Frequency modulation

From Glossary of Meteorology

frequency modulation

Changing the frequency of a carrier wave by some means so that the resultant waveform transmits a signal conveying information.

For example, if the carrier wave with angular frequency ω is cos(ωt), the corresponding frequency-modulated signal is
where a is a constant and
. Frequency modulation is used in some continuous-wave radars to permit range determination, but it is not generally used in weather radars.
Compare amplitude modulation.