A radar sensor includes: a transceiver unit for emitting a radar beam along a beam path in an outgoing direction and receiving radar radiation along the beam path in an incoming direction; and a reference object placed in the beam path to redirect part of the outgoing radar beam in the incoming direction. The reference object is one of a plurality of reference objects placed in the radar beam. A size of the reference objects in at least one dimension is smaller than a wavelength of the radar beam.