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

Nicola Berndt nb at komeda-berlin.de
Fri Oct 24 08:25:52 UTC 2008


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..



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