Lidar Measures Wake Vortices
Brent D. Johnson
When a large passenger aircraft takes off or lands, it displaces a sizable volume of air that trails off the wings in a phenomenon known as a wake vortex. This invisible wake whirls up to 100 mph, leaving behind a horizontal tornado with a position and magnitude that pilots and air traffic controllers can only guess at. The scale of this hazard is demonstrated by the National Transportation Safety Board's speculation that wake vortices played a role in the crash of American Airlines Flight 587 last year in Belle Harbor, N.Y.
To enhance the use of runways in inclement weather, laser technology and sophisticated image processing techniques are being used at San Francisco International Airport to estimate the position and strength of wake vortices.
The established method of avoiding wake vortices is to keep a buffer of four to six miles between airplanes. However, even after the crash of Flight 587, there is a perception that these buffers are somewhat conservative and could be narrowed to improve traffic patterns.
The simultaneous offset instrument approach, a near-term procedure under consideration at San Francisco International Airport, could help cut delays during bad weather by allowing two planes to land simultaneously on runways closer than 4300 feet, the current Federal Aviation Administration specification. Before this approach can be considered, it will require measurements and simulations of wake vortices to develop safety criteria. Once those criteria are developed and validated at San Francisco, simultaneous offset instrument approach procedures could be implemented with no wake monitoring required.
The man developing those standards is George Greene, an FAA officer based at NASA's Langley Research Center. Part of his role is to validate computer simulations, which requires a sensor to measure the vortices. In September 2001, he set up the WindTracer system from Coherent Technologies Inc. to begin collecting data at the airport.
Doppler lidar measures the radial velocity of the wind moving toward or away from the detector at altitudes up to about 5000 meters and out to horizontal distances in excess of 10,000 meters along the approach and departure corridors at the airport. The WindTracer bounces laser pulses off aerosols in the atmosphere that work like micromirrors. When the aerosols are immersed in a wake vortex, they move in a characteristic fashion, which induces a Doppler frequency shift onto the light backscattered to the lidar.
The system, built by CLR Photonics Inc., a division of Coherent, relies on a pair of lasers. The first is a continuous-wave device specifically designed for lidar applications. Its output power is in excess of 50 mW in a single-longitudinal mode, or 150 mW in multilongitudinal-mode operation at wavelengths of 2020 to 2030 nm. The second is a solid-state 1-W Tm:LuAG pulsed transmit laser that operates at 2 µm. It produces 2-mJ pulses 500 times per second. The Tm:LuAG laser probes the atmosphere, including the wake vortex.
Backscattered light is collected by a 4-in. superhemispherical scanner, which has a resolution of 0.01° and a scan speed of 20° per second. A real-time advanced signal processor converts the analog signal from the transceiver to a digital signal and computes several types of data products, including radial wind speed and backscatter profiles. By applying sophisticated image processing techniques, it produces estimates of wake vortex position and strength. It is this information that FAA investigators are using.
Greene said that the way to avoid wakes is to make enough measurements to determine safety under all weather conditions. However, the aircraft spacing will be somewhat conservative because of weather variability. At some point, regulators probably will actively monitor wakes to reduce spacing when weather conditions permit.
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