The time scale UTC
The time scale UTC (Coordinated Universal Time) serves as a worldwide reference time scale for the determination of time in everyday life and in scientific and technical fields such as astronomy, geodetic surveying, navigation and telecommunication. UTC is realised by the joint effort of more than 70 time institutes based in many countries. These institutes have about 400 atomic clocks at their disposal. The International Bureau of Weights and Measures (BIPM) in Paris coordinates the acitvities of these institutes and the dissemination of UTC.
The individual time institutes such as PTB use their atomic clocks to realize a local time scale - in our case referred to as UTC(PTB) - which is kept in as good agreement with UTC as possible. During the last years, the difference between UTC and UTC(PTB) has always been less than 10 ns (1 ns = 1/1000000000 s).
The legal time in Germany, CET(D) or CEST(D), is defined by adding one or two hours to UTC(PTB). In the same way the local times in other time zones are defined by the respective national time institutes.
Properties of UTC
The time scale UTC (Coordinated Universal Time) owes its existence to the CCIR (International Consultative Committee of Radiocommunications) of the ITU (International Telecommunications Union) which proposed to broadcast time signals worldwide in a "coordinated" way, i.e. by reference to a common time scale. If the atomic time scale TAI had been used for this purpose, the time signals would appear shifted with respect to a Universal Time clock (UT1 = mean solar time at 0 degrees longitude). For navigation based on the position of celestial bodies, however, it is necessary to know UT1, and the broadcasting of time signals should serve this purpose. As in the 1960s astronomic navigation was still of greater importance than today, it was decided to introduce UTC as a compromise: UTC and TAI have the same scale unit but the difference between UTC and UT1 is limited to less than 0.9 seconds by inserting leap seconds.
Like TAI, UTC is published only in the form of calculated time differences relative to the time scales UTC(k) realized at the individual time institutes. In 1975, the 15th General Conference for Weights and Measures recommended that UTC be used as the basis of public time, and accordingly time signals and standard frequencies should be disseminated in a coordinated way in UTC. This explains the requirement that the individual time scales UTC(k) should be in as good agreement as possible with UTC and thus with one another. Presently, a deviation of less than 10 ns is aimed for. For this purpose, the rate of a particularly stable clock selected for the realization of UTC(k) is evaluated and subsequently adjusted in order to obtain maximum agreement with UTC. This is achieved rather easily owing to the very good predictability of UTC and the quality of the clocks available. In the year 2013, 25 time scales UTC(k) with deviations UTC-UTC(k) of less than 10 ns existed. For example, the difference UTC-UTC(PTB) was 0,8 ns on January 2, 2013.