhone

The following is an excerpt from the new O'Reilly book "iPhone: The Missing Manual, second edition".

If any phone can tell you where you are, it's the iPhone. It has not one, not two, but three different ways to determine your location.

GPS.

First, in the iPhone 3G, there's a traditional GPS chip, of the sort that's found in automotive navigation units from Garmin, TomTom, and other companies.

Don't expect it to work as well as those car units, though. The main problem is that there's not nearly as much room for an antenna in the iPhone as there is in one of those single-purpose, dedicated-GPS car units.

Still, Apple's designers pulled every trick in the book to maximize the iPhone's sensitivity, including using the tiny metal ring around the camera lens as part of the GPS antenna. If the iPhone has a good view of the sky, and isn't confounded by skyscrapers or the metal of your car, it can do a decent job of consulting the 24 satellites that make up the Global Positioning System and determining its own location.

But what if it can't see the sky? Or what if you have an original iPhone, which has no GPS chip? Fortunately, both the original iPhone and the 3G have two other fallback location features.

Skyhook's Wi-Fi Positioning System.

Metropolitan areas today are blanketed by overlapping Wi-Fi signals. At a typical Manhattan intersection, you might be in range of 20 base stations. Each one broadcasts its own name and unique network address (its MAC address -- nothing to do with Mac computers) once every second. Although you'd need to be within 150 feet or so to actually get onto the Internet, a laptop or phone can detect this beacon signal from up to 1,500 feet away.

A company called Skyhook had a huge idea: Suppose you could correlate all those beacon signals with their physical locations. Why, you'd be able to simulate GPS--without the GPS!

So for 5 years, 500 full-time Skyhook employees have been driving every road, lane, and highway in major cities around the world, measuring all those Wi-Fi signals, noting their network addresses and locations. (Neither these vans nor the iPhone ever has to connect to these base stations. They're just reading the one-way beacon signals.)
So far, Skyhook's database knows about 50 millio