Locationgate: A Cell Phone GPS Explainer For Senator Al Franken

Here's how your cell phone's location system works:

GPS, or global positioning system, is something we all take for granted (maybe too much so), but it's actually a fantastic thing if you think about it: A tiny battery powered device can listen to radio signals from a bunch of satellites hundreds of miles up in space, and then work out where you are to an accuracy of a few paces. The tech to make this work is also fantastic--fantastically clever, and it relies on some neat math to work out your position. The algorithm to calculate your position isn't an instant one, since it relies on listening to the faint radio signals for a little while to polish the calculation down to a precise location--and this is why your old in-car GPS system took ages to actually show you a map. Hence, GPS works well, but it can be slow, and in areas where your equipment can't sense the satellite signals (among tall buildings, or indoors) it doesn't work.

Then along came A-GPS, where the "A" stands for assisted, usually found on 3G smartphones. It allows the system to work even in areas where there's not particularly good GPS signal quality, and it speeds up the calculation of your position very much. This is because it uses another source of location data to come up with an first set of coordinates--simplifying the GPS algorithm's job. This data can be well-known cell phone tower positions or Wi-Fi networks, and it explains why Google's Street View vehicles also recorded wireless data (although a number of other companies offer the service too, notably Skyhook, and Apple's indicated it records anonymized data like this so it can build future services). The same kind of speedy, approximate location fixes A-GPS enables are actually vital for apps like Foursquare, and a host of other location-based systems. You'd be way less inclined to "check in" if you had to do it outside a venue, and it took 20 seconds. The thing is, to store all this "assistance" data in your smartphone makes no sense--so it's typically delivered to the phone over the 3G data network on the fly, as a kind of "where am I?"..."you're approximately here" digital dialog with a remote database. Throwing away that data immediately once it's used also makes no sense, as your phone may lose a GPS lock or you may move to a new, nearby location and re-activate your GPS (a typical maneuver for some smartphone apps).