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        The science of accurate time

          When it comes to time, “give or take a few minutes” is of little consequence
          for the man in the street, but unknowingly, the availability of precise time does
          actually impact on everyone. Communication systems, electrical power grids and
          financial networks are amongst the many systems that rely on precision timing for

          synchronisation and operational efficiency.

             oday, even laptops require      quantum transitions of atoms. Accurate time is required as signals from each antenna
             some software applications      will reach the data processing centre at different times due to the difference in distance
        Tfor accurate time. New              from each antenna. Correlation and beam forming of the signals are necessary in order
        developments like 5G and all the     to make the antenna operate as one big receiver to obtain a high-resolution image of the
        “G’s” that will follow, will make    sky. Correlation relies on precise time tagging of the received signals. The correlating
        demands on access to accurate        beam former must be very accurately synchronised to the master time reference of the
        time, with accuracy of under 10      data centre, which itself must be synchronised to UTC.
        nanoseconds (a nanosecond or “ns”      “To meet the SKA requirements, we added hydrogen atomic clocks to the caesium
        is a billionth of a second).         atomic clocks which have been in operation for some time. A caesium atomic clock
           In South Africa, accurate time is   employs a beam of caesium atoms. The clock separates caesium atoms of different
        kept at the National Metrology Institute   energy levels and then uses a magnetic field to trigger a state transition. The NMISA
        (NMISA) laboratories situated at the   Caesium clocks measure frequency with a relative frequency accuracy of between
        Council for Scientific and Industrial   one and two parts in 10  (between 1 10  and 2 10  times the frequency), i.e.
        Research (CSIR) campus in Pretoria.   the 10 MHz output signal is accurate to 0,0000015 Hz; this corresponds to a time
        NMISA keeps the South African        measurement accuracy of 13 nanoseconds per day or one second in 210 000
        National Time (known as South        years. Adding the hydrogen atomic clocks improved this to typically less than 2 10 .
        African Standard Time – SAST)        The best caesium clocks in the world can generate a frequency accurately to a
        and contributes to the calculation of   relative frequency of five parts in 10 . It is the most accurate realisation of a unit that
        Coordinated Universal Time (UTC).    mankind has yet achieved.”
           NMISA has nine atomic clocks.
        Chris Matthee, NMISA manager of time,
        frequency and fibre optics, explained
        that a minimum of four clocks are
        required, three for keeping accurate
        time and one as a backup. “The fact
        that we landed up with nine atomic
        clocks is historical. We added hydrogen
        atomic clocks to the older caesium
        clocks and we currently use the
        combination to our advantage.”
           The Square Kilometre Array placed
        more stringent requirements on accurate
        time and signed an MoU with NMISA in
        2015 to upgrade the time reference from
        ~5000 ns to cutting edge accuracy of
        ~10 ns. Precise frequency is necessary
        to form clear images when using an
        array of antennas. Time and frequency   Chris Matthee with the NIMISA time laboratory. The three hydrogen clocks are on either
        constitute the best-known measurement   side of the caesium clocks. Several GPS receivers are on the top with antennas on the lift
        standards as they are based on       shaft of the building.

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