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

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Oct 30 17:49:22 UTC 2012

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.

I'm hoping that it wouldn't take NTP too long to fix the clock.  I see 
the situation as someone being out with some portable kit - how long 
should they wait before assuming the time is correct?

It strikes me that a receiver with no battery - like mine - is a poor 
choice for out portable!  I have a similar receiver with battery backup 
on order, so I hope it's better  You know where: 251164946899.
Web: http://www.satsignal.eu

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