Cell phone GPS - how effective?

Factual Questions
1

For certain things I use a handheld GPS. It is a great tool. You turn it on, have a clear section of sky, and in maybe a minute, but sometimes much longer, sometimes a little shorter, you have a lock on enough satilites to get your position. If you put your GPS in your pocket you could easily loose the lock. Also I have one that is known to have one of the best receptions, locking faster and holding a lock better then most other handhelds. The antenna is about 2 inches by about 3/4 on an inch (triangular prism), external and somethng called a quadrectifier antenna.

My cellphone has that new e911 GPS feature, I have the option of using it all the time or only for 911 calls. For something that I usually have in my pocket, how could this little cellphone, with what I assume is a small patch GPS antenna, possible get a lock fast enough. It seems like (from handheld GPS experence) it would take 5 minutes in an open field to get a position (from the size of it). How could this feature possible work fast enough to be useful?

Some other quetions:
How come I can’t use the phone as a GPS, if it as a GPS built in, I would think it could easily give me my coordinates?

Does anypart of the GPS function if I have the phone set to (use GPS) 911 only?

How much additional power does the ‘GPS for every call’ feature use?

2

Just out of curiosity…who’s your carrier on the cell phone?

3

Allow me to expand upon that.

I work for one of the major cellular carriers that has the kind of functionality that you speak of. In my job capacity, I have extensive knowledge about the way this all works, as well as the potential applications of the tool.

If you’re using the carrier I work for, I can likely provide strong insight to any of your questions, while at the same time managing to not turn into a commercial :wink: God knows ours are annoying enough!

4

This is purely a WAG, but since the cell system can narrow down your location significantly, the receiver in the phone may not need the usual number of satellite locks to pinpoint you within the cell. In fact, it’s conceivable that the system isn’t using real GPS at all, but is using the cell antenna system to triangulate your location within the cell.

5

I have a Verizon phone (LG VX4400) with GPS capability, and as far as I can tell, the service isn’t even available here. None of the GPS test screens work.

My understanding is that most of the real GPS functionality is in the tower, and the phone doesn’t do nearly as much work as a GPS receiver, it just gathers a small amount of data which the tower combines with its own data to determine your location.

6

If they are using the systems that I’ve read of that were designed for such use, there really isn’t much processing that goes on in the phone itself.

The systems that I read about that were to be developed for cell phone emergency use were to use a server based system. The cell phone itself has only a simple receiver that picks up the GPS signal - but does not attempt to decode it. Instead, the receiver in your cell phone concentrates on enhancing the received signal as much as it can - there are techniques to pick data streams out of noise, but they take processing power which a normal receiver needs to do the decoding and position calculations.

Your cell phone GPS receiver cleans up the signal as best it can, and then transmits the raw signal (digitized) back to your cell phone carrier’s server. That computer then applies still more processing power to cleaning up the signal, and then calculates your position.

The advantage to all of this is that you can get a position out of a truly shitty signal. The articles I read mentioned tests that had the GPS receiver inside a building with concrete walls (with rebar) and they could reliably get a position from the GPS receiver. The position wouldn’t be all that accurate because of reflections and crap, but if you take a normal receiver inside such a building (away from any windows) you won’t get any position at all.

With these systems, your cell phone doesn’t know where it is but the cell phone service can locate you quite well in an emergency.

This kind of system won’t work worth a damn for tracking a user, though. The bandwidth needed for the raw GPS signal is too high - it takes way too long to transmit it for you to be able to do much else on the phone system.

Another advantage to this system is that the GPS receiver sleeps all the time and only gets turned on in an emergency - thus saving battery power.

A conventional GPS receiver has to periodically take a look around and figure out where you are so that it can be ready in case someone asks for the location - this wastes battery power because 99.99999% of the time nobody is going to ask.

The company I used to work for looked into a lot of GPS solutions while trying to find a lower power rig that we could connect to the microphone/speaker jacks of portable two-way radios. The system I described above would have been perfect because of its ability to locate you from a dead start - but we couldn’t use it because of the bandwidth requirements.

They’ve finally gone to work using a very small, extremely low power conventional unit with a variable duty cycle and some other tricks to get power usage down.

7

GPS (like celestial navigation) works by assuming you already have a rough idea of where you are and then refining that. that is the reason when you first turn on the receiver it can take several minutes until it finally gets a fix. So, with a conventional GPS receiver you need to have it on continually if you want to have quick access to your position. With a combined cell phone and GPS receiver the first rough estimate which takes the longest is unnecessary because the cell tower itself can tell the GPS receiver roughly where it is so that most of the work is already done. Furthermore, the system lends itself to using differential decoding which will give you much more precision than general GPS. As the tower know its exact location the differential location (distance and direction from the tower) can be calculated (either by the tower or by the phone) and yield much more precision.

8

I got a pretty good idea how it works, but any additional insite is appreciated ** Mr. Cynical**. I use Verizon wireless and an Audiovox 8300.

9

i know I am ignorant on this sbject, but I don’t know what the initial GPS means. I do hav a Verizon cell phone with a GPS. However, no matter what the commercials say, I do not get a signal at my house. I would like this to be my only phone, since I use a phone rarely, but I have to drive to a graveyard down the road to use it.

Would it be practicle for me to buy a GPS only and where do you get one?

10

GPS is Global Positioning System and works with satelites. You can buy a receiver at any boating store, camping or online. Just search the Net. And google will yield plenty of info on GPS and DGPS = Differential GPS.

11

See How Stuff Works
How Location Tracking Will Work

12

Why does Verizon say it can be picked up anywhere. You know “Can you hear me hear, can you he me hear” and on and on. It seems to be false advertising.

13

They don’t say it can be picked up anywhere; they say they have the largest network, which is essentially true. That doesn’t mean they cover the entire country, or that they cover every area that every other provider covers. The first thing you should do when choosing a provider is see how well they cover the areas where you’re likely to use the phone.

14

Heard today: all cells will have GPS by 2005.

15

I actually got the phone for when I am on the road for help, or to stop along the way to tell my Mom (age 83) I’m almost to her house. LOL So the phone is doing its job.

I wish I could replace my home phone with it someday. I don’t do many phone calls, so I doubt if I would have extra charges. Someday, there will be reception out in my boon docks. My neighbor stands out in the middle of the road to use his. There must be some reception there. LOL That is an idea for in the summer when it isn’t raining. I can just see the whole neighborhood standing out there on their cell phones.