Metrics of Astrometric Variability in the International Celestial Reference Frame: I. Statistical analysis and selection of the most variable sources

TL;DR

This study analyzes the astrometric variability of ICRF3 sources using VLBI data, identifying sources with the most and least positional stability and examining how variability correlates with declination and observational factors.

Contribution

It introduces a comprehensive statistical framework to quantify and compare the astrometric variability of ICRF3 sources, highlighting factors affecting positional stability.

Findings

Excess variance depends on declination, influenced by ionospheric delays.

Identified sources with high and low positional stability.

Variability measures correlate with observational geometry and ionospheric effects.

Abstract

Using very long baseline interferometry data for the sources that comprise the third International Celestial Reference Frame (ICRF3), we examine the quality of the formal source position uncertainties of ICRF3 by determining the excess astrometric variability (unexplained variance) for each source as a function of time. We also quantify multiple qualitatively distinct aspects of astrometric variability seen in the data, using a variety of metrics. Average position offsets, statistical dispersion measures, and coherent trends over time as explored by smoothing the data are combined to characterize the most and least positionally stable ICRF3 sources. We find a notable dependence of the excess variance and statistical variability measures on declination, as is expected for unmodeled ionospheric delay errors and the northern hemisphere dominated network geometries of most astrometric and geodetic observing campaigns.