A self-propelled harvesting machine for harvesting a crop field has a ground drive including multiple working units, a control system and a sensor system. The sensor system periodically emits transmitted pulses of electromagnetic transmission beams in at least one transmission direction onto the crop field and the transmitted pulses are reflected on the crop field and are received as echo pulses by the sensor system. For at least one portion of the transmitted pulses, different partial beams of a single transmission beam are reflected by plants of the crop field lying one behind the other with a time offset, so the particular resultant echo pulse is composed of time-offset partial echo pulses. The control system determines a value for the crop density on the basis of a time correlation within the resultant echo pulse, and controls the ground drive and/or the working units on the basis of the determined crop density.