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

Martin Burnicki martin.burnicki at meinberg.de
Thu Nov 6 10:17:52 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.

If "Cold" means there are no satellite data stored in the receiver then this
can not be true unless some assumptions are made which may or may not be
correct.

The GPS timing frames are using GPS time, not UTC. 

GPS receivers can compute UTC from GPS time if they have received the UTC
parameter set from the satellites, which contains the current UTC offset in
seconds, plus a possible leap second announcement, plus some coefficients
for a polynomial required to compute small offsets in a
nanosecond/microsecond range.

This UTC data set is part of the satellite's navigational messages which is
repeated once every 12 minutes, so it may take up to 12 minutes after
powering up the GPS receiver in cold mode until those parameters are
available and thus the correct UTC time can be computed.

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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