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Abstract-Frequency differences between major national timing centers are being resolved with uncertainty of less than 1 part in 1014',using satellites of the Global Positioning System (GPS) in common-view. Portable clock and GPS time differences are in excellent agreement. Around the world GPS measurement between three laboratories had a time residual of 5.1 ns.

In interferometry ideal plane or spherical waves are supposed to propagate between the reflecting surfaces. Diffraction effects however, due to the finite diameter of the beams, cause distortions of the wavefronts, changing fringe position and contrast of the interference pattern. Corrections for length measurements with a two beam interferometer have been calculated. If for example the wavelength of a He-Ne-laser, stabilized by saturated absorption of methane at 3.39 μm, is determined by a precision length measurement in a Michelson interferometer with a beam diameter of 25 mm and a path difference of 400 mm a correction of Δλ/λ = 5.10-9 was found.

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