NTp sources that work in a datacenter (was Re: Is latency equivalent

>in every PoP to do measurements. In that case, the difficulty isn't in
>measuring one-way latency, it's in synchronizing the time on all the
>servers. And with fairly cheap GPS and CDMA clocks that is a lot
>easier/cheaper than it once was.

  a robust mesh of strat-2 chimers gives one more resilence
  and more accuracy than syncing off a single source.

But what GPS clock can you install in a datacenter? AFAIK, they all
require roof (or at least window) access in order to install the
antenna. (At least, all the GPS based ntp servers I've looked at do).
Is that not true of CDMA servers?

  some GPS, some PPS, and an atomic source here and there
  give great diversity and only a few need roof access.

How have others solved this issue? (Short of owning their datacenters.)

  Use NTP, run most systems as strat-2