Where Does a Cell Tower Get Its Coordinates From?

I think the older satellites from the later 70's and the early 80's were accurate within 100 feet but now that we have 27 with better technology in orbit they are more accurate. I found this information on equivalent range errors.

Error sources and analysis

Main article: Error analysis for the Global Positioning System

Source	Effect (m)
Signal arrival C/A	±3
Signal arrival P(Y)	±0.3
Ionospheric effects	±5
Ephemeris errors	±2.5
Satellite clock errors	±2
Multipath distortion	±1
Tropospheric effects	±0.5
C/A	±6.7
P(Y)	±6.0

Geometric Error Diagram Showing Typical Relation of Indicated Receiver Position, Intersection of Sphere Surfaces, and True Receiver Position in Terms of Pseudorange Errors, PDOP, and Numerical Errors

The term user equivalent range error (UERE) refers to the error of a component in the distance from receiver to a satellite. User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, , of about 1 meter. The standard deviations, , for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e., RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical error. Electronics errors are one of several accuracy-degrading effects outlined in the table above. When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (49 ft). These effects also reduce the more precise P(Y) code's accuracy. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally.

These UERE errors are given as ± errors thereby implying that they are unbiased or zero mean errors. These UERE errors are therefore used in computing standard deviations. The standard deviation of the error in receiver position, , is computed by multiplying PDOP (Position Dilution Of Precision) by , the standard deviation of the user equivalent range errors. is computed by taking the square root of the sum of the squares of the individual component standard deviations.

PDOP is computed as a function of receiver and satellite positions. A detailed description of how to calculate PDOP is given in the section, geometric dilution of precision computation (GDOP).

for the C/A code is given by:

The standard deviation of the error in estimated receiver position , again for the C/A code is given by:

The error diagram on the left shows the inter relationship of indicated receiver position, true receiver position, and the intersection of the four sphere surfaces.

When we used to do the survey work for the construction of a cell tower we provided the GPS coordinates far the center of the tower. But I was never told exactly how they transmitted that info between the tower and the phone.

And we usually put some form of permanent bench mark on the site for future use.

I always assumed that since your phone was in constant contact with more than 3 towers at any one time they triangulated your signal for your location. And I don't think cell phone GPS coordinates are considered very accurate. They are really only designed to get you close to the area of the cell phone user.

The US military can change the accuracy of GPS data at will and can make it useless in a War situation for a foe using GPS targetting (or me in my car driving somewhere....!!)

Error is introduced whenever you convert between coordinate systems. GPS produces coordinates in XYZ from the center of the Earth (Earth-Centered-Earth-Fixed, or ECEF). That's what the satellites base their orbits (ephemeris data) on, and that's the basis for calculating the intersection of spheres used in calculating position.

Other navigation systems use differential movement, as in "we're now 100 feet from where we were in 'this' direction." Well, if you move along a curved surface, your heading relative to any fixed reference point on that surface changes. The direction you thought you were moving in is now irrelevant. So you must continually update your heading - or ignore it completely and just keep moving in a straight Great Circle line. But I digress - that's got nothing to do with knowing where you are right now.

Most humans think in lat-long-alt or two-dimensional coordinates, which must be converted from ECEF. Consider that the Earth is not a perfect shape, let alone a perfect sphere, and perfection is a fantasy. Longitude depends on latitude - see the pinching of longitudinal lines at the poles.

It's impossible to accurately project a curved planetary surface onto a two-dimensional map, so every map you look at is wrong. If we all worked in the same coordinate system, like ECEF, we could know where we are and where we're going with much better precision.

I am including a few sample coordinates that I scraped from my phone log. If you copy the coordinates directly into Google Maps or Bing Maps (better 3d view), you can see the discrepancy that I am talking about. What I find interesting is that in all but one case, the reported location of the tower is always East by between 25ft and 60ft(rough estimates). I will get more samples of different towers over the next week. I will also look to see if the tower coordinates change from day to day. If they change, that would point to a GPS receiver on the tower. If they do not change, we would have to assume that the coordinates are surveyed and therefore "hardcoded" into the transmission protocol. If that is the case (surveyed data), why the discrepancy? If it turns out that there is a GPS receiver on the tower, I have a much more interesting line of thinking to persue...

