[time-nuts] distirbuted sync

[Chris Albertson]

I don't think those requirements are hard.  You can build a system
that works in three cases
1) GPS is available full time
2)  GPS is available intermittently.
3) there is not GPS system, world war III has destroyed it.

I think what you want is a system that is failure tolerant and can
make use of the best available source of timing and degrade
performance gracefully.  And you need this is be automatic with the
only control maybe being a status LED that shows free/yellow/red

Each system has a GPS receiver that disciplines a crystal oscillator.
This oscillator is used for timing.  I think it's clear that this
handles cases #1 and #2.

Then use you Blue Tooth or whatever other short distance
communications system you have to support an IP network.  TCP/IP over
zBlueTooth works well and is a standard now.  Using this you can
configure a NTP based network of "peers".  Each of the above systems,
when they are close enough will share timing with the other peers.
The system runs on a "consensus time".  If one or more systems has
access to GPS those system will supply timing to  any other system in
range of the blue tooth.   If there is no GPS at all the systems will
form what they call an "orphan network" and will remain synced to each
other untill some outside source of time connects and puts them all
back on GPS time.   NTP is pretty good at handing the case where
timing sources come on and off line and where network connects connect
and then go away.  It is very failure tolerant.

What you'd have is a kind os graceful degradation.  When GPS is
visible to all units they are all "dead-on" and running well above you
specs.  If GPS is hidden (perhaps in an urban canyon or you happen to
be inside a tunnel) the systems ail remain in spec for many hours or
even days depending on how much money you spent on the crystal  (or
Rubidium) oscillators

Finally if there is no GPS at all but several systems are within blue
tooth range that can sync to each other at the few millisecond level.
but because you did spend $$ on a god crystal will stay sync'd for
hours even when out of bluetooth range with no GPS.

The good thing is that you only need to integrate existing technology
to make this happen. The  software, hardware and all the parts are
available.   You'd not have to pay to advance the state of the art.

You would have to balance things like crystal oscillator stability vs.
power.  Ovenized units suck up power.  TCOXs use less power but maybe
hours and not days of hold over time is enough.

The LED's color depends on the estimated timing error, NTP is good at
computing that based on if GPS is connected and working, the network
status and so on.  So it might be green for microsecond level, yellow
for millisecond level error and red for "few seconds" level, flashing
for no-sync

On Tue, Jul 23, 2013 at 7:55 AM, Jim Lux <jimlux at earthlink.net> wrote:

> It *is* an interesting problem.. It's sort of weird, though, as I write the
> requirements..
>
> High frequency accuracy (1E-10, 1E-11) ideally.. or high stabiity over 1-100
> seconds, with a way to get "knowledge".
>
> But relatively low timing accuracy: 1-3 milliseconds over the same 100
> second interval (1E-5)
>
> Often you have a time requirement that is commensurate with the frequency
> requirement.
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-- 

Chris Albertson
Redondo Beach, California