Patent Number: 
Section: claims

1. A position measurement system for measuring a position of an object, the position measurement system comprising:a first incremental measurement unit configured to measure a first number of first distance steps in a distance between a reference frame and the object, wherein the first number equals a first integer value plus a first fraction,a second incremental measurement unit configured to measure a second number of second distance steps in a distance between the reference frame and the object, wherein the second number equals a second integer value plus a second fraction,wherein the position measurement system is constructed and arranged to initialize the second incremental measurement unit on the basis of the first number and the second fraction. 2. A position measurement system according to claim 1, wherein during initialization of the second incremental measurement unit, the second integer value is determined on the basis of a predetermined relationship between the first integer value, the first fraction, the second integer value and the second fraction. 3. A position measurement system according to claim 1, wherein the position measurement system is constructed and arranged to calibrate the second incremental measurement unit on the basis of a position measurement by the first incremental measurement unit. 4. A position measurement system according to claim 2, wherein an output signal of the first incremental measurement unit Xout1 substantially equalsXout1=IC1+(N1+φ1+ε1)·p1wherein:p1=a size of the first distance stepIC1=an initialization constantN1=the first integer valueφ1=the first fraction,ε1=a measurement error,and wherein an output signal of the second incremental measurement unit Xout2 substantially equalsXout2=IC2+(N2+φ2+ε2)·p2wherein:p2=a size of the second distance stepIC2=an initialization constantN2=the second integer valueφ2=the second fraction,ε2=a measurement error. 5. A position measurement system according to claim 4 wherein the predetermined relationship or the calibration is obtained by equating the output signal Xout1 of the first incremental measurement unit at a measurement position to the output signal Xout2 of the second incremental measurement unit at the measurement position. 6. A position measurement system according to claim 4 wherein the predetermined relationship is obtained bycalibrating the first incremental measurement unit at a first measurement position thereby initializing IC1 and N1,initializing the second incremental measurement unit at a second measurement position such that the output signal Xout2 of the second incremental measurement unit at the second measurement position corresponds to the output signal Xout1 of the first incremental measurement unit at the first measurement position, thereby initializing IC2. 7. A position measurement system according to claim 4, wherein the initialization of the second incremental measurement unit comprises equating the second integer value N2 by a round off operation. 8. A position measurement system according to claim 1, wherein the first incremental measurement unit comprises a grating and a first encoder head constructed and arranged to co-operate with the grating, wherein the second incremental measurement unit comprises a second encoder head constructed and arranged to co-operate with the grating and wherein the first distance step is a function of a pitch of the grating. 9. A position measurement system according to claim 8, wherein the grating is mounted to the reference frame and the first and second encoder heads are mounted to the object. 10. A position measurement system according to claim 8, wherein the grating is mounted to the object and the first and second encoder heads are mounted to the reference frame. 11. A position measurement system according to claim 8, wherein the second incremental measurement unit comprises a further grating constructed and arranged to co-operate with the first encoder head or with the second encoder head. 12. A position measurement system according to claim 1, wherein the first incremental measurement unit comprises a reflective surface constructed and arranged to reflect a radiation beam of an interferometer of the position measurement system to a first optical sensor of the first incremental measurement unit or to a second optical sensor of the second incremental measurement unit and wherein the first distance step is a function of a wavelength of the radiation beam. 13. A position measurement system according to claim 12, wherein the reflective surface is mounted to the object and the optical sensors are mounted to the reference frame. 14. A position measurement system according to claim 12, wherein the reflective surface is mounted to the reference frame and the optical sensors are mounted to the object. 15. A position measurement system according to claim 12, wherein the second incremental measurement unit comprises a further reflective surface constructed and arranged to reflect a radiation beam of the interferometer of the position measurement system to the first optical sensor or the second optical sensor. 16. A position measurement system according to claim 1, wherein the position measurement system is constructed and arranged to initialize the first incremental measurement unit on the basis of the second number and the first fraction. 17. A lithographic apparatus comprising:an illumination system configured to condition a radiation beam;a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;a substrate table constructed to hold a substrate;a projection system configured to project the patterned radiation beam onto a target portion of the substrate; anda position measurement system for measuring a position of the support or the substrate table, the position measurement system comprising:a first incremental measurement unit configured to measure a first number of first distance steps in a distance between a reference frame and the support or the substrate table, wherein the first number equals a first integer value plus a first fraction,a second incremental measurement unit configured to measure a second number of second distance steps in a distance between the reference frame and the support or the substrate table, wherein the second number equals a second integer value plus a second fraction,wherein the position measurement system is constructed and arranged to initialize the second incremental measurement unit on the basis of the first number and the second fraction. 18. A lithographic apparatus according to claim 17, wherein during initialization of the second incremental measurement unit, the second integer value is determined on the basis of a predetermined relationship between the first integer value, the first fraction, the second integer value and the second fraction. 19. A lithographic apparatus according to claim 17, wherein the position measurement system is constructed and arranged to calibrate the second incremental measurement unit on the basis of a position measurement by the first incremental measurement unit. 20. A lithographic apparatus according to claim 17, wherein the first incremental measurement unit comprises a grating and a first encoder head constructed and arranged to co-operate with the grating, wherein the second incremental measurement unit comprises a second encoder head constructed and arranged to co-operate with the grating and wherein the first distance step is a function of a pitch of the grating. 21. A lithographic apparatus according to claim 17, wherein the first incremental measurment unit comprises a reflective surface constructed and arranged to reflect a radiation beam of an interferometer of the position measurement system to a first optical sensor of the first incremental measurement unit or to a second optical sensor of the second incremental measurement unit and wherein the first distance step is a function of a wavelength of the radiation beam. 22. A lithographic apparatus according to claim 17, wherein the position measurement system is constructed and arranged to initialize the first incremental measurement unit on the basis of the second number and the first fraction.