Narrowband Frequency modulation is used exclusively, so that the capture effect will select the stronger of two transmitters wherever one predominates. While frequency control down to 1-2 hertz accuracy is achievable, this is undesirable, since the cancellation of signals would wipe out the signal at the receiver for up to a second.
Narrowband Frequency modulation is used exclusively to ensure that the strongest transmitter is selected by the capture effect, even if one is more dominant than the other. While precise frequency control down to 1-2 hertz is possible, this level of accuracy would cause signal cancellation at the receiver for up to a second.