[time-nuts] Noob question, NTP stratum 1.

[Paul Theodoropoulos]

On 7/22/19 09:46, Achim Gratz wrote:
> All rasPi I've bought so far (5 of them, all different models from three
> different sources) were ~10ppm slow at RT and within -6ppm and 0ppm at
> the apex temperature point of the crystal (around 60°C).  It is possible
> to keep the temperature within about 0.2K of that point by using the CPU
> itself as a heater and thermometer and the resulting frequency within
> +-20ppb over a day when disciplined via pps-gpio.  The residual drift is
> initially positive and gets smaller over time, mine are down to about
> 20~30ppb per week.
I've also found that my Raspi's give the best results with board temp 
right around 60°C. However, I found that the 'CPU as heater', while it 
does help dramatically compared to nothing at all, it introduces a lot 
of jitter on its own. Much better results by keeping them in an 
insulated enclosure in a relatively stable location (for me, a closet!), 
and 'tuning' the insulation as needed to balance venting against retention.
> I've never tried to hold over since it's really hard to figure out when
> to start holding and when to switch back.  A 24 hour holdover is pretty
> severe, but I guess you could keep it within double-digit ms territory
> without getting too fancy if you've got that right.
>
> Instead, by having enough independent NTP servers (all with their own
> antenna) the clients figure out by themselves when one is off by more
> than about a millisecond and switch to another one.  Each server
> monitors the other ones and if it finds itself off from that bunch too
> much will drop down from stratum-1 to stratum-2.  When exactly that
> happens depends a bit on how fast the drift rate is, but usually between
> 3ms and 5ms.
Something that just occurred to me, and I'm curious if I'm off in the 
weeds here - while a GPS failure could take many forms, I wonder if 
there's a good argument to be made to having at least one or two peers 
who are geographically quite *far* from one's location - e.g. the other 
side of the world - to at least ensure that some number of satellites 
might be available when others might not. Thinking in terms of a 
Carrington event scenario. Wouldn't the GNSS on the far side of the 
earth possibly fare better in such an event - all other things being 
equal? I guess it would depend on how long it took for the mass ejection 
to pass the earth.

Yeah, I think I'm off in the weeds.
> While I support that sentiment, after initial problems with off-brand
> cards the ones I have now are all getting into their third year with no
> signs of problems (knocks on wood).  In a datacenter you'd almost
> certainly net boot the rasPi instead of giving them a microSD card.
>
I can echo that sentiment re SD cards. I'm running Samsung EVO Plus and 
Sandisk Ultra cards in my devices, never a problem. Considering that the 
'sweet spot' for pricing on SD cards is now the 32gb ones, it provides a 
great buffer for the controller to handle internal block allocation for 
longer life.

-- 
Paul Theodoropoulos
www.anastrophe.com