Navigation signals emitted by a navigation transmitter, in a first signal path, have a time reference generated and, based on this time reference, navigation signals are generated which are subsequently emitted by way of a transmitting device. The wiping element is cooled down so that the liquid coming into contact with this wiping element and bristles preferably provided thereon assumes a higher viscosity or even solidifies and consequently remains adhered together with the particles contained in the liquid to the wiping element or its bristles. The outer auger blade and inner auger blade are arranged along a first helical path.