A pulse per second (PPS or 1PPS) is an
electrical signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
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 standard
A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision
Accuracy and precision are two measures of ''obser ...
s, other types of precision
oscillators and some
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
receivers.
'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 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 clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betw ...
s usually have an external PPS output, although internally they may operate at 9,192,631,770 Hz.
Physical representation
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, 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 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 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
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...
-0 source, a common use for the PPS signal is to connect it to a PC using a low-latency, low-jitter
In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...
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
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.
A clock s ...
* Inter-Range Instrumentation Group The Inter-Range Instrumentation Group (IRIG) is the standards body of the Range Commanders Council (RCC). The group publishes standards through the RCC Secretariat at White Sands Missile Range.
The best known IRIG standard is the IRIG timecode u ...
(IRIG) time codes
* Precise Time and Time Interval (PTTI)
* Square wave
A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...
* Timecode
A timecode (alternatively, time code) is a sequence of numeric codes generated at regular intervals by a timing synchronization system. Timecode is used in video production, show control and other applications which require temporal coordinatio ...
References
{{Reflist
External links
Sites that describe how to use the PPS signal to set precise time on a PC:
* In OpenBSD
OpenBSD is a security-focused operating system, security-focused, free and open-source, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by fork (software development), forking N ...
4.
the nmea(4) line discipline
ca
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
PPS pulse and PPS pulse distribution
Measurement
Time