Airspeed indicator or airspeed gauge is an instrument used in aircrafts to display the craft’s airspeed, typically in knots. The airspeed indicator is used by the pilot during all phases of flight, from take-off, climb, cruise, descent and landing in order to maintain airspeeds specific to the aircraft type and operating conditions as specified in the Operating Manual. A new way to measure an aircraft’s air speed using ultraviolet lasers has been developed by the ATC. By using the unique properties of light, this technique works at any altitude and even at low speeds where conventional methods struggle.

At present, pilots rely on devices called ‘Pitot’ tubes to tell them how fast their aircraft is moving. These tubes stick out into the airflow around the craft and work by sensing differences in air pressure. At low speeds their accuracy falls so for hovering jets or helicopters, air speed measurement can be a problem. Protruding fittings such as Pitots can also compromise the radar stealth of a military jet.

The new laser air speed system gets round these problems by bouncing light off air molecules and analysing the changes in reflection caused by the air’s motion relative to the aircraft. “The idea is to point a beam of light out of the aircraft and look at the back-scattered light” says Leslie Laycock, optical systems specialist at the ATC. “The returning light has a frequency ‘Doppler shift’ that is proportional to the air speed. Measure the Doppler shift and you can calculate the air speed” he says.

The Doppler shift for light is similar to the effect of the rising and falling in pitch heard from a police siren as it passes. The faster the vehicle moves, the greater the shift in pitch. “The advantage of this method is that the Doppler shift is directly proportional to the air speed whereas the Pitot tube measurements are non linear and inaccurate at low speeds. The laser system can even measure backward air speed – important if you’re in a helicopter” says Leslie.

Similar methods have been tried in the past that relied on light reflected from moisture and dust particles suspended in the air but these were only effective at low altitudes and favorable atmospheric conditions. The innovation of the ATC approach is to use light reflected from the air molecules themselves. This works at any level in the atmosphere. Laser light can be projected from small lenses which lay flush with a fuselage or wing without jutting out into the airflow. This is an important advantage over the Pitot tubes, not only for radar stealth but it also reduces slightly the overall drag and hence increases fuel efficiency of the plane. With oil prices set to remain at record levels and stringent environmental carbon emission targets to be met, this feature is a welcome bonus.

LASSI (Laser Air Speed Sensor Instrument) is a compact remote airspeed measurement system which uses a UltraViolet (UV) laser. This system is designed to replace the Pitot tube technology currently in use. LASSI is a collaborative project sponsored by the DTI. BAE Systems has teamed up with innovative laser manufacturer AOT Ltd. and Hull University to develop this new system.