Which technique uses retro-reflectors on satellites to measure distance by sending a laser pulse and timing its return?

• A. Lidar
• B. Satellite Laser Ranging (SLR)
• C. Very Long Baseline Interferometry
• D. DORIS

Answer: (B)

Distance to a satellite is measured by sending a short laser pulse to a satellite that carries retro-reflectors, and timing how long it takes for the pulse to return. Since light travels at a known speed, the round-trip time directly gives the distance to the satellite. Repeating these measurements from multiple ground stations and over time lets us determine the satellite’s orbit with high precision and tie together a global reference frame.

This technique is distinct from lidar, which measures distances to Earth surfaces rather than to satellites; from very long baseline interferometry, which uses radio waves and interferometric timing to determine Earth orientation and baselines; and from DORIS, which relies on Doppler beacons rather than laser timing.