Very Long Baseline Interferometry in Celestial Reference Frame Analysis

Very Long Baseline Interferometry (VLBI) constitutes the principal technique for establishing and maintaining the International Celestial Reference Frame (ICRF), the foundation of high-precision astrometry and geodesy. By linking radio telescopes separated by intercontinental distances, VLBI measures the time delays of extragalactic radio signals with microarcsecond precision. These measurements yield accurate angular positions of compact active galactic nuclei that serve as fixed fiducial points on the sky. Continuous observations over decades have enabled successive realisations of the ICRF, refining its axes through careful selection of defining sources and rigorous modelling of propagation effects, Earth orientation parameters and instrumental delays. Recent advances address systematic errors arising from source structure, ionospheric and tropospheric delays, and the solar system’s galactocentric acceleration. Dual- and multi-frequency observations at S, X, K and Ka bands have been introduced to mitigate frequency-dependent biases and achieve sub-milliarcsecond stability. The resulting celestial frame underpins spacecraft navigation, deep-space tracking and the alignment of optical and radio astrometric catalogues. Its stability ensures global interoperability of astronomical datasets, supports tests of fundamental physics and fosters synergies with space missions such as Gaia, which connects the optical and radio reference frames with unprecedented fidelity.

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Very Long Baseline Interferometry in Celestial Reference Frame Analysis publication trend

The graph below shows the total number of articles in very long baseline interferometry in celestial reference frame analysis across all publications each year (not limited to Nature Index journals).

Technical terms

Very Long Baseline Interferometry (VLBI): A technique that combines signals from widely separated radio telescopes to measure time delays and derive microarcsecond-accurate source positions.

Celestial Reference Frame (CRF): A set of extragalactic radio-source coordinates defining a non-rotating, inertial coordinate system for astrometry and geodesy.

Defining Sources: A selected subset of compact, stable radio-loud active galactic nuclei used to fix the axes of the celestial reference frame.

Group Delay: The measured time lag between wavefront arrivals at different telescopes, fundamental for deriving source position estimates.

Closure Delays: Combinations of delays around closed telescope triangles that are insensitive to station-based errors and sensitive to source structure.

References

1. Metrics of Astrometric Variability in the International Celestial Reference Frame. I. Statistical Analysis and Selection of the Most Variable Sources. The Astrophysical Journal Supplement Series (2024).
2. Mitigating the effect of source structure in geodetic VLBI by re-weighting observations using closure delays and baseline-to-jet orientation. Journal of Geodesy (2024).
3. The third realization of the International Celestial Reference Frame by very long baseline interferometry⋆. Astronomy & Astrophysics (2020).
4. Measurement of the solar system acceleration using the Earth scale factor. Astronomy & Astrophysics (2018).