Pulse-per-second signal

A Pulse per second (PPS) is an electrical signal that very precisely indicates the start of a second. PPS signals are output by various types of precision clocks, including 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

GPS and other time receivers that provide a PPS signal:

• 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

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

Specific how-tos for Garmin GPS 18 LVC model:

• Using a Garmin GPS 18 LVC as NTP stratum-0 on Linux 2.6
• Adding a FreeBSD 5.4 NTP server based on a GPS 18 LVC device
• Stratum 1 NTP, Garmin GPS 18 LVC on FreeBSD 8.0