OPTOELECTRONICS FOR AVIATION: Laser Doppler system for small aircraft measures wind shear
Swan International (Sydney, Australia), a startup firm developing laser-based wind-shear-detection systems for small aircraft, has moved a step closer to its goal of launching a commercial version of this technology in the United States, having received a U.
Swan International (Sydney, Australia), a startup firm developing laser-based wind-shear-detection systems for small aircraft, has moved a step closer to its goal of launching a commercial version of this technology in the United States, having received a U.S. patent covering the system and its primary application. According to the company, the system operates in the eye-safe 1.5- to 1.8-µm wavelength range and is designed for airborne wind-shear detection in fixed-wing, general-aviation, business, regional, and transport aircraft.
In general, wind shear results from rapidly changing wind conditions. In the context of aviation science, wind shear refers to a wind speed or direction change experienced by an airplane over a particular distance or length of time. This definition covers a wide range of meteorological phenomena, including convective turbulence, gust fronts, microburst, internal gravity waves, vertical shear, and terrain-influenced flow. 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; an aircraft exposed to low-level wind shear may suffer a critical loss of airspeed and altitude, thus endangering its ability to remain airborne.
Studies have shown that pilots generally need 10 to 40 seconds of warning to avoid wind shear; fewer than 10 seconds is not enough time to react, while more than 40 seconds is too long because atmospheric conditions can change in that time. An airborne forward-looking wind shear detector can thus play a critical role in helping to avoid airplane crashes by warning a pilot that he or she is approaching a wind shear hazard in time for the pilot to do something about it. Several such advance-warning systems have been developed and tested over the years, including microwave radar, Doppler lidar, and IR detection; today, the predominant system in the commercial aviation field is radar.
Safety for smaller craft
The compact Swan laser system is designed to do for smaller aircraft what radar-based wind-shear-detection systems now do for larger commercial and military airplanes. Mounted on the nose of the airplane, the laser-transmitter module generates a pulsed beam (currently from a 1550-nm-emitting erbium laser) that is directed out in front of the aircraft to measure wind during takeoff and landing by determining the Doppler shift of the backscattered radiation from the aerosols the beam encounters (see figure).
The detector module incorporates a pulse-delay module, a preamplifier, and a dual-differential Mach-Zehnder interferometer. Highly sensitive fiber-coupled avalanche-photodiode detectors detect very low light levels in the return signal. The interferometer measures the frequency of the collected scattered light versus that from a sample from the transmitting laser; the resulting frequency shift determines the presence of wind shear in the anticipated trajectory of the aircraft. If wind shear is detected, the warning module alerts the pilot. The system also incorporates a global-positioning system to determine position and ground speed.
While the company declines at this point to discuss the system’s laser source in detail, pending additional design and patent applications, Chief executive officer Brian McGuire says their strategy from the beginning has been to work with near-IR lasers that can be tested, certified, and safely used without goggles. In fact, Swan engineers have based the company’s laser system on the same IEC (International Electrotechnical Commission) standard (60825) used in free-space optics.
Swan has developed several prototypes of the wind-shear-detection system and has been evaluating the system in ground-test mode at Bankstown Airport (Sydney) for the past year. The company has also modified an aircraft for airborne testing and expects to begin that testing in the next few months, once the prototype has been upgraded to pass the relevant environmental tests.