Coherence limits in VLBI observations at 3‐millimeter wavelength

Very long baseline interferometry (VLBI) experiments at a frequency of 89 GHz (3.4‐mm wavelength) using hydrogen maser frequency standards show that under good atmospheric conditions coherence can be maintained for times up to 700 s, corresponding to an Allan standard deviation of approximately 10 −14 . The stability appears to be largely limited by the phase noise resulting from fluctuations in delay through the troposphere. Methods of estimating the interferometer fringe amplitudes and closure phases in the presence of large phase fluctuations are examined. The spectrum and the Allan standard deviation of the observed phase fluctuations are derived.