Abstract
Aims. We propose to estimate the accuracy of current very long baseline interferometry (VLBI) catalogs. Methods. The difference of source position estimated from two decimation solutions was analyzed to estimate the scale factor and noise floor for the formal error of radio source positions by two different methods. In one method, we investigated the weighted root-square-mean (wrms) scatter of source positional differences versus the number of observed sessions; for the other one, we compared the wrms difference versus the formal error. Based on the estimated noise floor and scale factor, we determined the realistic error of radio source positions in the standard solution and compared it with that of Gaia DR2 and ICRF2 catalogs. Results. The estimated scale factors from two methods are rather consistent, which is of ∼1.3 in both coordinates. As for the noise floor, it is estimated to be 20–25 μas for sources observed in at least ten sessions, and it could reduce down to ∼10 μas for sources which have been observed more than 1000 times. The inflated median formal error of our solution is of the same order as the Gaia DR2 catalog in declination and the direction of major axis of the error ellipse, but smaller by a factor of two in right ascension. With respect to the ICRF2 catalog, our solution yields an improved accuracy by a factor of about three. Conclusions. Currently, the VLBI radio source catalog still provides source positions with the best accuracy which is about 20–25 μas. Moreover, the noise floor of VLBI catalogs could potentially reach 10 μas with more observations in the future.
Highlights
Geodetic very long baseline interferometry (VLBI) measures positions of thousands of radio sources since 1979.0
If we assume that the formal errors in ICRF2 and Gaia DR2, and inflated errors in OPA solution are realistic, we could conclude that current VLBI solutions still provide source positions with the best accuracy
We proposed to estimate a realistic error of source positions in VLBI solution based on all the up-to-date VLBI observations
Summary
Geodetic very long baseline interferometry (VLBI) measures positions of thousands of radio sources (quasars) since 1979.0. After inflating the formal error, it is interesting to investigate whether the realistic error of VLBI positions for radio sources could still be better than that of the Gaia DR2 To answer this question, we need to determine the realistic accuracy of current VLBI radio source catalogs by estimating the noise floor and scale factor. Gordon et al (2016) shows that the re-observation of the so-called VCS sources (VCS-II) reduced down the average positional uncertainties for the re-observed VCS sources by nearly a factor of five These new observations lead to most of recent astrometric improvements with respect to the ICRF2 and possibly contribute significantly to reducing the noise floor of the generation of ICRF (ICRF3; Gordon 2017).
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