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If my understanding is correct, a GPS satellite produces a pretty simple signal which is basically composed of its location and its time. Given 4 of these signals, then one can uniquely solve for the X,Y,Z position (and as a by product, time) of the hand-held GPS unit.

Question: Why don't we use use cell towers like we use GPS satellites? It's much more easy to tell the position of a cell tower than a satellite (they don't move). And they can be given atomic clocks just like GPS satellites are given. Then we would have more redundancy, more availability, and more accuracy in determining location.

Note: I know that the E911 service uses cell tower to triangulate the position of cell phones, but this technology is based upon measuring the signal strength of the tower and is therefore nowhere nearly as accurate as GPS.

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you can only hang so much on a single tower, and the providers wouldn't want to give up existing bandwidth to the communication. Cost –  Brad Nesom Jan 6 '11 at 6:52
    
How about the simple fact of coverage, getting global coverage is much easier with satellites than towers. –  r_ahlskog Feb 7 at 14:46
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7 Answers

Cell Towers do not cover the globe or remote/rural areas, Global Positioning System does - however in dense urban areas cell triangulation can be better than GPS.

http://en.wikipedia.org/wiki/GSM_localization#Network-based

Atomic clocks are expensive to make. A GPS receiver works by measuring the relative time delay of signals from a minimum of three, but usually more GPS satellites, each of which has three or four onboard caesium or rubidium atomic clocks

http://en.wikipedia.org/wiki/Atomic_clock

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+1 Good point about the price. A cheap atomic clock costs $50K (clocktypes.com/buy_atomic_clocks.html ) and a cell tower costs $75K to $200K (squidoo.com/build-a-cell-tower ), so three clocks would usually cost more than the tower itself. Given that geolocation is an incidental purpose of cell transmission, such a cost increase would be hard to justify. I also suspect the signal corrections for transmission close to the ground would be more challenging to carry out than satellite corrections. –  whuber Jan 6 '11 at 3:36
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@Mapperz Interesting point about price that I had overlooked.(...which is a shame. I thought my next pocket watch was going to be a "Super Cool Caesium-133".) However, let's get around this by having the towers constantly reestimating their time based upon their connection to GPS. They wouldn't be nearly as accurate, but they would be accurate enough to provide more information. –  John Berryman Jan 6 '11 at 12:58
    
@John Berryman - as Brad Nesom mentions Bandwidth Costs - Cell Towers are reaching their limits with 'Data Transfer' requests - Ever had the 'Network Busy'? re-estimation will cause this occurrence even more. –  Mapperz Jan 6 '11 at 15:20
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Reminds me of a comment a co-worker once made (paraphrased): we can do anything, given unlimited time and funding. –  Jaime Soto Jan 6 '11 at 15:23
    
@John Berryman Brilliant idea! It wouldn't take much processing or bandwidth to obtain and retransmit times (stick them in dead space in packet headers or wherever). But I now find myself wondering how that time information would work. Doesn't it need to be accurate (and consistent with other signals) to about a nanosecond? Wouldn't the digital reprocessing and packaging for transmittal take far more than that and be subject to perhaps microseconds of variation? Maybe a retransmitted clock time wouldn't really improve on the triangulation we can already do today... –  whuber Jan 6 '11 at 18:16
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I would suggest this is due to cost. The Mobile Telecommunication Market is very cut throat and every service provider, manufacturer and software providers are generally taking losses on handset sales in preference for Plan based monthly payment plans. Because of this the overhead of upgrading towers and handset design to include the required hardware and software is prohibitive until we as consumers demand it.

In saying this localised positioning systems are used, here in Australia the Harbour of Whyalla uses a similar idea using radio receivers at points of known location and there by the changes in the wave length can be used to assign the distance.

Could Work but not likely with current Infrastructure.

Have Fun, CDB

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There is something in use quite like it though. It's called LORAN (LOng RAnge Navigation). It is a terrestrial radio navigation system using low frequency radio transmitters in multiple deployment (multilateration) to determine the location and speed of the receiver. It fell out of favor though when GPS gained widespread adoption. It still lives on as eLORAN ( enhanced LORAN ) and is supposed to be a backup to GPS. Its future however, is uncertain.

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Well, they actually are used for geolocation (mostly for 911 calls and law enforcement/intelligence surveillance):

But for the reasons listed in other people's answers, it's really not a replacement for GPS, but more of a hack that the Feds can use when they need to stalk someone they don't like, and can't place a GPS device on their vehicle.

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While not a direct answer to the question, cell towers are already being used in improving the GPS collected in the field. Google search for GPS-RTK networks and you'll discover how people are using this technology. Essentially you have communication between a GPS rover and a GPS base station with the cell phone as the middle man. With the correct GPS equipment, the result can be real-time, survey-grade, centimeter accuracy. Pretty amazing stuff!

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There are many more radio reflecting surfaces and obstacles on ground level. The same signal is going to be reflected from different surfaces and received with different delays. I'd suspect GPS signal isn't designed to handle multi path propagation that well.

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Yes, but this is an issues with GPS anyway. If you try to use GSP in an urban canyon, you can't get a good fix on location due to multi path propagation. –  John Berryman Mar 13 '11 at 18:11
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Assisted GPS technologies used for E911 do use techniques similar to GPS to determine position. Techniques such as Advanced Forward Link Trilateration and Enhanced Observer Time Difference use the timing of signals from cell towers to calculate more accurate positions than simple signal strength estimates. Accuracy of around 50m to 200m is possible.

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