A grain cleaning system for a combine harvester having a transmitter adapted to transmit a base signal at a known frequency and one or more spaced receivers for detecting signals of a different frequency as reflected from airborne grain and other materials within the duct of the grain cleaning system. An Electronic Control Unit modulates the base signal and the reflected signals to obtain Doppler signals or frequencies from which an average particle velocity is determined. The particle velocity is used as an input parameter for the generation of control signals for the adjustment of various working units of the combine harvester including, by way of example, the fan and sieves.