[time-nuts] Time syncing question

Dave Andersen dga+ at cs.cmu.edu
Thu Aug 31 11:29:49 UTC 2006


1)  They actually only need to be correct to within 10us, according to
the spec.

2)  Companies like endrun (www.endruntechnologies.com) make cute ($$$-y,
though) and symmetricom make little CDMA time receivers that will get
you ~10us accuracy indoors.  Nice toys to have in your arsenal if you
maintain time in machine rooms and don't like drilling through the ceiling.

  -Dave

Daun Yeagley wrote:
> Good explanation!
> 
> Daun
> N8ASB 
> 
> -----Original Message-----
> 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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