Sidebar: How Lidar Detects Turbulence
Like radar, lidar (light detection and ranging) can calculate objects’ distances, speeds, and rotation rates by directing electromagnetic pulses at them and measuring the pulses that are reflected back. In the case of turbulence detection, the objects are tiny atmospheric particles. But unlike conventional radars, which send radio waves, lidar uses laser light, with wavelengths 10,000 to 100,000 times shorter.
The advantage of lasers is that laser light rays travel parallel to one another in a tight beam, as opposed to a radar’s radio waves, which diffuse in all directions. The concentration increases the odds that the laser light will hit and reflect off of dust and other minute particles—known as aerosols—that lie directly in a lidar beam’s path.
Lidar is thus ideal for detecting clear air turbulence, which has only tiny particles, not large water droplets, to reflect radiation. But for the very same reasons, lidar cannot help in examining the interior of a storm; the laser light would be reflected entirely by the outermost layers of clouds or rain. To see through moisture, conventional radar works better.
To detect CAT, lidars shoot laser pulses into the air ahead of the aircraft, where aerosols are being carried in the same direction and at the same velocity as the wind. The speed of the aerosols is measured by observing the Doppler shift of the laser reflections. If the aerosols are moving away, the returning light waves will have a lower frequency and longer wavelength than those of the original laser light; the shift will be toward the higher frequencies and shorter wavelengths if the aerosols are approaching the laser. By comparing the relative motions of aerosols in a beam’s path, computers on aircraft can predict when CAT is imminent.