Lunar Laser Ranging experiments

Lunar Laser Ranging (LLR) is the practice of measuring the distance between the surfaces of the Earth and the Moon using laser ranging. The distance can be calculated from the round-trip time of laser light pulses travelling at the speed of light, which are reflected back to Earth by the Moon's surface or by one of several retroreflectors installed on the Moon. Three were placed by the United States' Apollo program (11, 14, and 15), two by the Soviet Lunokhod 1 and 2 missions, and one by India's Chandrayaan-3 mission.

Although it is possible to reflect light or radio waves directly from the Moon's surface (a process known as EME), a much more precise range measurement can be made using retroreflectors, since because of their small size, the temporal spread in the reflected signal is much smaller.

Laser ranging measurements can also be made with retroreflectors installed on Moon-orbiting satellites such as the LRO.