Cell phone GPS?

Pred8tr · 11-30-2002, 12:41 PM

I recently bought a Motorola T720. It has a built in technology called "aGPS". Does anyone know whether or not I could use this phone with the data cable in place of a normal GPS?

Telek · 11-30-2002, 09:03 PM

Short answer: Most likely not. It's up to the manufacturer to provide you with that data output on the cellphone, and they have no incentive to do so, and actual reason not to.

Long answer: Most likely not. The cellphone GPS is really not a full GPS, only partial (because it's designed to need to work indoors as well when you wouldn't get a GPS signal)... It uses a system called "Differential GPS" which basically means that since the cellphone towers are stationary you can know with extreme precision where these towers are located. Since GPS signals have a great deal of error in them you can use this method to reduce the error.

Normal GPS recievers attack this problem by combining the output from multiple GPS satellites and then calculating what a possible total error correcting value might be. Unfortunately even with several satelites because of the great deal of distance travelled for the signal it's difficult to reduce the error unless you already know your position, but then if you do it's pointless to use GPS, isn't it? If there was only some way to know what the error correcting factors were already for the GPS signals, then you could correct each one individually and come up with a very accurate position.

Enter Differential GPS. Since the cellphone towers are in a fixed position they use the information that they already know on their location to attack the problem backwards: They can determine the error in the signal because they already know their position. By combining this with the signal that the cellphone recieves you can reduce the innacuracy of your calculations greatly.

This is a long winded way of saying that it's not a normal GPS in your cellphone. IF the above method fails because you can't get any GPS signal on your cellphone (which will be frequent even outdoors) they have to rely on signal strength from multiple towers (because with digital cellphone technology all signals much reach the tower at the same strength, so the cellphone has to adjust it's broadcast power to compensate for distance, thus the cellphone has to know how far away from the tower it is). If you have more than 2 towers then you can triangulate the position the old-fashion way. Even if you only have 2 then you have 1 of 4 locations where the person could be (since we are talking about spheres here), but 2 will usually be either in the middle of air or ground.

In any case, the cellphone has to do a lot of calculations here to determine the position and thus you aren't going to find an NMEA compatible cellphone

... Additionally since this data is meant only to be accessible to the cellphone providers to track your location (er, I mean to pinpoint your location in the event of emergency, yeah that's it) it's not in their best interest to provide that data to you.

Read up on differential GPS here.

Pred8tr · 12-01-2002, 01:39 AM

Thanks for the very informative info.

Ron

Ohgary · 12-01-2002, 12:43 PM

differential GPS is not a self contained GPS system by itself. It cant work without the GPS system. If you cant hear the satellites, diff GPS isnt going to help much.

DIff GPS sites are position to a known very accurate location. Then it listens to the GPS satellites. The DiFF GPS transmitter (tansmits on an entirely differnt band than GPS sat signals) sends out a correction. I am at X,Y,Z but the GPS calulcated postion says I am off by .01,-.4,-,6. These corrections are applied to your GPS signals.

Telek · 12-01-2002, 04:09 PM

I never said that it was a self-contained system, actually I did say otherwise, but perhaps I was unclear: "By combining this [the DGPS information] with the signal that the cellphone recieves [the normal GPS signal] you can reduce the inaccuracy of your calculations greatly".

It's not quite that simple (DGPS it deals with *timing* errors, not *position* errors), but yes that's the idea.

However with the cellphones the requirements state that you must still be able to give information for (a) cellphones that don't have GPS recievers built in and (b) cellphones that do but aren't within range of satellites. Thus there is some additional stuff built in that uses signal strengths of the cellphone signal to determine the location/enhance the data provided by the GPS unit. This is provided by the cellphone and also by the towers (for instances where there is no GPS unit built in). However it's a lot easier for the cellphone to do that math than it is to corollate multiple towers information (mainly because the cellphone already knows all of this information), and this info can be used to suppliment information that the cellphone already has.