The lidar (Light Detection And Ranging) technique is a remote sensing measurement technique using the scattering properties of light by gases, liquids, and solids in order to infer their physical or chemical properties. This technique has many applications from either the ground, aircraft or space such as atmospheric sensing, medicine, oceanography, and topography.
A laser beam is sent into the atmosphere. The light is scattered by the atmospheric molecules and particles, and a fraction is collected back on the ground with a telescope. Knowing the speed of light, the distance to a scattering molecule or particle is deduced from the travel time of the photons on their way upward and then back to the lidar. Since the lidar comprises the light source itself, the lidar technique is known as an ?active remote sensing? technique, in contrast with other remote sensing instruments such as radiometers and spectrometers. The precise timing of the lidar-measured samples and the high-speed electronics available today yield a high vertical resolution (from a few meters to a few hundred meters) compared to most passive instruments which are sensitive to light reaching the instrument without precise temporal information of its origin. The light collected by the lidar telescope is geometrically and spectrally separated (e.g., with optical filters and beam splitters) and detected with photosensitive devices (photomultipliers, abbreviated PMTs) where it is converted to electro-photon counts, the so-called ?lidar signals?. The signals are sampled in time (i.e., distance) and after various corrections are proportional to the product of the number of photons emitted by the number of backscattering molecules. This proportionality is expressed by the so-called ?lidar equation?. This equation is the starting point for the retrieval of many atmospheric properties.
There are currently 30 lidar instruments worldwide contributing to NDACC. These instruments use three different lidar techniques to retrieve atmospheric temperature, ozone and aerosol properties:
+ Rayleigh backscatter integration technique for temperature (typically 30-80 km, down to 10 km if using Raman channels):
Eureka (Canada), Thule (Greenland), Ny-Alesund (Spitzbergen), Sondrestromfjord (Greenland), Andoya (Norway), Hohenpeissenberg (Germany), Haute-Provence (France), London (Canada), Table Mountain (California), Mauna Loa (Hawaii), Reunion Island (Indian Ocean), STROZ mobile system (USA), AT mobile system (USA).
+ Rayleigh/Raman backscatter technique for aerosols and clouds (typically 20-40 km):
Eureka (Canada), Ny-Alesund (Spitzbergen) , Thule (Greenland) , Sondrestromfjord (Greenland), Andoya (Norway), Hohenpeissenberg (Germany), Haute-Provence (France), Boulder (Colorado), London (Canada), Tsukuba (Japan), Table Mountain (California), Mauna Loa (Hawaii), Reunion Island (Indian Ocean), Lauder (New Zealand), Dumont d?Urville (Antarctica), McMurdo (Antarctica), STROZ mobile system (USA) , AT mobile system (USA), MARL mobile system (Germany), ComCAL mobile system (Germany).
+ DIAL technique for ozone (typically 10-45 km in the stratosphere, 1-10 km in the troposphere):
Eureka (Canada), Ny-Alesund (Spitzbergen), Andoya (Norway), Hohenpeissenberg (Germany), Haute-Provence (France), Huntsville (Alabama), Tsukuba (Japan), Table Mountain (California), Mauna Loa (Hawaii), Reunion Island (Indian Ocean), Lauder
(New Zealand), Rio Gallegos (Argentina), Dumont d?Urville (Antarctica), STROZ mobile system (USA).
Other techniques are also used within NDACC. Polarization and Raman backscatter for aerosol and cloud properties. Vibrational Raman backscatter for water vapor and lower stratospheric temperature, and rotational Raman backscatter for tropospheric temperature. Water vapor Raman lidars have also been accepted for inclusion into NDACC. Contributing instruments are still being added.