Patent ID: 7978339

Claim:
A method to determine an actual position of a first part of a lithographic apparatus with respect to a position of a second part of the lithographic apparatus, the method comprising: measuring along a first measurement path a position of the first part of the lithographic apparatus relative to a first measurement device of the lithographic apparatus, the first measurement path being between a first source and receiver of the first measuring device and the first part of the lithographic apparatus; measuring along a second measurement path a position of the second part of the lithographic apparatus relative to a second measurement device of the lithographic apparatus, the second measurement path being between a second source and receiver of the second measuring device and the second part of the lithographic apparatus measuring, by a first plurality of sensors located along nozzles of an air shower along the first and second measurement paths, a temperature of a gas flowing out of the respective nozzles; measuring, by a second plurality of sensors located along the first and second measurement paths, a physical parameter of a medium along the first and second measurement paths; and correcting the measured relative position of the first part with respect to the second part by taking into account the measured temperature of the gas and the measured physical parameter of the medium in order to determine the actual position of the first part with respect to the second part.