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Is your 3G's GPS always accurate?

Whimsicool

Whimsicool

macrumors member
Original poster
Jul 11, 2008
35
40
Washington DC
Mine sure isn't.

I can be in the same room with someone else with a 3G and maps/loopt will show as almost a mile apart. Sometimes it is spot on with its accuracy, most of the time I am shown as being located up to a mile away from where I actually am (not very good when the gf has loopt too and wonders why you are not where you said you were).

So I am wondering --- What can affect a GPS reading or cause it to give a wrong location? Is 3G better than Edge for precise location (because there is a virtual lack of good 3G service in my area)? Can the GPS system in my phone be damaged?
 
Whimsicool said:
Mine sure isn't.

I can be in the same room with someone else with a G3 and maps/loopt will show as almost a mile apart. Sometimes it is spot on with its accuracy, most of the time I am shown as being located up to a mile away from where I actually am (not very good when the gf has loopt too and wonders why you are not where you said you were).

So I am wondering --- What can affect a GPS reading or cause it to give a wrong location? Is 3G better than Edge for precise location (because there is a virtual lack of good 3G service in my area)? Can the GPS system in my phone be damaged?
Click to expand...


It's most likely due to the lack of GPS signal, so the phone use the "assisted" part which the location is determined by the cell towers around you. This results in a blue circle instead of a blue dot.
As for 3G vs. Edge, I believe that the location is determined by using all cell towers supported by the phone (GSM and UMTS), so they should be the same.
 
There are two modes for the location identification on the iPhone: true GPS, and assisted GPS.

GPS: Accurate, often within feet. It requires clear access to the sky which limits to situations where you are outside or near a window (still not great). It uses signals that are constantly beaming down from GPS satellites. Your phone never contacts these satellites. No GPS device talks to the satellites -- the signal is always one way. Cell reception doesn't matter. Google Maps needs to pull the data from a server to draw the map, but the phone knows where it is.

A-GPS: Uses cell towers to estimate your location. You 3G setting doesn't matter. If your phone can only see one tower it knows that it is within the coverage area of that tower. If it can see two towers it determines that it is in the intersection of two areas, and so on for three and more towers. The more towers visible the smaller than intersection becomes and the more accurate the estimate gets*. It can use signal strength for some additional tweaking, but that is the basic idea.

This means that your A-GPS accuracy is directly related to the density of the towers where you are. If you are in the middle of nowhere you are going to get poor accuracy since you may only be in range of one tower. If you are in a heavily covered area it could be correct within a block or two.

*It always draws this as a circle even though with two towers the shape is a lens, and with more towers the geometry becomes less regular.
 
Whimsicool said:
Mine sure isn't.

I can be in the same room with someone else with a 3G and maps/loopt will show as almost a mile apart. Sometimes it is spot on with its accuracy, most of the time I am shown as being located up to a mile away from where I actually am (not very good when the gf has loopt too and wonders why you are not where you said you were).

So I am wondering --- What can affect a GPS reading or cause it to give a wrong location? Is 3G better than Edge for precise location (because there is a virtual lack of good 3G service in my area)? Can the GPS system in my phone be damaged?
Click to expand...

GPS needs line of sight to the satellites (some signal may leak through). If you are inside how do you expect that to happen? Try moving your DirctTV/Dish/XM antenna inside and see how well they pick up.

As a backup the phone uses cell tower triangulation as a backup but this can be all over the board depending on how many towers and how far you are.

Go outside and I bet you will get accurate reading every time.
 
Tall buildings around you can affect the accuracy, the signal just can't get through as cleanly. So far both of ours have been pretty accurate.
 
The handful of times I've used it, it has been fairly accurate.. even tried it out the other day while driving down the road and not only was it accurate there was very little lag when tracking.. granted we were only driving about 25mph down a side street..
 
Mine seems relatively accurate most of the time. The one time it was way off was the one time I needed to use it. When it failed I was in the middle of relatively flat former farm land not far from the tower used by local weather stations. Besides this one baffling time it works pretty well most of the time, never dead on though, although I don't expect it to be.
 
Mine has been spot on everytime. I'd say accurate within probably 10-20 feet. I've also used the maps feature to track myself as I've ridden in cars and it gets every turn perfect. The only time it isn't 100% accurate is if I'm in a building and it has to use the cell towers to triangulate. Then I'd say it's accurate within about 150 yards.
 
chinl said:
There are two modes for the location identification on the iPhone: true GPS, and assisted GPS....

GPS...

A-GPS...
Click to expand...
I thought that A-GPS was the combination of the two? Assisted GPS, ie GPS + Cell towers.
 
chinl said:
A-GPS: Uses cell towers to estimate your location. You 3G setting doesn't matter. If your phone can only see one tower it knows that it is within the coverage area of that tower. If it can see two towers it determines that it is in the intersection of two areas, and so on for three and more towers. The more towers visible the smaller than intersection becomes and the more accurate the estimate gets*. It can use signal strength for some additional tweaking, but that is the basic idea.

