Quasar

A quasar (quasi‑stellar radio source) is an extremely luminous and distant active galactic nucleus powered by a supermassive black hole accreting matter. Quasars emit energy across the electromagnetic spectrum and often outshine their host galaxies. In geodesy, quasars are used as nearly fixed reference points in the sky because of their immense distance, making them essential for defining the International Celestial Reference Frame (ICRF). Very Long Baseline Interferometry (VLBI) observations of quasars provide precise positions that underpin global geodetic measurements, Earth orientation parameters, and satellite navigation systems.