From the Spanner paper:

> The underlying time references used by TrueTime are GPS and atomic clocks. TrueTime uses two forms of time reference because they have different failure modes... TrueTime is implemented by a set of time master machines per datacenter and a timeslave daemon per machine. The majority of masters have GPS receivers with dedicated antennas; these masters are separated physically to reduce the effects of [GPS] antenna failures, radio interference, and spoofing. The remaining masters (which we refer to as Armageddon masters) are equipped with atomic clocks. An atomic clock is not that expensive: the cost of an Armageddon master is of the same order as that of a GPS master.

Source: https://static.googleusercontent.com/media/research.google.c...

Here is a good article: http://www.bluetreble.com/2015/10/time-travel/

Here's a very rough (and non-practical due to the large time error) implementation of TrueTime that I wrote a while back that uses the regular linux apis:

https://stackoverflow.com/questions/18384883/why-is-googles-...

I suppose if you spent some serious effort with ntpd (maybe hook it up to a PCI ClockCard), you could get this approach to work.