>GPS, or at least the civilian version of it, is only accurate to within
>about 300 feet or 100 meters. I once did a "site survey" using a
>Motorola M12+T GPS timing receiver. The software I used plotted
>something like 10,000 position readings on the map. The result was a
>strip about 10 meters wide and 100 meters long and oriented ENE-SSW. My
>antenna was more or less in the middle of this mess.

There's also something called Differential GPS, which uses a transmitter
on the ground to broadcast corrections. That supposedly improves
the reading, if and where a DGPS transmitter is available.

>The military uses a different set of signals from the same satellite and
>gets accurracy good enough for weapons targeting. This level of GPS is
>available only to the military and certain defense contractors. Us
>lowly civilians can't get it.

I thought this involved mostly the same signals, except that there
are deliberately introduced small errors in the (civilian) signal,
and the military can use a special encrypted transmission that
identifies the errors (so they can be corrected for).

It is also my understanding that in broad areas (e.g. the Middle
East) the errors (Selective Availability, or SA) can be turned on
or shut off by the military. There was a shutoff around May 1,
2000 but I don't know whether it has been turned on since then.

I believe it's also possible to get military-precision positioning
by doing long-term averaging of position readings from a fixed
point. The military doesn't consider this to be much of a problem
as using GPS to guide a bomb carried by someone on foot stopping
for an hour every few meters to get a new position reading isn't
very practical.

>As far as I know, a cell phone tower has no means of determining the
>direction your signal is coming from.

But I think it does have a way of determining distance (e.g. ping
time). With 3 sufficiently separated cell towers, you can determine
a point (and get an estimate of your accuracy). Actually, 2
sufficiently separated cell towers give you two points, and if
you're tracking a moving target across several cell towers, that
might be enough.