Page 9 - EngineerIt March 2021

MEASUREMENT

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
-13
.
-13
.
Council for Scientific and Industrial one and two parts in 10 (between 1 10 and 2 10 times the frequency), i.e.
13
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 .
.
14
African Standard Time – SAST) The best caesium clocks in the world can generate a frequency accurately to a
16
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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