A method of tracking the position of an object, the method comprising: varying during a measurement period an optical frequency of at least one of (i) a first laser, providing a first output beam, and (ii) a second laser, providing a second output beam; simultaneously directing the first and second output beams into both (a) a reference interferometer having arms with a known optical path difference and (b) a measurement interferometer having arms with an unknown optical path difference, the unknown optical path difference giving rise to an interferometer phase difference for the first output beam and an interferometric phase difference for the second output beam, each interferometric phase difference consisting of an absolute phase offset and an unwrapped phase; obtaining, in the measurement period, reference interference data from the reference interferometer and measurement interference data from the measurement interferometer; distinguishing, on the one hand, interference data resulting from the first output beam from, on the other hand, interference data resulting from the second output beam, in both the reference interference data and the measurement interference data; determining for both the reference interferometer and the measurement interferometer the difference between the absolute phase offset of the first output beam and the absolute phase offset of the second output beam; using the reference interference data from the first output beam, the reference interference data from the second output beam, the measurement interference data from the first output beam, the measurement interference data from the second output beam, and knowledge of the difference between the absolute phase offset of the first output beam and the absolute phase offset of the second output beam for both the reference interferometer and the measurement interferometer to calculate a parameter indicative of the absolute phase offset of the measurement interferometer for the first output beam; and using the calculated parameter to calculate the ratio of the optical path differences of the measurement interferometer and the reference interferometer. varying during a measurement period an optical frequency of at least one of (i) a first laser, providing a first output beam, and (ii) a second laser, providing a second output beam; simultaneously directing the first and second output beams into both (a) a reference interferometer having arms with a known optical path difference and (b) a measurement interferometer having arms with an unknown optical path difference, the unknown optical path difference giving rise to an interferometer phase difference for the first output beam and an interferometric phase difference for the second output beam, each interferometric phase difference consisting of an absolute phase offset and an unwrapped phase; obtaining, in the measurement period, reference interference data from the reference interferometer and measurement interference data from the measurement interferometer; distinguishing, on the one hand, interference data resulting from the first output beam from, on the other hand, interference data resulting from the second output beam, in both the reference interference data and the measurement interference data; determining for both the reference interferometer and the measurement interferometer the difference between the absolute phase offset of the first output beam and the absolute phase offset of the second output beam; using the reference interference data from the first output beam, the reference interference data from the second output beam, the measurement interference data from the first output beam, the measurement interference data from the second output beam, and knowledge of the difference between the absolute phase offset of the first output beam and the absolute phase offset of the second output beam for both the reference interferometer and the measurement interferometer to calculate a parameter indicative of the absolute phase offset of the measurement interferometer for the first output beam; and using the calculated parameter to calculate the ratio of the optical path differences of the measurement interferometer and the reference interferometer.