So I’m interested in location, and for this reason will be buying the wifi+3G iPad, which has a comprehensive suite of location-awareness technologies, rather than the wifi iPad, which is also location-aware but less comprehensively so.
I keep seeing absurd fallacies being promulgated about the iPad and Assisted GPS. I think “promulgated” is a word that is now entirely reserved for absurd fallacies. Do you think anyone is out there promulgating enlightenment? If they are, they’re not posting to the Wired Gadget Lab weblog comment threads, anyway.
So here for your edification is the truth about A-GPS vs. GPS vs. wi-fi triangulation.
Note: this is dull, don’t bother reading it. I just had to get this rant down to stop me boring people with it in person.
GPS uses satellites to find your location. It’s quite accurate but it takes a long time to get a satellite fix, especially if you are in a new location. So A-GPS was invented – “a system which can improve the startup performance of a GPS satellite-based positioning system“. A-GPS is GPS, plus extra functions to make it faster. It uses a database of cell-tower locations to get a very rough fix, which makes finding the real satellite fix much faster. It saves battery life, speeds up your GPS fix, and does not damage accuracy at all. It’s not a special iPhone thing, just about any cellphone with GPS now uses A-GPS, because it’s better.
A-GPS does not require data service, just cell tower locations and GPS satellites. Google Maps requires data to download its maps, but if you only have wifi you can cache them and they’ll still be available; or you can use another app that keeps its maps on the device.
[UPDATE: as Aram points out below, it does require a data connection every few days to update its database of satellite locations. However just as with caching the Google Maps tiles, connecting to a wifi network every so often takes care of that so you can, as I say, get away without buying data service.]
The original iPhone did not have GPS or A-GPS. It uses skyhook, which triangulates from a database of known wifi base station locations. This is very accurate if you’re in a densely populated city with lots of wifi base stations around. It’s so accurate because when it can, it will send pings out to three base stations, time how long it takes to get a response from each of them, and triangulate your position. It’s how GPS works, but instead of satellites it uses wifi base stations. It’s pure genius. In the city it’s actually better than GPS because it can be hard to get line-of-sight to three satellites when you’re surrounded by skyscrapers or underground. However in less densely populated areas where wifi is sparse it becomes inaccurate. This skyhook service is what the wifi-only iPad is using. Fantastic in the city, useless in the country.
The iPhone 3G, 3GS and wifi+3G iPad all use A-GPS. As I mentioned earlier, this is GPS. It uses GPS satellites. But because it’s Assisted, it’s faster than unassisted GPS. This means that they can get an accurate position fix anywhere in the world, except a few locations where the US government intentionally blurs GPS (mostly war zones). I expect Apple also has skyhook running on these, so even underground in cities where GPS can’t reach they can still get a location fix through wifi triangulation.
Now, as a bonus, a look into the future: location services will get even better. The Smart Internet Technology CRC Australia developed a system that will triangulate location from known fixed bluetooth signals. This means in super-dense environments like shopping malls and convention centres, you could have centimetre-accurate location. Also, if you happen to be in Japan rejoice: a small constellation of Japan-specific satellites is going up, to bring a super-accurate GPS-augmenting local system to the home islands.
So, please stop writing that “A-GPS” is “fake”. It’s not; it’s the real deal. A-GPS plus skyhook is absolutely the best (fastest, most accurate, most comprehensive) location service available at the moment. The wifi+3G iPad will be, like the iPhone 3G and 3GS, an unparalleled device for location-based services – for the brief time before the rest of the industry catches up.