A geodetic technique using large, ground-based, parabolic-dish radio telescopes to observe quasars (the most distant objects in the cosmos).

• A. Very Long Baseline Interferometry (VLBI)
• B. Global Positioning System
• C. Doppler Orbitography and Radiopositioning Integrated by Satellite
• D. Lunar Laser Ranging

Answer: (D)

This item is about very long baseline interferometry, a geodetic technique that uses multiple large ground-based radio telescopes to observe distant quasars. By recording the arrival times of the same radio signal at widely separated antennas, scientists cross-correlate the data to measure tiny time delays. These delays translate into precise measurements of the baselines between telescopes, which in turn determine the Earth's orientation in space, the rotation of the Earth (UT1), and tectonic motions with extraordinary accuracy. Quasars serve as fixed, distant reference points, so their signals provide a stable celestial frame for linking the telescope network.

The described method relies on coordinating several big radio antennas to capture the same quasar signal and then compare the timing, which is exactly how VLBI operates. The other options involve different techniques: GPS uses satellites and ground receivers to triangulate position, DORIS tracks satellites via Doppler shifts, and Lunar Laser Ranging measures distance to the Moon with laser pulses. None of those use large parabolic-dish radio telescopes observing quasars.