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

TL;DR

This paper introduces a novel 'merged' VLBI astrometric analysis technique that combines advantages of existing methods, improving accuracy and applicability, especially for weak sources, and demonstrates its effectiveness through comparison with traditional approaches.

Contribution

The paper presents a new merged analysis method for VLBI astrometry that enhances model correction and source measurement capabilities, outperforming traditional techniques in accuracy and weak-source analysis.

Findings

The merged technique yields superior astrometric results compared to phase-referenced maps.

It enables astrometric measurements of sources too weak for parametric model fits.

Application to IM Pegasi supports the Gravity Probe B mission.

Abstract

When VLBI observations are used to determine the position or motion of a radio source relative to reference sources nearby on the sky, the astrometric information is usually obtained via: (i) phase-referenced maps; or (ii) parametric model fits to measured fringe phases or multiband delays. In this paper we describe a "merged" analysis technique which combines some of the most important advantages of these other two approaches. In particular, our merged technique combines the superior model-correction capabilities of parametric model fits with the ability of phase-referenced maps to yield astrometric measurements of sources that are too weak to be used in parametric model fits. We compare the results from this merged technique with the results from phase-referenced maps and from parametric model fits in the analysis of astrometric VLBI observations of the radio-bright star IM Pegasi (HR 8703) and the radio source B2252+172 nearby on the sky. In these studies we use central-core components of radio sources 3C 454.3 and B2250+194 as our positional references. We obtain astrometric results for IM Peg with our merged technique 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 to the phase-referenced mapping technique. We used our merged technique to estimate the proper motion and other astrometric parameters of IM Peg in support of the NASA/Stanford Gravity Probe B mission.