Time synchronisation glossary 125 terms
The key terms from the world of time and time synchronisation, clearly defined. From NTP and PTP to GNSS, holdover and oscillators. Every term has its own page.
1
A
- Accuracy vs precision Accuracy = how close to the true time; precision = how reproducible the measurement is.
- Allan deviationADEV The standard measure for the frequency stability of an oscillator across different time scales.
- Allan variance The mathematical basis used to quantify the frequency stability of oscillators.
- Atomic clock A clock that derives time from the vibration of atoms, extremely accurate and stable.
B
C
D
- Daylight saving time Setting the clock one hour forward in the summer half-year (DST).
- DCF77 German long-wave time signal (77.5 kHz) that can be received across large parts of Europe.
- Dilution of precisionDOP A measure of how the satellite geometry affects accuracy.
- Disciplining Continuously adjusting a local oscillator to an external reference such as GNSS.
- Distribution amplifier Device that splits one time or frequency signal into many outputs without loss of quality.
- Drift The gradual deviation of a clock caused by the imperfection of its oscillator.
- DUT1DUT1 The difference between astronomical time UT1 and atomic UTC.
E
- eLoran The modernised, digital successor to LORAN-C, as a robust backup for GNSS.
- Ephemeris The precise orbital data a satellite broadcasts about its own position.
- Epoch The chosen zero point of a time scale from which time is counted.
- Equinox The moment when day and night are equally long everywhere, twice a year.
F
- Fallback server A backup time source that takes over when the primary time server fails.
- FINRAFINRA US securities regulation prescribing clock synchronisation for trading.
- Frequency The number of cycles per second of a signal, expressed in hertz (Hz).
- Frequency standard A source of an extremely accurate and stable reference frequency.
G
- Galileo The European satellite navigation system, under civilian control.
- Genlock Synchronising multiple video sources to one common timing signal.
- GLONASS The Russian satellite navigation system, one of the four GNSS constellations.
- GMTGMT Greenwich Mean Time, the historical world time on the prime meridian.
- GNSS Collective name for satellite navigation systems: GPS, GLONASS, Galileo and BeiDou.
- GNSS antenna The antenna that receives the satellite signals; the most underrated part of a time server.
- GPS American satellite navigation system that also provides a highly accurate time reference.
- GPS time The continuous time scale of the GPS system, without leap seconds.
- GPS week rollover The overflow of the GPS week counter, which can only count a limited number of weeks.
- GPSDOGPSDO A GNSS-disciplined oscillator: combines satellite time with a stable local oscillator.
- Grandmaster The master clock that provides the reference time for all other clocks in a PTP network.
- Gravitational redshift Clocks higher in a gravitational field run faster: time depends on altitude.
- Gregorian calendar The solar calendar in worldwide civil use, introduced in 1582.
H
I
- IEC 61850IEC 61850 The international standard for communication and time synchronisation in power substations.
- IEEE 1344IEEE 1344 An extension of IRIG-B with year, time-zone and leap-second information.
- International Date Line The imaginary line near 180° longitude where the calendar date jumps by one day.
- Ionospheric delay The signal delay in the ionosphere, a major error source in GNSS.
- IRIG-B Serial time code that distributes the time-of-day over a single signal, widely used in industry and defence.
- ISO 8601ISO 8601 The international standard for writing date and time (e.g. 2026-07-06T14:30:00+02:00).
J
L
- Latency The delay between sending and receiving a signal or message.
- Leap second An extra second occasionally added to UTC to keep pace with the Earth's rotation.
- Leap year A year with an extra day (29 February) to keep the calendar aligned with the solar year.
- LORAN-CLORAN-C A ground-based long-wave navigation and timing system, precursor and backup to GPS.
- LTC SMPTE timecode as an audio signal, recorded on a sound track.
M
- Marine chronometer The accurate ship's clock that let sailors determine longitude from the 18th century.
- Master clock Central clock that determines and distributes the time for an entire system or building.
- Measurement uncertainty The quantitative margin within which the true value of a measurement lies.
- MIDI timecodeMTC A variant of SMPTE timecode for synchronising music and audio equipment.
- MiFID IIMiFID II European financial regulation requiring traceable timestamping of transactions.
- MTIEMTIE Maximum time interval error: the largest time error a clock builds up within a window.
- Multicast and unicast Two ways in which PTP or NTP messages are sent across the network.
- Multipath Reception errors because a satellite signal reaches the antenna via reflections along several paths.
N
- NMEA 0183 Serial data standard in which GPS receivers output position and time as text sentences.
- NTP The most widely used protocol for synchronising clocks over an IP network.
