Patent ID: 7268888

Claim:
A method, comprising: directing a first measurement beam along a first path contacting a measurement object to determine a first interferometric phase related to a position of a measurement object; directing a second measurement beam along a second path contacting the measurement object to determine a second interferometric phase related to the position of the measurement object; determining a contribution to the first interferometric phase due to perturbations in refractivity along the first path based on the second interferometric phase, wherein the contribution to the first interferometric phase due to perturbations in refractivity at a first time is related to a contribution to the second interferometric phase due to perturbations in refractivity at a second time different from the first time and a difference between the first time and the second time is related to a velocity of a gas in the first path; and adjusting a relative position between the measurement object and another object based on the determined contribution.