Beam me up, and down

Laser technology designed originally to detect submarines is to be trialled to help increase airport safety and capacity.

The laser ‘listens’ for traces of dangerous wake vortices forming on an aircrafts’s wingtips as it lands or takes off. The US government has approved funding for tests this year at either Denver, Colorado, or Louisville, Kentucky.

Any aircraft caught in another’s vortices can lose aerodynamic control. There have been 45 such accidents in the last five years, seven of these fatal. Light aircraft flying behind an airliner are especially vulnerable.

The only effective way to prevent vortex-related accidents has been to keep aircraft far apart, especially in poor weather, separating them by six miles rather than the usual three. But this halves the amount of aircraft that can land or take off at a busy airport.

Named SOCRATES (Sensors for Characterising Ring-eddy Atmospheric Turbulence Emanating Sound), the new system relies on the fact that vortices (invisible in normal conditions) emit a distinct sound signature.

Managing the project is US company Flight Safety Technologies. Its president, WilliamCotton, said that SOCRATES consists of four lasers positioned below the flightpath. Positioned 100m away are four reflectors that turn the light back towards detectors.

A vortex coming from the aircraft makes a distinctive sound above the laser beams.said Cotton: ‘As a soundwave passes through the laser, the movement of the air changes its index of refraction.’ The index of refraction refers to how much a medium can ‘bend’ waves travelling through it.

Programme manager Walt Werner from partner company Lockheed Martin NavalElectronics & Surveillance Systems, said: ‘A very small change in the index of refraction means a change in the phase of light.’ A phase shift means that the timing of the wave peak cycle changes, though frequency remains constant.

The phase change is picked up after the laser beam is reflected back into the detectors.

Complex software sorts out the relevant information from background noise to detect where dangerous vortices are forming, then tracks them and assesses how hazardous they are.

The information is then sent to air traffic control, which ultimately decides on appropriate aircraft separations. Though wind and bad weather do affect it, Cotton estimated that with SOCRATES in place, controllers would need to enforce large separations 10 times less than before.

Since 1997, the team has been researching the sound properties of wake vortices, having collected data from JFK airport. In 2000, a NASA Boeing 757 was used to test an early SOCRATES prototype. The technology itself is indirectly derived from a classified US Navy project from the late 1960s that aimed to use laser detection in conjunction with sonar. Werner said the team was continually improving the system, with the next step being to reduce its cost.

It would be around 2008 before the system is ready for commercialisation. ‘We are actively pursuing other applications,’ said Werner, though he declined to name them.