Operating similar to a sonar the Doppler SODAR (Sonic Detection and Ranging) directly measures the atmospheric wind speed and direction at multiple altitudes simultaneously. These systems utilize the Doppler effect imposed on the received acoustic signal echoed by the atmosphere for the wind measurements. The Doppler effect is the common observation that the acoustic signal frequency increases (decreases) for motion toward (away) from a receiver. By applying this principle and considering the geometry of the Doppler SODAR system it becomes a cost effective and easily maintained “invisible tower” for wind measurements.
Wind shear is a generic term referring to any rapidly changing wind currents. This change can be associated with either the wind speed or direction. A type of weather phenomenon called 'microbursts' can produce extremely strong wind shear, posing great danger to aircraft. These are local, short-lived downdrafts that radiate outward as they rush toward the ground. As a downdraft spreads down and outward from a cloud, it creates an increasing headwind over in airspeed, and the plane lifts. If the pilots are unaware that this speed increase is caused by wind shear, they are likely to react by reducing engine power. However, as the plane passes through the shear, the wind quickly becomes a downdraft and then a tailwind. This reduces the speed of air over the wings, and the extra lift and speed vanish. Because the plane is now flying on reduced power, it is vulnerable to sudden loss of airspeed and altitude. The pilots may be able to escape the microburst by adding power to the engines. But if the shear is strong enough, they may be forced to crash.
About 500 fatalities and 200 injuries from wind shear related crashes occurred between 1964 and 1985. This involved at least 26 civilian aircraft. Since 1985 wind shear related accidents have been reduced but numerous near accidents continue to occur in which the aircraft recover control prior to ground contact.
Wind shear poses the greatest danger to aircraft during takeoff and landing, when the plane is close to the ground and has little time or room to maneuver. During landing, the pilot has already reduced engine power and may not have time to increase speed enough to escape the downdraft. During takeoff, an aircraft is near stall speed and thus is very vulnerable to wind shear.
Microburst related wind shear often occurs during thunderstorms. But it can also arise in the absence of rain near the ground.
A brief statement of airport applications for doppler SODAR systems