Phase delay connection in differential VLBI

Differential Very Long Baseline Interferometry (VLBI) is a technique to measure the angular separation between two nearby celestial-radio-sources accurately by observing each source alternately. The connection of fringe phases among consecutive scans is recognized as an important issue to improve the measurement accuracy in the differential VLBI. Fluctuations of fringe phase limit the gap length over which fringe phases can be connected. Thus an antenna-switching period should be less than this limit length. Therefore the maximum limit of gap length as well as source strength is a critical parameter for preparing an observation schedule. This parameter thought to be strongly related to weather condition and site location. A number of researchers investigated statistical characteristics of atmospheric phase fluctuations by using a radio interferometry including VLBI. Some hints regarding a gap limit may be obtained from their results of Allan variance and/or structure function, but it is not a straightforward one and the location of antenna is different from our concerned one. We have therefore carried out a series of VLBI sessions dedicated to investigation of fringe phase fluctuations in order to obtain a realistic scan gap limit. At first we developed a simple method to connect phase delays (fringe phases) between two consecutive scans. Then the relation between a scan length and a gap length was investigated statistically. Results will be reported for both the X and S bands.