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W.F. Brisken (Princeton University), W.M. Goss (NRAO), E. Fomalont (NRAO)
Phase referencing is a radio interferometry technique that allows accurate measurements of the separation between a calibrator source and the source being studied. While this technique removes the bulk effect of the ionosphere and troposphere, gradients across the sky remain uncorrected. At low frequency the uncalibrated ionosphere dominates, producing distorted images. The delay resulting from the ionosphere is dispersive, allowing multifrequency observations to model and remove the remaining ionosphere's effect. This technique was applied to a pulsar astrometry project observed at 20cm with the VLBA. Pulsar positions accurate to less than a milliarcsecond were achieved which will allow a parallax measurement to be made with milliarcsecond accuracy.
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