Terrestrial Time — TT(BIPM)
The annual realization of Terrestrial Time for scientific applications requiring the highest levels of frequency accuracy and long-term stability.
Understanding TT(BIPM)
A realization of Terrestrial Time for precision science
Terrestrial Time (TT) is the time scale defined for scientific applications at or near the Earth. International Atomic Time (TAI), computed by the BIPM, provides a realization of TT based on atomic clocks and frequency standards.
TT(BIPM) is an independent realization of TT calculated retrospectively by the BIPM. This allows all available evaluations of the frequency of TAI from primary and secondary frequency standards to be taken into account, including evaluations that become available after the corresponding period.
The BIPM publishes an updated realization each year for scientific applications requiring particularly accurate and stable time and frequency over long periods.
The key distinction
Calculated retrospectively using more complete data
Unlike UTC and Rapid UTC, which are calculated with a relatively short delay using the data available at the time, TT(BIPM) is calculated retrospectively, when more complete frequency-standard evaluations are available.
This makes TT(BIPM) particularly valuable for scientific measurements and observations extending over long periods.
The annual realization
From atomic time to TT(BIPM)
Each annual realization draws on evaluations of the frequency of TAI to provide a highly accurate reference for long-term scientific applications.
TAI
International Atomic Time provides the atomic time-scale information used in the calculation.
Frequency evaluations
Primary and secondary frequency standards provide evaluations of the frequency of TAI.
BIPM calculation
The BIPM uses the available evaluations to calculate an updated realization of Terrestrial Time.
TT(BIPM)
The updated realization is published annually for use in precision science.
Precision science
Where TT(BIPM) is used
TT(BIPM) provides a stable reference for comparing precision measurements and observations across extended periods.
Pulsar timing
Observations extending over many years require an exceptionally stable reference so that very small effects can be identified and compared.
Long-baseline interferometry
Precision observations made at widely separated locations depend on stable timing over extended periods.
Deep-space navigation
Long-duration missions and precise navigation calculations benefit from a highly stable reference over long time intervals.
Fundamental physics
Precision tests may depend on detecting very small effects that accumulate or become measurable over long periods.
TT(BIPM) results and documentation
Access the published annual realizations and supporting technical information.
TT(BIPM) results
Access the annual realizations of Terrestrial Time published by the BIPM.
Access TT(BIPM) results →Technical information
Learn more about the data, methods and frequency evaluations used to calculate TT(BIPM).
Explore technical information →Frequently asked questions
Find quick answers about Terrestrial Time and the annual TT(BIPM) realization.
What is International Atomic Time (TAI), and how does it relate to TT(BIPM)?
International Atomic Time (TAI) is a continuous time scale produced by the BIPM based on atomic clock data and evaluations from primary and secondary frequency standards. TT(BIPM) is an independent realization of Terrestrial Time computed retrospectively by the BIPM using available evaluations of the frequency of TAI.
Does TT(BIPM) replace UTC?
No. UTC remains the international reference time scale supporting synchronization and civil timekeeping. TT(BIPM) serves a different purpose and is principally intended for scientific applications.
Why is TT(BIPM) computed retrospectively?
Retrospective computation allows the BIPM to take into account all available evaluations of the frequency of TAI, including evaluations that become available after the corresponding period. This provides a particularly accurate and stable reference for scientific applications requiring measurements to be compared over long periods.
Who uses TT(BIPM)?
TT(BIPM) is principally used for scientific applications requiring long-term frequency stability and high frequency accuracy, including pulsar timing and other demanding long-term studies.
How often is TT(BIPM) updated?
The BIPM computes and publishes an updated realization annually.
Are leap seconds applied to TT(BIPM)?
No. Leap seconds are adjustments made to UTC to maintain its relationship with time based on the Earth’s rotation. TT(BIPM) is continuous and is not adjusted by leap seconds.
What are primary and secondary frequency standards?
They are highly accurate atomic frequency standards used to realize and evaluate the SI second.
Explore the BIPM Time Department
Learn more about UTC, Rapid UTC, future developments in timekeeping and the wider work of the BIPM Time Department.
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