Pulse per second
Encyclopedia
A pulse per second 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 clock
s, radio clock
s 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.
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:
Specific how-tos for Garmin GPS 18 LVC model:
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
repeats once per second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
(i.e. with a frequency of 1 Hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
). PPS signals are output by various types of precision clocks, including atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
s, radio clock
Radio clock
A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...
s 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 NTPNetwork Time Protocol
The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
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
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
wire connection
Electrical connection
An electrical connection between discrete points allows the flow of electrons . A pair of connections is needed for a circuit.Between points with a low voltage difference, direct current can be controlled by a switch...
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.
External links
GPS and other time receivers that provide a PPS signal:- Arbiter Systems, Inc. GPS clocks 40ns to 500ns accurate PPS outputs
- Trimble Precise Timing GPS products, Offered 5-Hz formats typically within 25 nanoseconds.
- Garmin GPS 18, LVC (barewire) edition only. Offered in 1-Hz and 5-Hz formats.
- GPS 18x LVC and GPS 18x 5 Hz, a high-sensivity replacement for deprecated GPS 18 models
- Meinberg GPS and DCF77 Receivers, dedicated GPS and DCF77 time receivers
- hopf Elektronik GPS and DCF77 Receivers, dedicated GPS and DCF77 time receivers
Sites that describe how to use the PPS signal to set precise time on a PC:
- In OpenBSDOpenBSDOpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley. It was forked from NetBSD by project leader Theo de Raadt in late 1995...
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
Specific how-tos for Garmin GPS 18 LVC model: