VLBA Scientific Memorandum n. 31: Astrometric calibration of mm-VLBI using "Source/Frequency Phase Referenced" observations

TL;DR

This paper introduces the Source/Frequency Phase Referencing (SFPR) method for high-precision astrometry in mm-VLBI, overcoming atmospheric limitations of conventional techniques, demonstrated at 86 GHz with the VLBA.

Contribution

The paper presents a novel SFPR technique combining fast frequency-switching and source switching to enable accurate astrometry at mm-wavelengths, where traditional methods fail.

Findings

Successful demonstration of SFPR at 86 GHz VLBA observations.

Equivalent results to conventional methods at cm wavelengths.

First effective astrometric application at mm-wavelengths.

Abstract

In this document we layout a new method to achieve "bona fide" high precision Very-Long-Baseline-Interferometry (VLBI) astrometric measurements of frequency-dependent positions of celestial sources (even) in the high (mm-wavelength) frequency range, where conventional phase referencing techniques fail. Our method, dubbed "Source/Frequency Phase Referencing" (SFPR) combines fast frequency-switching (or dual-frequency observations) with the source switching of conventional phase referencing techniques. The former is used to calibrate the dominant highly unpredictable rapid atmospheric fluctuations, which arise from variations of the water vapor content in the troposphere, and ultimately limit the application of conventional phase referencing techniques; the latter compensates the slower time scale remaining ionospheric/instrumental, non-negligible, phase variations. For cm-VLBI, the SFPR method is equivalent to conventional phase referencing applied to the measurement of frequency-dependent source positions changes ("core-shifts"). For mm-VLBI, the SFPR method stands as the only approach which will provide astrometry. In this memo we layout the scope and basis of our new method, along with a description of the strategy and a successful demonstration of the application of this new astrometric analysis technique to the highest frequency VLBA observations, at 86 GHz. Our previous comparative astrometric analysis of cm-VLBI observations, presented elsewhere, produced equivalent results using both methods.