Instrumental delay calibration with zero baseline interferometry for VLBI time comparison

A highly accurate zero baseline interferometry (ZBI) method for measuring the instrumental delay in VLBI time comparisons has been developed. ZBI is a kind of portable clock method but adds a separately calibratable portable reciever as a reference. Instrumental delay is an important factor in determining the accuracy of time-and-frequency comparisons. Fundamental experiments using the ZBI method show a measuring accuracy of closer than 0.2 ns.