How secure are station clocks (using DCF77)?
The DCF77 signal transmits bits by dropping the amplitude of the 77&nbsp;kHz carrier wave to 15&nbsp;% of its normal value for a specific amount 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&nbsp;1&nbsp;bit to a&nbsp;0&nbsp;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.)