[ntp:questions] Slow convergence of NTP with GPS/PPS
Richard B. Gilbert
rgilbert88 at comcast.net
Sat Oct 25 03:03:05 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.
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!
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