National and International time keeping
National Time Keeping
The Swedish national time scale UTC(SP) is a national realisation of the Coordinated Universal Time UTC. UTC(SP) is maintained using atomic clocks, cesium clocks and hydrogen masers, and is continuously compared with the international atomic time scales TAI and UTC (see below). The comparison is obtained through measurements against the satellites within the GPS-system and coordinated by the International Bureau of Weights and Measures (BIPM) located in Sèvres outside Paris. Swedish standard time is UTC + 1h and and UTC + 2h during daylight savings time. The changes between standard time and daylight savings time is decided by the Swedish government through a directive from the European Community (EC).
The distribution of UTC within Sweden is obtained via different channels. Partly via UTC(SP) and the time synchronisation services that SP offers, but also via satellite systems like GPS and the long wave transmitter DCF77 in Germany.
International Time Keeping
UTC, which is the basis of world time distribution, is a combination of the time scales TAI and UT1. The International Atomic Time scale TAI is calculated and maintained by the International Bureau of Weights and Measures (BIPM) as a weighted average of data from some 230 atomic clocks kept in over 60 national laboratories worldwide. Universal Time UT1 is a dynamic time scale derived from observations of the rotation of the Earth, and is proportional to the angle of rotation of the Earth about its axis. This implies that UT1 follows the rise and the set of the sun, which TAI does not necessarily do.
By definition UTC has the same metrological properties as TAI, which is an atomic time scale. In addition it follows the rotation of the Earth within less than one second by the addition or subtracting of so-called leap seconds. The difference between UTC and TAI is therefore always a complete number of whole seconds and the difference between TAI and UT1 is always less than 0.9 seconds. This is sufficient for the purposes of for example astronomical navigation. Leap seconds are usually introduced by the end of June or December, a decision taken by the IERS (International Earth Rotation Service).
TAI and UTC are so-called virtual time scales or "paper clocks". UTC is realised physically through the local time scales of maintaned in national time laboratories, in Sweden through UTC(SP). Due to the procedure in which UTC is determined, the realisation of the SI-unit one second and the official time scale of the world does not depend on one single clock. This is very important for the continuity and the stability of UTC.
You can read more about time keeping and different time scales here.
The Swedish national time scale UTC(SP) is based on cesium clocks and hydrogen masers and is continuously compared with the international time scales TAI and UTC through coordinated observations against the satellite system GPS.
The atomic time scales TAI and UTC are calculated by the International Bureau of Weights and Measures (BIPM) in Paris as a weighted average of approximately 230 atomic clocks kept in over 60 national laboratories. Some of these clocks are located at SP.
Coordinated Universal Time UTC is based on the International Atomic Time TAI. Leap seconds are introduced to UTC to follow the rotation of the Earth. The time for introduction is decided by IERS with help from national astronomy observations. In Sweden the Onsala Space Observatory is part of the measures of the rotation of the Earth.
Leap seconds are introduced when the official time scale UTC follows the rotation of the Earth by the introduction of leap seconds.