[time-nuts] Time syncing question
daun at yeagley.net
Thu Aug 31 00:32:53 EDT 2006
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf
Of David I. Emery
Sent: Thursday, August 31, 2006 12:01 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Time syncing question
On Wed, Aug 30, 2006 at 07:53:19PM -0500, Didier Juges wrote:
> I have observed that some cell phones set their clock when you power
> them up, and others set it at regular time. Some automatically change
> time zone as you travel and some don't, maybe due to the same process.
> Didier KO4BB
I assume most members of this august group know that all CDMA cellphones
MUST know the correct time to within about 1 us or so in order to correctly
spread and despread the forward and reverse channel signals.
There are mechanisms (pilot signals) built into the signaling format
that allow a CDMA cellphone with cheap TCXO time base to acquire the necessary
frequency and then time lock when it is first turned on or first sees a signal.
But a CDMA cellphone locked up on a base-station (its normal
state) should know the CDMA system time with microsecond accuracy and also be
able to correct its TCXO time base and lock it to the system reference so it too
should be very accurate.
And essentially ALL CDMA systems lock system time and frequency to
GPSDOs at the cell sites (easiest way of keeping a whole network of them locked
together so handoffs work) and keep system time set to UTC.
So the only thing preventing the clock on a CDMA cellphone from being
essentially arbitrarily accurate is laziness or sloppiness in the GUI firmware
that handles the visible clock display - in engine room in the bowels of the
phone there is VERY accurate time of day.
> Glenn wrote:
> > _Most_ cell phones set their time to "network time." Usually within
> > one second. Although I have seen cell phones set themselves and be
> > off by nearly a minute.
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> time-nuts at febo.com
Dave Emery N1PRE, die at dieconsulting.com DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten 'For Rent'
sign still vainly flapping outside on the weed encrusted pole - in celebration
of what could have been, but wasn't and is not to be now either."
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