Lidar

Lidar (Light Detection and Ranging) is an active optical sensing technique that emits short laser pulses and measures the round-trip travel time of returned photons to determine precise ranges and characterise surface or atmospheric properties at high vertical and spatial resolution.

Laser wavelengths span UV (355 nm), visible green (532 nm), and near-infrared (1064 nm) depending on application. Range is derived from the round-trip photon travel time: distance = (speed of light x time) / 2. Four principal subtypes operate in EO. Altimetry sensors time pulse returns from land, ice, or ocean surfaces to determine elevation and vegetation canopy height (ICESat-2 ATLAS operates at 532 nm photon-counting with approximately 17 m footprint and centimetre-level elevation precision). Atmospheric backscatter lidars profile aerosols, clouds, and boundary layer structure using dual-wavelength depolarisation to discriminate particle types (CALIPSO, 532+1064 nm). Doppler wind lidars use coherent detection of the Doppler frequency shift in aerosol returns to retrieve atmospheric wind vector profiles from 0 to 30 km altitude (ADM-Aeolus, 355 nm UV). Differential Absorption Lidar (DIAL) transmits at paired wavelengths straddling a molecular absorption line to retrieve trace gas column or profile concentrations (MERLIN, planned 1645 nm methane IPDA lidar).

Representative missions include ICESat-2 (NASA, 532 nm photon-counting, ice and vegetation altimetry, launched 2018), CALIPSO (NASA/CNES, aerosol and cloud profiling, 2006-2023), ADM-Aeolus (ESA, Doppler wind lidar, 2018-2023), GEDI (NASA, 1064 nm waveform lidar for global forest canopy structure on the ISS, 2019 onward), and EarthCARE (ESA/JAXA, ATLID 355 nm backscatter lidar, launched 2024).

Principal applications are ice sheet and glacier elevation change and mass balance, above-ground biomass and forest canopy height (GEDI), global aerosol and cloud vertical profiling, atmospheric wind profiles for numerical weather prediction assimilation, coastal bathymetry via 532 nm water penetration, and trace gas column retrieval via DIAL.

Lidar's strengths lie in its high vertical resolution (metres to tens of metres in altimetry and profiling) and active illumination enabling day-and-night operation. Core constraints are blockage below optically thick cloud layers, a narrow swath yielding low areal coverage per pass, high power demand that limits laser operational lifetime, and complex noise-filtering requirements for photon-counting data.