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

[Steve Francis <steve () expertcity com>]

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.)