[ntp:questions] Slow convergence of NTP with GPS/PPS

Unruh unruh-spam at physics.ubc.ca
Fri Oct 24 17:30:13 UTC 2008

nb at komeda-berlin.de (Nicola Berndt) writes:

>Richard B. Gilbert schrieb:
>> David Woolley wrote:
>>> Richard B. Gilbert wrote:
>>>> To turn your equipment on after months of downtime and expect it to 
>>>> lock on to the correct time with millisecond accuracy within seconds 
>>>> is asking for a hell of a lot.
>>> Not really.  He's starting a GPS receiver at the same time and that has 
>>> to lock to 50ns.
>>> Doing it on a general purpose computer is more difficult, but not 
>>> particularly impossible.
>> Even with GPS and a full four satellite fix, ten seconds to synchronize 
>> is extremely ambitious!!  You can set the time to within whatever 
>> precision the hardware and software support but that is only half the 
>> problem.  You also need to set the correct clock frequency.  On a cold 
>> start, the clock frequency is a moving target as the hardware warms up.
>> I would expect to wait at least thirty minutes for the system to 
>> stabilize with both the correct phase (time) and frequency.

>To transfer the full almanac of GPS it takes roughly 12 minutes from a 
>cold start. Then the receiver knows everything there is for it to know. 
>Some receivers (like mine) you can tell it's location, wich gets you in 
>the 10 s range for precise time. Then again, who claimed, it has to be 
>10 s? I would be very happy with these 12 mins..

For some receivers if they know their position, they can get the time
virutally instantly from "cold start". All you need is one sattelite. If the receiver has no idea where it is, it can take much longer. 
 Whether or not the receiver the OP has has that
capability I do not know. 

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