Baldur Norddahl <baldur.norddahl@gmail.com>:
Ok how many hours or days of holdover can you expect from quartz, temperature compensated quartz or Rubidium?
Sorry, I don't have those drift figures handy. I'm a programmer, not a large-site sysadmin - I've never had purchase authority with a budget large enough to buy a rubidium-oscillator GPSDO or any other device with holdover. Better to ask Mel Beckman or someone else with field experience.
Should we calculate holdover as time until drift is more than 1 millisecond, 10 ms or more for NTP applications?
If you want to go super-accurate, 1ms. If you want to go cheap, on sampling-theory grounds I'd say you want to vary your drift threshold from 1 to 5ms (half the expected precision of WAN time, think of it as the Nyquist rate) and look for a knee in the cost curve.
I am thinking that many available datacenter locations will have poor GPS signal so we can expect signal loss to be common. Some weather patterns might even cause extended GPS signal loss.
Weather won't do it, usually. Rain and fog and clouds are transparent to GPS frequencies. Standing water directly on an antenna can cause some attenuation, but with any serious GPS engine made more recently than 5 years ago I would be extremely surprised if that lost it lock. The newer ones handle down to 30 feet in ocean water on robot submarines. -- <a href="http://www.catb.org/~esr/">Eric S. Raymond</a>