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5 X-ray Pulsar-Based Navigation: Theories and Experiments
there are always differences between the atomic times at different places. For this
reason, a lot of the atomic clocks distributed at the laboratories of various countries
are compared with one another, and then the global unified atomic time system is
calculated by data processing, which is called International Atomic Time (TAI).
The TAI was initially established and maintained by the Bureau International de
l’Heure (BIH), and maintained by the Bureau International des Poids et Measures
(BIPM) since 1988. The time origin of the TAI is at zero hour of the UT on January 1,
1958, and the timescale is the atomic second defined by the SI units. It was initially
hoped that the UT and TAI had the same time origin, but for technical reasons, there
was a small difference of 0.0039 s between both. At present, the TAI is based on a
system consisting of about 270 laboratory-constructed atomic clocks. Signals from
these atomic clocks are transmitted to the BIPM and used to generate the TAI. The
Cesium fountain clocks now provide the SI unit’s second to an unprecedented level
of accuracy. These clocks are predicted to be offset by less than one second for more
than 50 million years.
5.3.2.7 Coordinated Universal Time
Although the implementation of the atomic time makes a qualitative leap in the
accuracy of time measurement, the atomic time cannot replace the application of the
UT. The UT based on the rotation of the Earth conforms to people’s daily habits and
still has a wide range of uses. However, due to the trend that the Earth’s rotation rate
slows down for a long time, the UT is about 1 s slower than the atomic time every year,
and the difference between the two is accumulating year by year. In order to avoid
excessive deviation between the published atomic time and the UT, a compromised
time system based on the atomic second length has been adopted since 1972, which
is well known as Coordinated Universal Time (UTC). The length of a second of
the UTC is exactly equal to that of the atomic time, and the leap second method is
used to make the UTC as close as possible to the moment of the UT. When the time
difference between the UTC and UT exceeds ±0.9 s, a leap second with the positive
or negative is introduced into the UTC, which is generally added at the end of June 30
or December 31 of each year. The need for additions of leap seconds is determined
by the International Earth Rotation and Reference Systems Service (IERS), located
at Paris Observatory. Except for the introduction of the leap seconds, the UTC is
essentially the TAI, so the establishment, measurement and comparison of the UTC
systems are basically as same as the atomic time. The UTC is calculated by the
weighted average of the BIPM’s global atomic clocks, with the highest long-term
stability and precision.
Currently, there are 72 laboratories that have established and maintained the UTC
systems around the world. According to the international agreement, the UTC of
each laboratory is expressed as UTC (k), where k is the abbreviation of the laboratory name. For example, UTC (NIM) stands for the UTC maintained by the National
Institute of Metrology in Beijing, China; UTC (NTSC) stands for the UTC maintained by the National Time Service Center at Lintong, Xi’an, China; UTC (USNO)
5 X-ray Pulsar-Based Navigation: Theories and Experiments
there are always differences between the atomic times at different places. For this
reason, a lot of the atomic clocks distributed at the laboratories of various countries
are compared with one another, and then the global unified atomic time system is
calculated by data processing, which is called International Atomic Time (TAI).
The TAI was initially established and maintained by the Bureau International de
l’Heure (BIH), and maintained by the Bureau International des Poids et Measures
(BIPM) since 1988. The time origin of the TAI is at zero hour of the UT on January 1,
1958, and the timescale is the atomic second defined by the SI units. It was initially
hoped that the UT and TAI had the same time origin, but for technical reasons, there
was a small difference of 0.0039 s between both. At present, the TAI is based on a
system consisting of about 270 laboratory-constructed atomic clocks. Signals from
these atomic clocks are transmitted to the BIPM and used to generate the TAI. The
Cesium fountain clocks now provide the SI unit’s second to an unprecedented level
of accuracy. These clocks are predicted to be offset by less than one second for more
than 50 million years.
5.3.2.7 Coordinated Universal Time
Although the implementation of the atomic time makes a qualitative leap in the
accuracy of time measurement, the atomic time cannot replace the application of the
UT. The UT based on the rotation of the Earth conforms to people’s daily habits and
still has a wide range of uses. However, due to the trend that the Earth’s rotation rate
slows down for a long time, the UT is about 1 s slower than the atomic time every year,
and the difference between the two is accumulating year by year. In order to avoid
excessive deviation between the published atomic time and the UT, a compromised
time system based on the atomic second length has been adopted since 1972, which
is well known as Coordinated Universal Time (UTC). The length of a second of
the UTC is exactly equal to that of the atomic time, and the leap second method is
used to make the UTC as close as possible to the moment of the UT. When the time
difference between the UTC and UT exceeds ±0.9 s, a leap second with the positive
or negative is introduced into the UTC, which is generally added at the end of June 30
or December 31 of each year. The need for additions of leap seconds is determined
by the International Earth Rotation and Reference Systems Service (IERS), located
at Paris Observatory. Except for the introduction of the leap seconds, the UTC is
essentially the TAI, so the establishment, measurement and comparison of the UTC
systems are basically as same as the atomic time. The UTC is calculated by the
weighted average of the BIPM’s global atomic clocks, with the highest long-term
stability and precision.
Currently, there are 72 laboratories that have established and maintained the UTC
systems around the world. According to the international agreement, the UTC of
each laboratory is expressed as UTC (k), where k is the abbreviation of the laboratory name. For example, UTC (NIM) stands for the UTC maintained by the National
Institute of Metrology in Beijing, China; UTC (NTSC) stands for the UTC maintained by the National Time Service Center at Lintong, Xi’an, China; UTC (USNO)