Where's Waldo?

October 29, 2009 10:54 AM

How does A-GPS work? It's not working for me and I'm wondering if it's my phone or my provider (Rogers in Vancouver)

I have a Nokia E71. With A-GPS turned on, I can't get a fix. With A-GPS off and just regular GPS enabled I can get a fix, but of course it takes much longer.

Would appreciate troubleshooting tips because the call centre drones at Rogers, of course, are no help.

posted by randomstriker to technology (8 answers total)

Since you say AGPS doesn't work at all, I don't understand why without AGPS you say it can take "much longer" for you. Much longer than what?

It probably means that your carrier's AGPS server sucks. I have experience with the Rogers one, but this wouldn't be the fifth time I've seen that "unassisted" GPS works better. A wild guess based on how many phone providers are out of their league with this stuff: they didn't plan for the high activity levels of so many phones constantly hammering the servers.

Think of AGPS like a caching service. It's more than that, but close enough: a good cache will make web pages etc seem to work faster by giving you some "cheating" information early. A bad or slow one will give you old/bad data or generally slow down your experience.

Unless you have some reason for needing GPS faster than a couple of seconds, I'd just leave it off and let the technology/infrastructure mature a bit.
posted by rokusan at 11:25 AM on October 29, 2009

Aah, let me clarify. A-GPS did work quite well for me up until about 2 months ago...i.e. instant position fixes. Then it started blanking out, and to rectify this I switched to unassisted mode which works fine for accuracy but takes much longer to initialize.
posted by randomstriker at 12:10 PM on October 29, 2009

I guess what I'm wondering is if A-GPS works off another signal besides GPRS or HSDPA. I.e. is there a receiver for another frequency that could be damaged in my phone (I do drop it from time to time). Aside from A-GPS, my phone appears to be in perfect working order.

Oh and with unassisted GPS, it actually takes about 30 seconds for my E71 to find the necessary 4 or 5 satellites.
posted by randomstriker at 12:13 PM on October 29, 2009

Every AGPS service I've seen uses regular cell data and a TCP/IP connection to the server. It sends what it "knows", like the locations/strength of the nearest cell towers, and gets a response back from the server much like a web server: very variable response time depending on load and network speed. So GPRS or 3G or whatever you're using for data is probably what Rogers is using for the AGPS data too.

(A guess, yes. If a Rogers person is in MeFi, they can correct?)
posted by rokusan at 12:22 PM on October 29, 2009

AGPS data is relayed via data connection to and from the assistant server in the network.
If your data works fine, chances are there is something going on with the network itself.

Front line troubleshooters sometimes are not the best people to talk to. Ask for your issue to be escalated to someone who knows what they are talking about.
posted by 7life at 1:00 PM on October 29, 2009

Select: Menu > Tools > Settings > General > Positioning.

What do you see for Positioning server's address? On a normal phone this should be supl.nokia.com if Rogers have changed it to their own server, try changing it back.

Also ensure that the "Access Point" is set to use your GPRS or 3G connection and is not set to a Wifi access point.

If these settings are correct Rogers may have network problems.
posted by Olli at 4:23 PM on October 29, 2009 [1 favorite]

wow that really fixed it! thanks, Olli.
posted by randomstriker at 7:26 PM on October 29, 2009

AGPS is a data connection which downloads the ephemera (satellite locations and velocities) to your device. With this information, your device knows what frequency to look for the GPS signals on and can therefore acquire the signals very quickly.

Without the ephemera, the receiver doesn't know the relative velocities of the satellites with respect to its approx current ground location and therefore the doppler shift is unknown, which means the receiver must scan across a fairly wide frequency range to try and acquire each satellite. Most small receivers can track all 12 satellites in view but will often have only a single correlator block to perform the scanning function so it can take a few minutes to find all the satellites in view. You need at least 4 (and with favourable geometry, i.e. all not off in one direction from you) for a position fix but more will be better.

If your device has been used in the last couple of days, its remembered almanac (collection of ephemera data) will not be too far out of date so the amount of doppler scanning is minimised and you can get a fix without AGPS pretty quickly.

Looks like Olli has your fix though.
posted by polyglot at 9:30 PM on October 29, 2009 [1 favorite]