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Time and Frequency Standards

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Time, Duration

second, s: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

The practical realisation of the second is based on electrical oscillators that are locked to the frequency of the transition in caesium 133 atoms. One of the major limitations in atomic clocks has always been the influence of temperature, which causes the clocks to wander randomly. In the last few years techniques have been developed that enable caesium vapour to be cooled to very low temperatures, so now the best primary standards produce the second to about 1 part in 1016.

In accordance with Einstein’s theory of general relativity clocks run slower at low altitudes – on earth the effect is about 1 part in 1016 per metre. Since everything else in the clock’s immediate environment also runs slow, the relativistic effects are no problem for most laboratory experiments. However when one has to link clocks at different places on Earth there is a problem. So at present the time of day is based on the average time as maintained by about 200 clocks held by different national standards laboratories (including MSL), with the relativistic corrections applied. This is called international atomic time (TAI). However the problem is more complicated than this, because we would also like the time to be consistent with day and night and the rotation of the Earth. Since the Earth’s rotation rate is gradually slowing down, occasional leap seconds have to be added to ensure that the sun crosses the Greenwich meridian within 0.9s of noon. This time is then called Coordinated Universal Time (UTC).

The advent of the Global Positioning System (GPS) with its satellites and orbiting atomic clocks has revolutionised time keeping. Now caesium clocks world-wide can be compared to each other and the global average maintained to within about 100ns over periods of months. The precision of atomic clocks may seem excessive, and for most applications that is true, however the rates of transmission of data on fibre-optic cables and high frequency communication systems are at present limited by the synchronicity of the clocks used to manage the data transfer. Fully automatic navigation and landing systems for aircraft have also only just become practical.

In the last couple of years handheld GPS receivers have become common, and in principle anyone who has a GPS receiver has direct access to UTC. However the accuracy of the time obtained in this way depends on a number of factors, including the type of receiver, and keeping track of the difference between GPS clocks and UTC. MSL is currently researching the performance of some of the low cost GPS systems, and developing ways of using them to transfer time to high accuracy.