In the Netherlands, we have clocks at train stations like on the one on the image below.

enter image description here

In order to get the time as precise as possible, the clocks read the time from the DCF77 signal. (As far as I know, for the sake of this question, let's say it is the case.) DCF77 is a longwave time signal from Germany used in mainland Europe.

My question is, how secure are these clocks?

What would happen if I would broadcast a completely other time on the same wavelength near such a clock? Would the time change?

Disclaimer: I'm absolutely not planning on doing this. (Otherwise, I would have found out myself.) I'm just curious about how secure these clocks are.

  • \$\begingroup\$ I read that the second hand makes a full cycle in 57 seconds, rather than 60. Each beat of the second hand is only 0.95 seconds long. THen resyncs every minute something like the 50 or more year old IBM clocks in classrooms which would sync to the master clock with a magnetic latch at 12. \$\endgroup\$ – Tony Stewart EE75 Aug 27 '16 at 23:40
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    \$\begingroup\$ @Kevin Well, the specification for DCF77 time format is public, so that would be a good place to start looking. My hunch is that there's no security except for the practical issues. \$\endgroup\$ – AndrejaKo Aug 27 '16 at 23:44
  • \$\begingroup\$ This is not an EE question. VTC! \$\endgroup\$ – Andy aka Aug 28 '16 at 0:11
  • \$\begingroup\$ I think that you won't be unnoticed if you build such large scale transmitter. \$\endgroup\$ – Marko Buršič Aug 28 '16 at 6:37
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    \$\begingroup\$ Even the Wikipedia page is clear enough for an answer. The DCF77signal has no security or even error checking features. \$\endgroup\$ – JRE Aug 28 '16 at 8:26

The DCF77 signal transmits bits by dropping the amplitude of the 77 kHz carrier wave to 15 % of its normal value for a specific amount of time:
amplitude modulated DCF77 signal as a function of time
(source: Wikipedia, CC0)

If you transmitted your signal so that the receiver sees it at the same amplitude as the original signal, all you could do is to change a 1 bit to a 0 bit, or to make the signal unusable.

But if your signal is strong enough, you would be able to override the original signal completely.

(The DCF77 signal is also phase modulated, but almost no receiver actually uses that.)


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