Coherent detection

From Glossary of Meteorology

coherent detection

The conversion of the intermediate frequency (IF) signal in a Doppler radar or lidar to I and Q channels so that the phase of the echoes is preserved.

Typically two mixers, fed by IF reference signals in phase quadrature, are used to convert the frequency components in the IF signal, centered at the IF frequency, to two video signals with frequency components centered at zero frequency, without modifying the information content.