What technique defines the International Celestial Reference Frame (ICRF) and introduces inherent uncertainties?

Very Long Baseline Interferometry (VLBI)

The International Celestial Reference Frame (ICRF), which provides the foundation for the celestial coordinate system, is realized through observations using Very Long Baseline Interferometry (VLBI). This technique links radio telescopes across vast distances to achieve high angular resolution needed to locate the distant quasars that define the frame's fixed points. However, VLBI processing inherently involves systematic biases and uncertainties. If the mathematical models used to process this interferometric data contain systematic flaws, these biases are inherited by the ICRF, thereby imposing an unavoidable floor on the precision achievable by optical astrometry derived from it.