Having done some GIS mapping in my work, it seems that Lynn is correct that somehow you have a conversion error. That is usually the case when you have a group of coordinates that are offset from their expected location by the same bearing and amount.

But the distance is not consistent nor is the heading.

It's just not a matter of "somehow" getting conversion error, there IS conversion error.

How many maps do you know can guide you to a specific square inch of Earth's surface? I suggest that none of them can, because you always end up using dead reckoning to localize yourself. The map can only get you in the vicinity, then you take it from there.

Instead of saying that all maps are wrong, I should have said they're all approximations. Even in an ideal universe with a smooth, spherical Earth, you can't accurately project a 3D surface onto a 2D surface. At the center of projection you're accurate, but the farther you get towards the edge of the map the more unreal it is.

I'm frankly baffled that with millenia of time for the smartest human minds to solve the problem of mapping, location and direction, that we haven't done better. (But I can't figure out how to improve on the fundamentals.)

Take the basic impossibility of accuracy in the 3D-2D transformation, and add the likely conversion from ECEF to lat/long/alt to some other coordinate system or projection along the way, and the lack of precise and commonly accepted data about how high the Earth is at your point of interest.

If you were the US military, you'd probably be using the Precise Positioning Service to tell you where you are in ECEF (probably converting it once to geoid - lat/long/alt), and to tell you where your target is in ECEF (again as little conversion as possible), and thus making it possible for you to get within a few feet of where you want to go or deliver.

100 meters isn't that bad an error for a mapping process that you have no control over.

Cell towers (in the USA, can't speak for other countries) use GPS for two basic reasons: network timing and E-911 reporting.

The timing issue is important because the timing offset in the radio signal helps each tower in CDMA networks distinguish itself from its neighbors because neighboring cells usually operate on the same frequencies. In GSM networks, adjacent towers use different frequencies, but timing is still critical as GSM is time-division system.

As for 911 reporting, in the US it is a federal mandate, so all the carriers have to be able to identify where the cell phone is -- at least by relation to a tower. So, the tower location is important. GPS isn't required for this information: whatever your opinion of your service provider, they are usually pretty well aware of the exact location of their towers!

The real reason for the apparent inaccuracy goes back to what CNCNucckingfuts and Lynn.Wallace posted. The application that is getting the data may not be posting properly because the device doesn't know exactly where it is. The data it is using may be slightly off for a variety of reasons.

Almost every publicly available mapping tool that I have seen for cell towers (like www.antennasearch.com) and radio towers/antennas has been off by several meters -- and these are all stationary!

The GPS everyone (civilians) utilize is a "trickle down" technology from military origin. The accuracy of our commercially available signal is (as stated earlier) controlled by them and is purposefully less than "dead on" for obvious reasons. Electronic warfare is nothing new and I'm certain you wouldn't want an enemy using your system to send a projectile to your doorstep. Try calculating the circumference of a circle substituting pi with just 3.000... 'bout the same deal. ∞

Ok, so what I have deduced from all the comments and my observations is this:

1)Cell tower location originally fixed by GPS survey method

2)While the tower may have a GPS receiver, it is used for time derivation and not relayed to cell customers phone

3)Cell phone towers are located within a 60ft radius of actual location (as determined by an aerial map view)

Just imagine the accuracy we could have if they used an accurate survey and used those values for lat/lon.

Differential GPS calculates the error in earth received satellite data based on the knowledge of a surveyed point on the earth. The "offset" is then broadcast to users in the general vicinity and can be used to correct for atmospheric distortions, etc. If the cell tower broadcast the offset value to our cell phones, we could have very accurate GPS in our hands...