Pulse per second
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A pulse per second (PPS) 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 (see definition of second). 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.
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
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