[ntp:questions] What is the NTP recovery time from 16s step in GPS server?

Rob nomail at example.com
Tue Oct 30 17:59:46 UTC 2012

David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> I have discovered that on a cold start my Resolution SMT GPS receiver 
> outputs the time in GPS time, until it has downloaded enough information 
> to determine the GSP offset, when it switches to UTC.  This particular 
> receiver has no battery backup (it's an evaluation board).
> I'm feeding this into gpsd, from which NTP gets its coarse seconds. 
> This is no problem when I have other Internet or LAN servers telling NTP 
> what the time actually is - NTP isn't fooled and sticks with UTC.
> However, suppose I had /only/ the GPS receiver?  NTP has the GPS time 
> and the PPS signal for the exact second, syncs, and then a few minutes 
> in the time suddenly changes by 16 seconds.  I would hope that then 
> causes NTP to step the clock onto UTC rather than GPS time.  I realise 
> that the time for the GPS receiver to be sending UTC rather than GPS 
> time varies, but how long might it then take NTP to react to the 16 
> second change, and alter the system clock?  Both the PPS and the gpsd 
> are being polled at fixed 16 second intervals.

When I wrote that part of gpsd, I implemented the proper checks so
that the time is only put in the SHM after the GPS receiver has
indicated that it has a valid fix on the satellites.

I would hope that the receiver has collected the GPS offset by that
time, so that it will not put the wrong time value in SHM during a
short interval.

However, it has been a couple of years and many changes have been
made to the code, I cannot tell you if it may have been broken.

Do you really see the problem you describe above, or is it only a
hypothesis of what might happen?

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