Lithographic Method and Apparatus

A lithographic method for measuring a position of a target grating with a mask sensor apparatus which comprises a plurality of detector modules each comprising a diffraction grating located at a mask side of a projection system of a lithographic apparatus and an associated detector, the method comprising a first step of measuring first intensities of a combination of diffraction orders diffracted from the target grating while the mask sensor apparatus is moved relatively to the target grating along a first direction; a second step of displacing the mask sensor apparatus relative to the target grating in a second direction, wherein a size of the relative displacement is proportional to a spatial frequency of a potential error; and a third step of measuring second intensities of the combination of diffraction orders diffracted from the target grating while the mask sensor apparatus is moved relatively to the target grating along the first direction.