Laser radar scans the sky

The flight of a modern jet aircraft generates strong vortices in its wake. These spiralling air masses trail off each wing tip and are a natural consequence of the lift generated by wings.

Strong circulating vortices generated behind aircraft exist for many minutes after the aircraft has passed. Smaller aircraft can be in danger if they encounter wake vortices during landing. Rapid rotating flows of air have led to aircraft crashing during the final stages of landing and aviation authorities have mandated minimum separations between aircraft subject to air traffic control.

But there is a limited understanding of how wake vertices are generated, evolve and dissipate in the airport environment. Wind tunnel measurements allow the early stages of wake vortices to be studied under ideal conditions, but do not develop a knowledge of how environmental features affect the dissipation of vortices around airports.

At the DERA, a Laser Radar, or Lidar system has been developed that can image the wake vortices trailing off landing aircraft in real time. It can monitor the size and strength of the wake vortices and even follow how the wake vortex moves under the influence of prevailing winds and ground features like trees and buildings.

Laser Radar works much like conventional radar, but the scene is illuminated with a safe laser beam. Minute particles in the atmosphere, like dust, pollen, and water droplets are swirled around by the air movement and scatter the invisible laser beam. This scattered light is collected by the Laser Radar and the Doppler shifted signal is analysed to yield information about the particles motion, and thus the air movement. Scanning the laser beam across the sky produces a picture of the wake vortex.

Recent work funded by the Civil Aviation Authority at Heathrow Airport has produced unique information about wake vortices from a range of civil aircraft. Preliminary analysis has shown that under most conditions, wake vortices weaken and fall toward the ground out of the flight path in a few minutes. Occasionally, one of the two vortices generated by every aircraft can initially fall and then rise up back into the path of an incoming aircraft under the influence of local ground effects, such as trees. Separations between aircraft landing must ensure that such occasions do not lead to danger.

In the past, it was believed that the strength of the wake vortex generated by an aircraft only depended on the weight, speed and wingspan of the aircraft. But the work at Heathrow Airport has indicated that there are noticeable and consistent differences between the wake vortex structure of each type of aircraft which could determine the rate at which the vortices decay.

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