Some or all of the Navy VLF transmitters may at any time be using a form of code known as MSK (minimum-shift-keying). This is a type of frequency-shift-keying in which the bit frequency is equal to twice the difference frequency. In consequence, if a bit is transmitted at frequency #1, then one bit at frequency #2, and a third bit at frequency #1 again, the phase of frequency #1 during the third bit will differ by 180° from what it was during the first bit. Therefore, there is no coherent "carrier" at either of the two frequencies. By a process of frequency doubling, those transmissions at zero phase and those at 180° are restored to equal phase at the second harmonic frequency. Phase tracking of the 2nd harmonic with a VLF tracking receiver is then possible. INSTALLATION