Radio Source

A radio source is an astronomical object that emits detectable radio frequency radiation, such as quasars, active galactic nuclei, or other distant extragalactic objects. Many of these sources are extremely far away and can be considered effectively fixed with respect to the celestial reference frame.

In geodesy, compact extragalactic radio sources are observed by Very Long Baseline Interferometry (VLBI) to realize and maintain the International Celestial Reference Frame (ICRF). Their stable positions provide the fundamental reference for determining Earth orientation parameters, linking terrestrial and celestial reference frames, and supporting precise global positioning and Earth rotation studies.