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 hav

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.com/en//pubs/archive/39966.pdf https://static.googleusercontent.com/media/research.google.c...