About SST

Time is a universally important parameter that impacts on all aspects of life. The metrology of time involves the establishment of a precise standard time interval, a suitable time scale for dating events and a means for synchronisation of time. Precise synchronisation of time is essential for broadcasters, telephone companies, financial institutions, controllers of air traffic and operators of computer systems and networks. From manufacturing plants to laboratories, data and events may have to be recorded with known time to meet legal and statutory requirements. A traceable time scale is also critical for coordinating defense activities.

Unit of Time

The basic unit of time or time interval in the International System of Units (SI) is the second. It is equal to 9 192 631 770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of caesium-133 as realized in a caesium atomic clock. The stability of the caesium atomic clock is typically better than 1 part in 1014 of a second, which translates to an error of 1 second in every million years.

International Time Scales

The International Atomic Time (TAI) maintained by International Bureau of Weights and Measures (BIPM) is a time scale established by addition without dead-time of seconds given by the caesium atomic clocks. Exact multiples of the seconds are used to define the time interval of what we know today as minutes, hours and days. TAI provides a continuous, uniform and precise time scale required in scientific applications and communication systems that form the infrastructure of modern society.

However, many of our everyday activities are linked to the rotation of the earth, even though the rate of the earth rotation is much more erratic compared to the rate of the atomic clock. A widespread opinion was that, while the unit of time or time interval should be based on an atomic transition, the worldwide time scale adopted should be based on the rotation of the earth.

A compromise was reached with the creation of a new practical time scale known as the Coordinated Universal Time (UTC). The UTC time scale changes at the same rate as TAI but differs by an integer number of seconds. The UTC time scale is adjusted by the insertion or deletion of seconds (known as leap seconds) to ensure approximate agreement with the astronomical time scale, designed as UT1, which is based on the rotation of the earth. The difference between UTC and UT1 would never exceed 0.9 seconds. At present, leap seconds have been inserted into the UTC time scale at average intervals slightly longer than one year. To date, 32 leap seconds have been added to the UTC.

UTC time scale is also equivalent to the historical Greenwich Mean Time (GMT), which is a 24-hour astronomical time system based on the local time at Greenwich, England. The difference between UTC and GMT is within fractions of a second. However, by international agreement, UTC is recommended for all general timekeeping applications, and the use of the GMT is discouraged. The UTC time scale is the basis for the synchronisation of worldwide activities. This is done through various modes of time signal dissemination.

Establishment and Dissemination of the Singapore Standard Time

An ensemble of caesium atomic clocks is maintained at the National Metrology Centre. These clocks are continuously compared with time standards maintained by other time laboratories through the Global Positioning System (GPS) which is a constellation of satellites orbiting the earth at a high altitude. Our time data are sent to BIPM as part of an international network that contributes towards the generation of the TAI and UTC. The time link network involves about 50 time laboratories in 30 countries all over the world.

The time scale kept by NMC, designated as UTC(SG), is the local realization of UTC in Singapore. Based on the comparison data, BIPM regularly reports the difference between UTC and each local time scale, such as UTC(SG). UTC(SG) is not expected to differ from UTC by more than 100 nanoseconds at any time. UTC(SG) is then offset by +8 hours to give the time-of-day in Singapore. This becomes the Singapore Standard Time (SST).

From March 1999, NMC started to disseminate the SST to the industry and public. This was then followed by automated computer time services (ACTS), network time service (NeTS) through the Internet.

For more information, please email: time@nmc.a-star.edu.sg