Laser Remote Sensing of the Atmosphere
Measurement of surface and atmospheric winds is best accomplished using a specific Laser Remote Sensing technique known as Laser Doppler Velocimetry (LDV).
Using this technique, laser pulses emitted from the sensor are reflected from microscopic particulates naturally entrained in the air. These particulates are comprised of microscopic desert sand, volcanic ash and cosmic dust that filters through the Earth’s atmosphere from space as well as other atmospheric scatterers. The effect of a laser scattering from these types of particulates can be observed by shining a laser pointer in the air at night or at any laser light show, it is in fact the only reason that the laser can be seen. Since the scatterers move with respect to sensor (either the sensor moving through the air on an aircraft or the wind moving past the ground based sensor), the color (wavelength) of the laser light that returns to the source is shifted slightly. This shift in color, known as a Doppler Shift, of the laser light is a measure of the speed at which the air is moving relative to the sensor. This Doppler Shift is measured by the sensor, giving a direct reading of the airspeed or wind speed. By measuring this shift in at least three non-collinear directions, a full three-dimensional picture of the aircraft movement through the air or the wind movement around the runway is created.
This same technique can be used to measure the speed and direction of hard targets such as cars, tanks, and other moving objects, or even the relative movement of an aircraft over the ground.