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

Nicola Berndt nb at komeda-berlin.de
Fri Oct 24 20:11:29 UTC 2008

Unruh schrieb:
> 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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U-Blox state on page 24 of this - 
- document a rate of 50 bits/second sent out by the satellites and a 
time of 12.5 mins to transmit the full almanach. I don't know, if really 
the entire almanac is needed for precise time, but I'd actually suspect 


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