NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?)

Michael.Dillon@radianz.com wrote:

I assume that it's fairly common for people to have Solaris or Linux boxes

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.

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