> But, how does it work? We know the position of the fixed towers. How
> does this software position the sellphone inside the Chrysler Building?
>
When Google announced the 'My Location' service for cell phones without GPS,
they explained they do approximation from the cell tower, determining
direction and signal strength. There were a couple of things that were not
explained.
According to the guy that designed the user IDs determination system,
licensed by three of the cellular companies, the tower and horn ID is
broadcast as part of the ID packet during transmission. Assume there are six
horns (antennas) on each tower. If the tower has been 'mapped', I can tell
from the tower and horn ID the 'pie slice' where you are located. Apparently
they also know the signal strength of your device.
The CEO of Growell in 2003 claimed that Sprint reads the device signal
strength, then increases the tower signal to compensate. He also claimed the
other methodologies increased the device signal, thus decreasing battery
life. I don't know if this is still the case. The point is, if you knew
signal strength you could estimate distance from the tower maybe as far away
as five miles BUT all kinds of radio issues could throw that estimate off. I
have used tower tracking on a HTC Hermes and iPhone but it does not work on
my Blackjack Version I not II.
Most, not all, devices are being tracked by two towers. The tower with the
strongest signal is >usually< the tower serving the device. If the base
station controller permits, mathematically you could overlay the pie slice
of the two towers to find an approximate location. If you have not
experienced this, they draw a blue circle of differing radius to alert you
of the possible area. I've never seen a better location determination than
100 yards. Frequently it's 1000+ meters.
If you are in an urban area, enter street addresses, intersections, business
names, etc. If you are in a rural area, traveling 60 mph, then a half mile
is 30 seconds of travel. You'll see the blue dot jump as you are passed from
one tower to another, giving an indication of travel.
Since you are focused on the technology, research the Popular Science
articles that will soon be provided for free in an online archive. An
inventor filed for a patent to do 'triangulation' from two points. PopSci
reported you can't >tri<-angulate from two points. He had to do a demo. He
successfully demonstrated location determination from two mountain tops in
Nevada, but the patent office was not satisfied. He arraigned for a test
from one of the space shuttles. His test was aboard the shuttle that blew up
at launch, not the one that came apart during re-entry.
I apologize for not being able to provide more technical info, I'm a
designer and marketing guy. I'd suggest Wikipedia and check to see if the
archives are online at
www.popsci.com.
Thurman
CellHelp.US
