Page "Decca Navigator System" Paragraph 8

When two stations transmit at the same phase-locked frequency, the difference in phase between the two signals is constant along a hyperbolic path.

Of course, if two stations transmit on the same frequency, it is practically impossible for the receiver to separate them ; so instead of all stations transmitting at the same frequency, each chain was allocated a nominal frequency, 1f, and each station in the chain transmitted at a harmonic of this base frequency, as follows: