Spanner provides the precise coordination and control of the vast network of computers and data centers that make up the global internet. (Google networking engineers named it “Spanner” because it enabled them to build a real-time synchronized computer network that “spans” the entire globe.)
The Spanner hardware relies on GPS Antennas and Atomic Clocks.
If you take a look at the data center server node below, the server node has 4 time servers:
2 of the time servers are connected to GPS Antennas to receive the globally-synchronized Atomic Time Signals from the GPS satellites; and
The 2 other (backup) time servers receive their signals from the 2 on-board Atomic Oscillators.
This article covers most of the internal concepts of Spanner. You can also refer the Google Cloud Next event videos linked below.