Место издания:Max-Planck-Institut fur Radioastronomie (Bonn) Bonn (Germany)
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Аннотация:Phase referencing techniques are used to image weak sources, and to measure sub-mas position changes of radio sources. The fast switching (or simultaneous observations) between a calibrator and target source decreases the effects of temporal fluctuations of phase, but does not remove persistent spatial phase difference between the sources. In March 2002 multi-calibrator test observations at 2.3 / 8.4 GHz were made with the VLBA on three different days. By suitably combining the phase measurements from two calibrators near the target source, higher quality images were obtained. The astrometric precision, as measured by the target position jitter among the three days, decreased from 40 μ arcsec using one calibrator to 20 μ arcsec using two calibrators. The systematic position difference between the two calibration methods was ≅ 130 μ arcsec, indicating strong coupling between the phase errors and position determination.