A sensor apparatus comprising an acoustic assembly arranged to transmit an acoustic signal to a substrate and receive at least part of the acoustic signal after the acoustic signal has interacted with the substrate, a transducer arranged to convert the at least part of the acoustic signal to an electronic signal, and, a processor configured to receive the electronic signal and determine both a topography of at least part of the substrate and a position of a target of the substrate based on the electronic signal. The sensor apparatus may for part of a lithographic apparatus or a metrology apparatus.