S/X

S/X‑band VLBI is a Very Long Baseline Interferometry technique that simultaneously observes radio signals at S‑band (~2–4 GHz) and X‑band (~8–12 GHz) frequencies. This dual-frequency approach allows correction for ionospheric delays, improving the precision of geodetic and astrometric measurements. In geodesy, S/X‑band VLBI is used to determine Earth Orientation Parameters (EOPs), maintain the International Celestial Reference Frame (ICRF), and measure station positions for the International Terrestrial Reference Frame (ITRF). It is the standard technique for high-accuracy global geodetic VLBI observations.