This means that your A-GPS accuracy is directly related to the density of the towers where you are. If you are in the middle of nowhere you are going to get poor accuracy since you may only be in range of one tower. If you are in a heavily covered area it could be correct within a block or two.

*It always draws this as a circle even though with two towers the shape is a lens, and with more towers the geometry becomes less regular.
Click to expand...


go to google to find out what a-gps really means.

the problem is that when you are not using the gps or a-gps, the phone attempts to triangulate your position from cell phone towers (the same technique that is used to locate earthquake epicenters -- but far less accurate in this example) the other way that they attempt to get your location is by a map of wireless router MAC addresses. (provided by Skyhook Wireless i believe) they basically have vans/trucks/dunebuggys/whatever that drive around collecting wireless router's MAC addresses and plot their location on a map. (ex. they see your wireless network pop up when they drive by your house and then enter your router's MAC address to a database with the location of where they found it)

if you try to get your location and it give you a large circle covering 1/2 - 2 sq miles then it is using the triangulation method. if you get a circle about the area of a city block or smaller it is likely using Skyhook's method. if you get a dot with a pulsating ring, that is gps or a-gps.

the problem i had with the Skyhook method, was when i moved it had my wireless router's MAC address stored at my old address, so if i am inside and cant get a gps lock, it looks at my network's MAC address and shows my location at my old address, about a mile away.

hope this helped
 
Cell id or WiFi locating are NOT repeat NOT the same as A-GPS.

GPS is about using satellites only. And A-GPS is only about assisting GPS.

A-GPS means the current satellite information is given to the device so it can more quickly and accurately obtain the signals and timing calculations necessary.

Yes, a starting location (determined most often by the carrier server by cell id) can be part of the assistance, but is not necessary.

Using other methods at the same time (which, for instance, some companies do), is called a Hybrid locating service.
 
kdarling said:
Cell id or WiFi locating are NOT repeat NOT the same as A-GPS.

GPS is about using satellites only. And A-GPS is only about assisting GPS.

A-GPS means the current satellite information is given to the device so it can more quickly and accurately obtain the signals and timing calculations necessary.

Yes, a starting location (determined most often by the carrier server by cell id) can be part of the assistance, but is not necessary.

Using other methods at the same time (which, for instance, some companies do), is called a Hybrid locating service.
Click to expand...

10-4 glad that there is someone else that knows
 
The best my phone seems to be able to do is ~50m uncertainty window (that transparent blue filled circle that spreads out from the blue dot). And more often than not, the center of this window, the blue dot, is like 20m, AT LEAST, from my actual location.

If I had to blind guess how far off it would be at any given time, I would say ~200m as that is most often how far it is off (when it has a gps fix).

I really hope this is mostly due to the software implementation and isn't a hardware limitation of some sort.
 
kdarling said:
Again, that method is not A-GPS.

But just as importantly, another correction: the cell id method does NOT triangulate. It uses information about just ONE tower.
Click to expand...

really?

did not know. i thought it triangulated because i dont know of any towers near my home and the center of the large circle was pretty close to dead on.

(this was on my 2G phone with wifi shut down)
 
pj rage said:
The best my phone seems to be able to do is ~50m uncertainty window (that transparent blue filled circle that spreads out from the blue dot). And more often than not, the center of this window, the blue dot, is like 20m, AT LEAST, from my actual location.

If I had to blind guess how far off it would be at any given time, I would say ~200m as that is most often how far it is off (when it has a gps fix).

I really hope this is mostly due to the software implementation and isn't a hardware limitation of some sort.
Click to expand...


i would do a restore and if that didn't solve anything, go to an apple store and get it replaced before the refurbs come roaring down the lines.
 
I found it to be very accurate. I was delivering for a pharmacy among other things and it worked very well for tracking me in the car, and me being able to use it for directions. Its a TAD bit behind me, but it catches up right away. I can see tom tom working on the iPhone easily.

When I say a tad i mean like 5 feet when im driving at 60MPH.
 
The whole a-gps thing is confusing. I recall at one point it was considered inferior, now all of a sudden it's superior to real GPS.

I think this may be due to a change in what a-gps means now compared to what it used to mean. Before, a-gps meant that a phone had a chip capable of doing location, but no processor powerful enough to do the calculations w/o hurting the performance of the phone. So the data was offloaded to your carriers servers to determine your location. This meant if your carrier didn't support the service or provide APIs for accessing it, you couldn't use location services on your phone. We used to rant about this a few years back with some of the windows mobile devices that had a-gps chips but we were unable to use them in the way we wanted.

I guess now a-gps is being used in place of the software we used to use that would update the phone w/ satelite locations for a faster lock. (forgot the name of it)

Very confusing.
 
@ kdarling

for a while i had navizon on my 2G iphone when it was jailbroken and the GUI made it look like it was using multiple towers to get the location. the location was usually the same as what i got in gmaps. was the GUI of navizon just making it look like triangulation, or was it really doing the deed?
 
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