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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. Precision clocks are sometimes manufactured by interfacing a PPS signal generator to processing equipment that aligns the PPS signal to the UTC second and converts it to a useful display. Atomic clocks usually have an external PPS output, although internally they may operate at 9,192,631,770 Hz. PPS signals have an accuracy ranging from a 12 picoseconds to a few microseconds per second, or 2.0 nanoseconds to a few milliseconds per day based on the resolution and accuracy of the device generating the signal.
PPS signals are usually generated as a TTL signal capable of driving a 1 kilohm load. Some sources use line drivers in order to be capable of driving 50 ohm transmission lines. Because of the broad frequency contents, a long transmission line can have significant impact on the shape of the PPS signal due to dispersion and after delivery effects of the dielectric of the transmission line. It is common to set t0 at the voltage level of the steepest slope of a PPS signal. PPS signals are therefore notoriously unreliable when time transfer accuracies better than a ns are needed, although the stability of a PPS signal can reach into the ps regime depending on the generating device.
PPS signals are used for precise timekeeping and time measurement. One increasingly common use is in computer timekeeping, including the NTP protocol. Because GPS is considered a stratum-0 source, a common use for the PPS signal is to connect it to a PC using a low-latency, low-jitter wire connection and allow a program to synchronize to it. This makes the PC a stratum-1 time source. Note that because the PPS signal does not specify the time, but merely the start of a second, one must combine the PPS functionality with another time source that provides the full date and time in order to ascertain the time both accurately and precisely.
Sites that describe how to use the PPS signal to set precise time on a PC:
- In OpenBSD 4.1 the nmea(4) line discipline can attach to a GPS timer and optionally use the PPS signal for low jitter and high accuracy in system time and NTP time
- gpsd — a GPS service daemon, required to activate PPS signal on some devices (works in conjunction with OpenBSD's nmea line discipline if installed via ports tree and a stand-alone service daemon for other operating systems)
- RFC 2783 Pulse-Per-Second API for UNIX-like Operating Systems, Version 1.0
-  M. Siccardi, About time measurements, EFTF 2012