Pulse per second

A pulse per second (PPS) is an electrical signal that very accurately repeats once per second (i.e. with a frequency of 1 Hertz). PPS signals are output by various types of precision clocks, including atomic clocks, radio clocks and some models of GPS receivers. Depending on the source, properly operating PPS signals have an accuracy ranging from a few nanoseconds to a few milliseconds.

Uses

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.

See also

• Precise Time and Time Interval (PTTI)

External links

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