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

David J Taylor david-taylor at blueyonder.neither-this-part.nor-this-bit.co.uk
Fri Oct 24 08:57:53 UTC 2008

Hal Murray wrote:
>> It isn't NTP which is the limit, but the GPS receiver acquiring lock
>> from scratch after an indeterminate period of being switched off.
>> The GPS could take minutes to lock and declare that it has valid
>> time.
> From the spec sheet for the Garmin GPS 18x:
>  Reacquisition: Less than 2 seconds
>  Hot:           Approx. 1 second (all data known)
>  Warm:          Approx. 38 seconds (initial position, time, and
>              almanac known; ephemeris unknown)
>  Cold:          Approx. 45 seconds
> I believe that's typical of modern GPS receivers.

Hal, thanks for those figures.

My own experience suggests that, at least with a hand-held GPS, it can be 
a lot longer than 45 seconds.  That figure may only apply if the GPS has a 
180-degree clear view of the sky.  But the GPS18 LVC does usually recover 
quickly enough (mine has a less than 180-degree sky FoV).

If we accept 45s, is that shorter than the typical system boot time from 
cold?  Could the system boot be delayed enough so that the GPS was 
guaranteed to be ready by the time it was needed?


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