[time-nuts] Cell timing error

Graham / KE9H TIMENUT at austin.rr.com
Sun Dec 16 16:40:47 UTC 2012

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?


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

--- Graham / KE9H


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