Introduction to Lidar

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These are two lidar trailers behind Building 21 at Goddard Space Flight Center. You can see the bright green laser light coming out of the roof of one of the trailers. Measurements are made at night; otherwise, sunlight would overwhelm the small signal of laser light scattered back to the instrumentation in the trailers.

Lidar as a Remote Sensing Instrument

Lidar is an acronym standing for Light Detection and Ranging. It is the direct, optical analog to Radar, which uses radio waves. In a typical Lidar experiment, laser light is transmitted into the atmosphere. The beam can interact with the atmosphere in a number of different ways;

• Laser light can be scattered elastically (i.e. with no change in wavelength or color) from molecules in the atmosphere (Rayleigh Scattering), and from particles (Mie Scattering).
• Laser light can be scattered inelastically from molecules in the atmosphere. In this instance, the wavelength of the scattered light is shifted, and the change in wavelength is dependent on the molecule which scattered the light. This is known as Raman Scattering.
• Laser light can be absorbed by a specific molecule in the atmosphere. In small molecules, this absorbed energy can then be radiated by the molecule at the same, or at another, wavelength. This process is known as fluorescence. Under certain circumstances, this can be particularly sensitive and specific, since the absorbed wavelength is molecule specific and so is the fluorescence wavelength.
• Laser light can be absorbed, but not radiate optically. This has the result of reducing the Rayleigh scattering.

Laser light that has been scattered in any of these ways can then be detected using a telescope and light sensitive detectors. Add some electronics to acquire and store the signals from the detectors, and you have a typical Lidar instrument. Distance of the scattering molecules or particles from the transmission site can be deduced if a pulsed laser is used; one simply measures the time from the laser pulse to the time that the Lidar signal is received. Instruments using these various scattering mechanisms have been designed and built within Code 916.

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