International Atomic Time (
TAI, from the
French name
Temps Atomique
International) is a high-precision atomic
coordinate time
standard based on the notional passage of
proper time on
Earth's
geoid. It is the principal realisation of
Terrestrial Time, and the basis for
Coordinated Universal
Time (UTC) which is used for civil timekeeping all over the
Earth's surface. , TAI was exactly 34 seconds ahead of UTC: an
initial difference of 10 seconds at the start of 1972, plus 24
leap seconds in UTC since 1972; the last
leap second was added on December 31, 2008.
Time coordinates on the TAI scales are conventionally specified
using traditional means of specifying days, carried over from
non-uniform time standards based on the rotation of the Earth.
Specifically, both
Julian Dates and the
Gregorian calendar are used. TAI
in this form was synchronised with
Universal Time at the beginning of 1958, and
the two have drifted apart ever since, due to the changing motion
of the Earth.
Operation
TAI as a
frequency standard is a weighted
average of the time kept by about 300
atomic clocks in over 50 national laboratories
worldwide. The clocks are compared using satellites. Many of these
are
caesium atomic clocks, which are the
standard by which the
SI second is defined. Due to the averaging it is far
more stable than any clock would be alone.
The participating institutions each broadcast, in real time (in the
present), a frequency signal with
time codes, which is their estimate of TAI. Time
codes are usually published in the form of UTC. The better
laboratories' signals are mutually synchronised to within less than
10
^{−7} s, but there are outliers up to
10
^{−5} s out.
These time scales are denoted in the form
TAI(NPL) (UTC(NPL) for the UTC form), where
NPL in this case identifies the National
Physical Laboratory, UK. Some laboratories also publish their own
atomic time scale, denoted in the form TA(USNO)
(USNO identifies the United States
Naval Observatory).
The clocks at different institutions are regularly compared against
each other. The
International
Bureau of Weights and Measures (BIPM) combines these
measurements to retrospectively calculate the weighted average that
forms the most stable time scale possible. This combined time scale
is published monthly in
Circular T, and is the
canonical TAI. This time scale is expressed in the
form of tables of differences UTC-UTC(
x) and
TAI-TA(
x), for each participating institution
x.
Errors in publication may be corrected by issuing a revision of the
faulty Circular T or by errata in a subsequent Circular T. Aside
from this, once published in Circular T the TAI scale is not
revised. In hindsight it is possible to discover errors in TAI, and
to make better estimates of the true proper time scale. Doing so
does not create another version of TAI; it is instead considered to
be creating a better realisation of
Terrestrial Time (TT).
History
Atomic timekeeping services started experimentally in 1955, using
the first caesium atomic clock at the National Physical Laboratory,
UK (NPL). The first formalised atomic time scale was the
A.1 scale defined by USNO in 1959. A.1 was defined by an
epoch at the beginning of
1958: it was set to read Julian Date 2436204.5
(1958-01-01T00:00:00) at the
UT2 instant JD
2436204.5 (1958-01-01T00:00:00) as calculated at USNO. This
synchronisation was inevitably imperfect, depending as it did on
the
astronomical realisation of UT2. At
the time, UT2 as published by various observatories differed by
several
centiseconds. A.1 was
extrapolated backwards to 1956.
In 1961, the
Bureau
International de l'Heure (BIH) (later superseded by the BIPM
and the IERS) constructed an atomic time scale named
AM
based on three atomic clocks. The clocks were compared by listening
to radio
time signals based on them. The
BIH's time scale was synchronised with A.1's epoch, and
extrapolated back to 1955 using time signals from the first caesium
clock at NPL. This time scale was soon renamed from AM to
A3.
Also in 1961, UTC began. UTC is a discontinuous time scale composed
from segments that are linear transformations of atomic time, the
discontinuities being arranged so that UTC approximates
UT1. This was a
compromise
arrangement for a broadcast time scale: a linear transformation of
the BIH's atomic time meant that the time scale was stable and
internationally synchronised, while approximating UT1 means that
tasks such as
navigation which require a
source of Universal Time continue to be well served by public time
broadcasts.
In 1967 the SI second was redefined in terms of the frequency
supplied by a caesium atomic clock.
More clocks were added to the A3 time scale from 1967, and it was
renamed to
TA. Finally in 1971 it was renamed
TAI.
In the 1970s, it became clear that the clocks participating in TAI
were ticking at different rates due to
gravitational time dilation, and
the combined TAI scale therefore corresponded to an average of the
altitudes of the various clocks. Starting
from Julian Date 2443144.5 (1977-01-01T00:00:00), corrections were
applied to the output of all participating clocks, so that TAI
would correspond to proper time at
mean
sea level (the geoid). Because the clocks had been on average
well above sea level, this meant that TAI slowed down, by about
10
^{−12}. The former uncorrected time scale continues to be
published, under the name
EAL (
Echelle Atomique
Libre, meaning
Free Atomic Scale).
The instant that the gravitational correction started to be applied
serves as the epoch for
Barycentric Coordinate Time
(TCB),
Geocentric Coordinate
Time (TCG), and Terrestrial Time (TT). All three of these time
scales were defined to read JD 2443144.5003725
(1977-01-01T00:00:32.184) exactly at that instant. (The offset is
to provide continuity with the older
Ephemeris Time.) TAI was henceforth a
realisation of TT, with the equation TT(TAI) = TAI +
32.184 s.
In the 1990s, annual periodic variations in the rate of some clocks
were traced to
blackbody
radiation that varies with the ambient
temperature. It became clear that a correction
for this was required. Accordingly, in 1997 the BIPM declared that
the definition of the SI second referred to a caesium atom at rest
and at
absolute zero temperature.
Temperature corrections were implemented in TAI from 1995 to 1998,
speeding TAI up by about 10
^{−14.3}.
See also
References
- BIPM clock comparisons
External links