{{Short description|Class of electrical signals}} {{distinguish-redirect|Pulse per second|Pulse repetition frequency}} {{refimprove|date=March 2016}} 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.<ref>[http://www.eftf.org/proceedings/proceedingsEFTF2012.pdf] {{Webarchive|url=https://web.archive.org/web/20150923234240/http://www.eftf.org/proceedings/proceedingsEFTF2012.pdf|date=2015-09-23}} M. Siccardi, ''About time measurements'', EFTF 2012</ref><ref>[https://navcen.uscg.gov/sites/default/files/pdf/gps/ICD-GPS-060B.pdf]''ICD-GPS-060B: GPS User Equipment (Phase III) Interface Control Document for the Pecise Time and Time Interval (PTTI) Interface'', Global Positioning System Joint Program Office, 2002</ref> 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.<ref name='Atomic clock internal frequency'>{{cite web|url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf|title=SI Units 2019 Brochure|author=BIPM|author-link=International Bureau of Weights and Measures|date=September 4, 2019|access-date=September 13, 2019|edition=9th}}</ref> PPS signals have an accuracy ranging from 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.
== Physical representation == PPS signals are usually generated as a TTL signal capable of driving a 1-kiloohm load. Some sources use line drivers in order to be capable of driving 50-ohm transmission lines. Because of the broad frequency contents, along the transmission line can have a 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 t<sub>0</sub> 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 nanosecond are needed, although the stability of a PPS signal can reach into the picosecond regime depending on the generating device.
== Uses == PPS signals are used for precise timekeeping and time measurement. One increasingly common use is in computer timekeeping, including NTP. 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== * Clock signal * Inter-Range Instrumentation Group (IRIG) time codes * Precise Time and Time Interval (PTTI) * Square wave * Timecode
==References== {{Reflist}}
==External links== * [http://pos.mgb-tech.com/insightpps/ PPS pulse and PPS pulse distribution] * [https://www.arbiter.com/files/product-attachments/pd0043200_timing_signals_overview.pdf Timing Signals, IRIG, 1PPS] * [https://ntrs.nasa.gov/api/citations/20190028909/downloads/20190028909.pdf Using GPS Receiver 1PPS Output to Verify Time Stamp Accuracy and Measure Propagation Delay]
Sites that describe how to use the PPS signal to set precise time on a PC: * In OpenBSD 4.1 [https://man.openbsd.org/nmea.4 the nmea(4) line discipline] can [https://man.openbsd.org/ldattach.8 attach to a GPS timer] and optionally use the PPS signal for low jitter and high accuracy in system time and NTP time * [https://web.archive.org/web/20080212004332/http://gpsd.berlios.de/ 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) * [https://datatracker.ietf.org/doc/html/rfc2783 RFC 2783 Pulse-Per-Second API for UNIX-like Operating Systems, Version 1.0]
Category:Measurement Category:Time