Re: NTP, possible solutions, and best implementation

[Michael Shields <shields () msrl com>]

In message <003501c38940$34098fe0$eea28c45@wifey>,
"Robert M. Enger" <enger () seka erols net> wrote:
> An alternate time source could be the GLONASS system.
> Receivers do exist, but I have never used one.

Note that GLONASS satellites are failing frequently, and unlike GPS
satellites, are not always being replaced.  Currently only eight are
operational out of a desired constellation of 24.
http://www.glonass-center.ru/nagu.txt

GLONASS is also subject to many of the same failure modes as GPS --
antenna failure, jamming, &c.

A very robust design would fall back to a local frequency standard
rather than to another transmitted one.  You need to decide how good
that standard needs to be based on your requirements (or maybe you
just want to install cesium or rubidium so your marketing department
can say you have atomic clocks).

> The US  FAA  is transmitting WAAS correction signals.  Depending on
> the algorithms in the GPS receiver, this may result a reduction in PPS
> jitter.   (although any such jitter is probably swamped by the jitter 
> at the IP layer)

http://216.239.51.104/search?q=cache:AZ97VsY3w10J:www.cmcelectronics.ca/products-services/custom-elect/Enhancing_GPS_Timing_Engines_using_WAAS_signals.pdf+waas+timing&hl=en&ie=UTF-8
suggests that WAAS can reduce your PPS jitter from about 50 ns to
about 20 ns, which sounds plausible to me.  This is beyond the
level of accuracy that is needed or useful for NTP.

Might I suggest that comp.protocols.time.ntp would have better advice
on this than NANOG.  Even 10 ms accuracy is usually enough for network
operations.  Beyond that we're talking about operating a service, not
a network.
-- 
Shields.