if you buy a clock like this one that
says radio controlled you just put a
battery in and it sets itself to the
correct time and it stays there
automatically for a year or more so it
can't be receiving signals from
satellites and there's no wi-fi
connection there's nowhere near enough
power in here for any of that instead it
uses much older technology here at the
radio transmission station in anthon the
national physical laboratory npl
broadcasts a time signal out to the uk
in a simple enough format that the cheap
electronics inside this clock can
understand it and run accurately for
over a year on a single double a battery
under npl's base in london i asked the
team how it all works what we have in
anthon are some very high end cesium
atomic clocks like the ones we have down
here we broadcast from there we monitor
the signals at npl and accordingly put
out steers to those clocks to correct
for any
any change because it's a very low
frequency at 60 kilohertz it has very
low susceptibility to atmospheric
effects we steer from npl
based on
measuring what anthem is broadcasting at
the level of nanoseconds that is the
nanosecond the distance light travels in
one nanosecond 30 centimeters
all that is taken into account so that
seemed simple enough the official time
scale is kept in london uh i wasn't
allowed anywhere near the official
atomic clocks i don't even know if
they're actually at npl's headquarters
or not they might be somewhere else they
told me nothing but up north here this
transmitter has separate atomic clocks
about 400 kilometers away from the base
or 1.4 light milliseconds away because
there's a light speed delay a latency of
about a thousandth of a second between
the transmitter here and the base in
london because the team at npl know
exactly what that delay should be they
can issue corrections if these clocks
start to drift even by nanoseconds
billions of seconds officially the
accuracy is much lower than that of
course partly because they keep a very
wide safety margin and partly because i
can introduce a one nanosecond drift to
this clock by doing that
what is really important when you're
transferring time
is not about the latency but
understanding the latency so you can
take that out of the equation from the
npl we disseminate to the uk over radio
broadcast essentially that offers the
entire uk mainland
several milliseconds of traceability to
utc we also have the fiber delivered
service for a regulatory compliance in
the finance sector guaranteed at the
microsecond and that's fine i can
understand all that but then that leads
to another question how does npl know if
their clocks in london start to drift
sure they've got many of them they cross
check and a second in the 21st century
is defined by the number of vibrations
of a cesium atom but still they can't be
perfect they're meant to show the
international standard utc coordinated
universal time so
what if npl's official source of
timekeeping truth drifts from that
standard the utc or coordinate to
universal time as the name suggests
is coordinated globally contributing
data from multiple atomic clocks to
generate what is the global time scale
utc
what we have here at npl is the uk's
timescale utc npl
we have a whole suite of atomic clocks
cesium clocks hydrogen masers and our
cesium fountain the bureau of weights
and measures based in france take the
data from the slots around the globe and
inform each of the national lands how
offset it is from utc and that's where
the buck stops in paris unless you start
getting into physics and relativity and
light cones and that's way beyond me
why does it matter well first of all
finance and high frequency trading but
also science
things like the square kilometer array a
planet-wide array of radio telescopes
that's being planned and built right now
when they're listening for incredibly
faint radio waves from space all the
telescopes around the globe have to be
synchronized with that level of
precision or the whole experiment could
fall apart so then i had a final
question when accurate time code is
being sent from so many places when the
public can get time that's more than
accurate enough for almost every purpose
from navigation satellites in space
why is this signal still important and
the team from npl had a rather serious
answer time really is an invisible
utility it underpins our digital
infrastructure whether it's the
the synchronization of the energy grid
or the telecom networks or finance we
are so dependent on gps and other
constellations one of the problems we
face is those signals are easily
disrupted it's the equivalent of a light
bulb on the moon one of the things we're
doing at npl is to ensure that the uk
is resilient for the future
standing by the ocean with a domestic
appliance
the weird thing is it's not the first
time
you