Some explanations for folks on how aGPS works on smartphones

Right now, I'm seeing three points of confusion on GPS-enabled smartphones (versus "real" GPSr devices like Garmins and TomToms):

a) Whether phones can store maps "off the cloud" for areas of poor cell coverage (most commercial packages can, including CoPilot Live and Navigon)

b) Whether phone GPS tools can use POIs (CoPilot Live can use .ov2 natively, Google Maps can use with some massaging, Navigon can use them with a special conversion program).

c) Whether a cell phone needs cell coverage to get GPS.

The last one is due to two things, IMHO:

a) Cell phone companies in the Bad Old Days did lock down GPS service to their own proprietary programs (Verizon used to be downright notorious for this); a class action lawsuit by Blackberry owners pretty much ended that dark period.

b) Most GPS-enabled cell phones use "aGPS" or "assisted GPS", and it's that last one that's causing the real confusion here.

First off, it's best to list what aGPS is not. It's not triangulation by cell towers (which is a different method of getting a "fix"), it's not triangulation by wifi (which can be used by some GPS devices).

What aGPS is, is a way for smartphones to get some info that ALL GPS devices use more quickly so as to get a fix more rapidly. To know how this works, you need to know first how non-aGPS GPSr devices (like a Garmin) get a first fix from a "cold boot".

All GPS devices (including smartphones, GPSr devices like Garmins and TomToms, the GPS chips you can buy at Sparkfun for $50, the GPS chips that expensive "APRS Ready" amateur radio equipment has built in, ad so on) all have to have a way to find the satellites in the sky in the first place.

GPSr devices and other devices that do NOT use aGPS will--as soon as they see and successfully contact a GPS satellite--try to get what's called the "GPS Almanac and Ephemeris". This is basically a list of what GPS satellites are in the sky over an area and where they are (yes, it is pretty similar to the Old Farmer's Almanac moon-rise and moon-set tables, but for satellites).

This is why non-aGPS GPS receivers will take a while to get a fix from a "cold start" if they've not been used in a while--they have to find the satellite, then get the list of satellites and where they are.

aGPS as used in smartphones is basically a way to speed this up--instead of just GPS satellites sending you their "GPS almanac and ephemeris", your cell company gets it from their OWN (continuously running) GPS receivers--and transmits this to your phone based on what cell you're closest to.

In essence, your local cell tower says "OK, here's the GPS satellites I'm seeing, here's my list of the ephemeris and almanac tables". And usually cells for cell towers are fairly small, so it's reliable enough that your smartphone can use this as a "kick start" to find those satellites and get a fix faster.

And yes, there are programs where you can use aGPS WITHOUT a cell tower--there were Windows Mobile tools where you could put in a location, connect to broadband (like a wifi connection or to your computer), have the ephemeris tables downloaded to your phone, and get an accellerated fix that way. (There's also an Android app that does this, GPS Tools.)

That said--all aGPS does is basically give a "kick start" file from another GPS receiver (in this case, the ones run by the cell phone company). Nearly all phones that can use GPS separately from a cell signal can happily use regular GPS and can grab ephemerides tables from the satellites (just like a Garmin or TomTom does, and it takes about the same time).

The big thing in knowing whether your phone can use "plain GPS" is just to make sure your phone can do this independently of having the cell-phone transmitter on. Most modern phones can (especially post-lawsuit by Blackberry users over locked-down Verizon phones).

For example, with my G2 there are separate checks under the "Location and Security" tab for "use wireless networks" (i.e. using triangulation by cell towers, which is NOT aGPS) and "use GPS satellites", and under "Wireless & Networks" where the main phone radio can be turned off via Airplane Mode. (There's also a separate check for "Wi-Fi"--yes, I can pretty much turn everything off and then turn wi-fi back on, presumably for T-Mobile's Wi-Fi Calling and for use of wi-fi in airplanes that have wi-fi service.)

I can turn everything off save for "Use GPS Satellites", and it'll take about twenty to forty seconds to lock on from a "cold boot", versus 4-5 seconds (if THAT) using aGPS and the "kickstart" from the ephemerides file.

(For comparison--before I had this, I had a Samsung Omnia that had originally been "GPS hobbled" by Verizon, then was "un-hobbled" after the class-action lawsuit by Blackberry users (Verizon decided to un-hobble things). Unfortunately, their "fix" managed to link turning on the cell-phone's radio and the GPS chip--so no GPS without having the radio for the phone on. This is more of a design flaw with the Omnia, though, and one that modern phones are going away from.)

So, Cliff's Notes version on aGPS: It is basically a very quick way of getting info on where satellites are (from a cell provider running GPS devices) so we don't have to wait to get that same info from the satellites themselves. Nothing more, nothing less.

More info from Wikipedia on aGPS: http://en.wikipedia.org/wiki/Assisted_GPS

More info on those ephemerides tables that all GPSr devices use: http://en.wikipedia.org/wiki/GPS_signals#Navigation_message (this gets techy, but is very useful info if you want to know the guts on how GPS devices even find the satellites in the first place!)