[time-nuts] Cell timing error

Dennis Ferguson dennis.c.ferguson at gmail.com
Sun Dec 16 04:11:09 UTC 2012

On 15 Dec, 2012, at 22:38 , Hal Murray <hmurray at megapathdsl.net> 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?

E911 requires the carrier to be able to figure out where the handsets
are.  I think GPS is used as a common timing reference so they can
triangulate to locate the phone using time-of-arrival measurements
of the handset's transmissions made at several cell towers.

GSM/UMTS carriers do it this way, at least.  CDMA2000 carriers instead
rely on the handsets to make the time-of-arrival measurements, both
of signals from cell towers and of GPS signals the handset can hear.

Dennis Ferguson

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