1. A lithographic apparatus comprising: a patterning device support configured to support a patterning device, the patterning device adapted to provide a patterned beam of radiation; a substrate support configured to support a substrate; a projection system configured to project the patterned beam of radiation onto the substrate; an encoder configured to measure a position of one of the supports, the encoder including a sensor and a grating; a controller configured to control a position of said one support based on measurements provided by the encoder; and an interferometer configured to measure a position of said one support, said measurement of said position of said one support being used to calibrate the encoder.