Which technique uses large radio telescopes to observe quasars for geodesy?

• A. DORIS
• B. Very Long Baseline Interferometry (VLBI)
• C. Lidar
• D. Global Navigation Satellite System

Answer: (B)

Observing quasars with a network of large radio telescopes is done with Very Long Baseline Interferometry. In VLBI, multiple distant radio dishes observe the same quasar at the same time, and the exact arrival-time differences of the radio waves are recorded and correlated. Those time differences let us treat the separated antennas as one giant telescope, giving extremely precise measurements of the distances and orientations between stations. This ties the terrestrial reference frame to the celestial reference frame defined by quasars, enabling accurate Earth orientation parameters (like UT1 and polar motion) and monitoring of tectonic plate motions.

Other techniques don’t use this combination of wide baselines and quasar references: DORIS tracks satellites with Doppler signals rather than quasar radio waves; Lidar uses laser ranging for local distance measurements; GNSS relies on signals from satellites and ground receivers rather than observing quasars with large radio telescopes.