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

Unruh unruh-spam at physics.ubc.ca
Sat Oct 25 06:02:32 UTC 2008


"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>Unruh wrote:
>> 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. 
>> 
>> 

>There is a subtle difference between *getting* the correct time and 
>*keeping* the correct time!  A GPS receiver can generate a Pulse Per 
>Second (PPS) signal accurate to within about 50 nanoseconds.  This does 
>not mean that you can get the same accuracy out of your computer!

Of course not ( and the GPS18 only gives 1us accuracy anyway) but the OP
wanted accuracy to 1ms, which is trivial for both the computer and the gps.




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