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  • 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,

  • there's no wifi connection,

  • there's nowhere near enough power in here for that.

  • Instead, it uses much older technology.

  • Here at the radio transmission station in Anthorn,

  • the National Physical Laboratory, NPL, broadcasts a time signal 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.

  • And at NPL's base in London, I asked their team how it all works.

  • - What we have in Anthorn 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.

  • It's a very low frequency, 60 kilohertz,

  • it has very low susceptibility to atmospheric effects.

  • We steer from NPL based on measuring what Anthorn is broadcasting

  • at the level of nanoseconds.

  • That is a nanosecond, the distance light travels in one nanosecond, 30 centimeters.

  • All that is taken into account.

  • - So that seemed simple enough. The official timescale is kept in London.

  • 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 four hundred kilometres away from the base.

  • Or 1.4 light-milli-seconds away.

  • 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, billionths 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 a fibre-delivered service

  • for 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 standad,

  • UTC, coordinated universal time,

  • so what if NPL's official source of timekeeping truth drifts from that standard?

  • - UTC or coordinated universal time, as the name suggests,

  • is coordinated globally, contributing data from multiple atomic clocks

  • to generate what is the global timescale, 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 a cesium fountain.

  • The Bureau of Weights and Measures based in France

  • take the data from the clocks around the globe,

  • and inform each of the national labs 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 Kilometre Array, a planet-wide array of radio telescopes

  • that's being planned and built now.

  • When they're listening for incredibly faint radio waves from space,

  • all the telescopes around the globe have to be synchronised

  • with that level of precision, or the whole experiment could fall apart.

  • So then, I had a final question:

  • when accurate timecode 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 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 the 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.

If you buy a clock like this, one that says "radio controlled",

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How does Britain know what time it is?

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    林宜悉 發佈於 2022 年 03 月 06 日
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