A pulse per second (PPS or 1PPS) is an electrical signal that has a width of less than one second and a sharply rising or abruptly falling edge that accurately repeats once per second.
PPS signals are output by radio beacons, frequency standards, other types of precision oscillators and some GPS receivers.
Atomic clocks usually have an external PPS output, although internally they may operate at 9,192,631,770 Hz.
It is common to set t0 at the voltage level of the steepest slope of a PPS signal.
Sites that describe how to use the PPS signal to set precise time on a PC: