High Precision Relative Astrometry by Means of '4-Element' VLBI Observations

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This research aims at measuring with high accuracy the relative positions of closely spaced radio sources by making use of simultaneous multi-view differential VLBI observations of those sources with '4-element' VLBI system. Since destructive effects of phase fluctuations occurring in the atmosphere and frequency standards must be largely compensated in this method of observations, one can expect to obtain very precise VLBI fringephase difference as an observable. For the purpose, we used 45-m mm-wave telescope and 6-m VLBI antenna at Nobeyama Radio Observatory/NAO, as well as 34-m and 26-m antennas at Kahshima Space Research Center of Communications Research Laboratory, forming dual baselines with length of about 200 km. After three preparatory experiments held in March and May of 1992, we successfully conducted the main 24-hour observations on June 27 - 28, 1992.10 pairs of radio sources with angular separation ranging from 0.074 to 10.35 degrees, as well as a gravitational lens cand … More idate PKS1830-211, were observed. Each pair was observed with two baselines alternately or by switching to a strong calibration source for the purpose to compensate the effects of the phase delay within the signal transmission systems. Cross-correlation processing carried out with K-3 correlator at Kashima revealed good fringes for almost all sources, thus showing the basic feasibility of the 4-element VLBI observations and yielding a number of multi-view differential VLBI data for the first time. The data were format-converted and now are analyzed on the main frame computer of NAO, Mizusawa.Along with the experiments, we successfully developed a statistical model of the atmospheric fluctuation effects upon the multi-view differential VLBI observations, and showed that the high accuracy is really achievable as far as the atmosphere is the primary limiting factor for the precise observations. We developed also an promising algorithm of long-time integration for relatively weak radio sources and models for phasedelays due to the distortion of the transmission cables. These results will be used in interpreting the analyzed data. Less