On 12/15/2012 9:38 PM, Hal Murray wrote:
GSM cell sites in the US have GPS because it is required to support E911
positioning.  I'm not sure if it is used for anything other than this, but
it doesn't have to be.
So it's cheaper to install and maintain GPS rather than make one measurement
and tell the setup where it is?



No.

In addition to knowing where the GSM cell site is, you time stamp the time of arrival of a specific feature in the cellphone signalling system. If the cellphone is heard by three (or more) cell sites, then you can calculate the location of the cellphone within the
cell site using the time-of-arrival and speed-of-light calculations.

The CDMA systems inherently depend on knowing time to sub microseconds in order to function. You can extract similar information from the signalling systems in CDMA.

Newer cellphones have a GPS receiver front end inside the phone, which allows greater accuracy than the time of arrival systems. Many times, cellular signals bounce off of things between the handset and the base station, introducing a path length change and therefore a time-of-flight delay in the signal which causes errors in the time of arrival
calculations.

--- Graham / KE9H

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