- NTP pool A worldwide group of volunteer time servers providing public NTP time.
- NTSNTS Secured NTP: authentication against forgery of time servers.
O
- OCXO Crystal oscillator in a heated enclosure at constant temperature, for high stability.
- Offset The difference between a clock's time and that of the reference at a given moment.
- Optical clock An atomic clock based on an optical rather than a microwave transition, the most accurate clock there is.
- Ordinary clock A PTP device with a single port: either the grandmaster or an end clock (slave).
- Oscillator The vibrating element that provides the base frequency from which a clock derives time.
P
- Pendulum The first accurate time regulator: a swinging motion with a fixed period, basis of the pendulum clock.
- Phase The position within a periodic cycle; the phase difference determines whether two signals run in sync.
- Phase noise The random, rapid fluctuation of an oscillator signal's phase.
- PLL Control loop that keeps an oscillator locked in fixed phase with a reference signal.
- PoE Power and data over a single network cable, handy for placing clocks without a separate supply.
- ppm and ppb Units for expressing the frequency deviation of a clock (parts per million or billion).
- Prime meridian The 0° longitude reference line through Greenwich, from which time and longitude are reckoned.
- Pseudorange The measured distance to a satellite, biased by the receiver's clock error.
- PTPIEEE 1588 Network protocol for time synchronisation with sub-microsecond accuracy.
- PTP Power ProfileIEC 61850-9-3 PTP profile for time synchronisation in power grids (smart grid).
- Pulsar time Using the extremely regular pulses of neutron stars as a natural clock.
Q
R
- Radio-controlled time Clocks that set themselves automatically to a long-wave time signal from an atomic clock.
- Relativity and time Einstein's theory that explains why time depends on speed and gravity.
- Rubidium oscillator Compact atomic oscillator that uses rubidium for top stability and very long holdover.
S
- Sagnac effect A time difference that arises when a signal travels over a rotating system such as the Earth.
- SBASSBAS Satellite augmentation that makes GNSS signals more accurate and reliable.
- Sidereal time Time measured by the Earth's rotation relative to the stars.
- SMPTE timecode Time code for labelling audio and video frames in broadcast and film.
- SNTP A simplified variant of NTP without the full filtering and error correction.
- Solar time Time determined by the position of the Sun; the basis of civil time.
- Solstice The moment of the longest or shortest day, when the Sun reaches its highest or lowest point.
- Spoofing Broadcasting fake GNSS signals to make a receiver accept a wrong time or position.
- Stratum The level of an NTP clock in the synchronisation hierarchy, counted from the reference source.
- Sundial The oldest time instrument: the shadow of a rod shows the solar time of day.
- Synchronous EthernetSyncE Frequency synchronisation via the physical layer of Ethernet, separate from the data clock.
- Synchrophasor A time-stamped measurement of voltage and current in the power grid.
- Syntonization Aligning the frequency (not the time) of two clocks.
T
- TAI Continuous atomic time scale without leap seconds, the basis underneath UTC.
- TCXO Crystal oscillator with temperature compensation, more stable than a plain quartz clock.
- TDEVTDEV Time deviation: a measure of a clock's time stability over an observation window.
- Terrestrial TimeTT An ideal, uniform time scale for astronomical calculations, tied to TAI.
- The second The SI base unit of time, defined via a transition of the caesium-133 atom.
- Time dilation The effect that clocks run slower due to speed or gravity, per relativity theory.
- Time zone A region that uses the same civil time, expressed as an offset from UTC.
- Timestamping Recording the exact moment a packet departs or arrives; hardware is more accurate than software.
- Traceability The demonstrable chain linking a time measurement to the national or international standard.
- Transparent clockTC PTP switch that measures its own transit delay and passes it on instead of resynchronising.
- Trilateration Determining position and time by measuring distances to several satellites.
- TWSTTTWSTT Two-way satellite time transfer: two labs exchange time signals for top accuracy.
U
- Unix time The number of seconds since 1 January 1970, the standard time representation in computers.
- UT1UT1 The time scale that follows the actual rotation of the Earth.
- UTC The worldwide time standard that clocks and time zones use as their reference.
- UTC offset The fixed difference in hours and minutes between a local time and UTC.
V
W
- Wander Slow variation of a clock signal over longer periods, by definition below 10 Hz.
- Water clock An ancient time instrument that measures time by a steady flow of water.
- White Rabbit An extension of PTP for sub-nanosecond synchronisation over optical fibre.
- WWVBWWVB The American long-wave time signal (60 kHz) that radio-controlled clocks synchronise to.
No term found.