The invention relates to a level sensor for use in a lithographic apparatus that determines a surface height of a substrate. The level sensor includes an emitter and a receiver, wherein the emitter is arranged to emit a signal directed to a predetermined position on the surface of the substrate, such that the signal is at least partially reflected by the substrate to render a reflected signal. The receiver is arranged to receive at least part of the reflected signal, and the level sensor is arranged to determine the surface height of the substrate with respect to the level sensor based on the emitted and received signal. The signal includes a pressure wave.