Coordinated Universal Time (UTC)

Author: Tianjiang Shuo

Website: https://cislunarspace.cn

Definition

Coordinated Universal Time (UTC) is the internationally adopted time standard. Based on the SI second of atomic time, UTC maintains approximate synchronization with Universal Time UT1 through a leap second mechanism (deviation not exceeding 0.9 seconds). UTC balances the high precision of atomic time with the correspondence between Universal Time and Earth's rotation, and is the most widely used time reference in daily life, communications, navigation, and aerospace engineering.

Core Elements

Coordination Between Atomic Time and Universal Time

Time SystemBasisCharacteristics
International Atomic Time (TAI)Cesium atomic transition frequencyExtremely uniform; does not reflect irregularities in Earth's rotation
Universal Time (UT1)Earth's rotationReflects actual Earth orientation, but non-uniform
Coordinated Universal Time (UTC)Atomic-time second + leap secondsBalances precision with astronomical correspondence

The relationship between UTC and TAI:

TAI=UTC+n (seconds)\text{TAI} = \text{UTC} + n \text{ (seconds)}

where nn is the cumulative number of leap seconds (37 seconds as of 2026).

Leap Second Mechanism

Because the Earth's rotation rate undergoes long-term slowing and irregular variations, a gradual offset accumulates between atomic time and Universal Time. When the deviation between UTC and UT1 approaches 0.9 seconds, the International Earth Rotation and Reference Systems Service (IERS) announces the insertion (or deletion) of a leap second at the end of June 30 or December 31, keeping the UTC-UT1 deviation within +/- 0.9 seconds.

Role of UT1

UT1 is Universal Time corrected for polar motion. It is generally used to establish the connection between celestial and terrestrial coordinate systems and is critical for ground station pointing. UTC maintains approximate synchronization with UT1 through the leap second mechanism.

Application Value

UTC is the time reference for global communications, navigation, and space missions. GPS system time and BeiDou system time both reference UTC. For cislunar missions, UTC is used for time synchronization of ground tracking and control stations, mission scheduling, and telemetry data timestamps. Onboard computers typically use GPS time or atomic time, which have known fixed offsets from UTC.

References

  • 郑伟, 安雪滢, 周祥, 何睿智. 空天飞行力学[M]. 国防科技大学, 2026.
  • 任萱, 肖峰. 人造地球卫星轨道力学[M]. 国防科技大学出版社.
  • IERS. IERS Conventions (2010)[S].