On 04/04/14 17:29, Saku Ytti wrote: > On (2014-04-03 21:25 -0700), Will Orton wrote: > >> There are commercially available NTP servers with GPS + Rb oscillators... >> for NTP >> use you could basically let it sync up a couple days, disconnect the GPS and >> let >> it freerun. You'd still be within a millisecond of GPS even after a couple >> years >> most likely. Reconnect it to GPS for a couple days every 1-2 years to resync >> it. >> More fun and cheaper to build your own I'd bet, if you had the time. > > Meinberg[0] pegs rubidium at ±8ms per year, if you need NTP to do say single > direction backbone SLA measurement you want to have microsecond precision.
Those two statements don't go together. Also outside the HFTers most of us don't care about a few milliseconds (sure an extra 50ms can be a pain, but is trivial to measure). It's one thing to be a time-nut (I'm certainly one), but recognise that straight NTP, well deployed, even syncing from the pool is sufficient for nearly all use cases not needing sub-millisecond precision. > I think most GPS chipsets today do Glonass also, maybe it's partly because > Russia has import sanctions for those who don't do, or maybe simply because it > gives better user experience as sync is found earlier. > > But is there NTP product which you can configure to GPS-only mode or > Glonass-only mode, which I think might be closer to Rob's idea of redundancy. > As if you use both, 'poisoned' source would break all of your kit. > But that is probably not easy to solve with two sources, if half is GPS and > half is Glonass, and Glonass starts sending bad timing information, what do > your NTP clients do, average to the middle? > > [0] http://www.meinbergglobal.com/english/specs/gpsopt.htm >
