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When VLBI observations are used to determine the position or motion of aradio source relative to reference sources nearby on the sky, the astrometricinformation is usually obtained via: (i) phase-referenced maps; or (ii)parametric model fits to measured fringe phases or multiband delays. In thispaper we describe a "merged" analysis technique which combines some of the mostimportant advantages of these other two approaches. In particular, our mergedtechnique combines the superior model-correction capabilities of parametricmodel fits with the ability of phase-referenced maps to yield astrometricmeasurements of sources that are too weak to be used in parametric model fits.We compare the results from this merged technique with the results fromphase-referenced maps and from parametric model fits in the analysis ofastrometric VLBI observations of the radio-bright star IM Pegasi (HR 8703) andthe radio source B2252+172 nearby on the sky. In these studies we usecentral-core components of radio sources 3C 454.3 and B2250+194 as ourpositional references. We obtain astrometric results for IM Peg with our mergedtechnique even when the source is too weak to be used in parametric model fits,and we find that our merged technique yields superior astrometric results tothe phase-referenced mapping technique. We used our merged technique toestimate the proper motion and other astrometric parameters of IM Peg insupport of the NASA/Stanford Gravity Probe B mission.

VLBI for Gravity Probe B. IV. A New Astrometric Analysis Technique and a Comparison with Results from Other Techniques