laserranging

Laser ranging is a measurement technique that determines distance by timing how long it takes laser light to travel to a target and back, or by measuring the relative phase of continuously emitted light. It uses the speed of light in air (and corrections for atmospheric index) to convert time or phase measurements into distance. The method can achieve millimeter-scale accuracy in specialized systems and is widely used in geodesy, surveying, and lidar applications.

There are two main approaches: pulsed laser ranging and phase-based continuous-wave ranging. In pulsed systems, a short laser pulse is fired and the time of arrival of the reflected pulse is measured with fast detectors and timing electronics. In phase-based systems, the phase difference between emitted and received light is tracked, yielding high precision but requiring coherent detection and unambiguous range through phase unwrapping.

Common platforms include satellite laser ranging (SLR) using large telescopes to send pulses to satellites with retroreflectors, lunar laser ranging (LLR) to measurements of distance to the Moon, and terrestrial lidar (or 3D lidar) for mapping surfaces, forestry, and architecture. Airborne and mobile lidar enable rapid topographic surveys; automotive lidar supports autonomous navigation.

Key components include a laser source (pulsed or CW with phase-locking), telescope or optics for transmitting and collecting light, fast photodetector or APD, timing system (time-to-digital converter or phase-locked loop), and data processing. Challenges include atmospheric attenuation and turbulence, background light, detector noise, and calibration.

History and organizations: Laser ranging emerged after the invention of the laser; major international programs include the International Laser Ranging Service (ILRS) coordinating SLR operations.