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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Oct 24 21:50:21 UTC 2008

Nicola Berndt wrote:

> 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. 

It needs more like 6 hours for that, as it can only get the detailed 
ephemeris when a satellite is above the horizon.  Of course, it doesn't 
really need to know the details of satellites until they actually come 
into view.

> 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..

You will need at least 30 seconds for full accuracy, for a warm start, 
but I can imagine that you could be well within 10 microseconds in 10 
seconds, if your almanac wasn't too out of date.

(Incidentally, it is possible to preload the current almanac data.)

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