New sensor measures aircraft velocity by bouncing lasers off air molecules

Known as the Laser Air Speed Sensing Instrument (LASSI), the new system was developed by scientists in Chelmsford in the UK(Credit: BAE Systems)

British defense and aerospace company BAE Systems has come up with some interesting technology over the years, including armored vehicles that used Formula 1 suspension tech, and army helmets that use bone conduction for comms. Now, it's looking to improve how we measure airspeed, replacing conventional, air-pressure-based systems with tech that bounces around ultraviolet lasers to get the job done.

Conventional air speed sensors take the form of small tubes, known as pitot tubes, that protrude from aircraft. These are combined with small holes positioned at right-angles to the direction of flight, positioned either somewhere on the pitot tubes themselves, or elsewhere on the aircraft.

The conditions inside the right-angle holes describe the normal conditions outside the aircraft, something that's known as "static" air pressure, while those inside the pitot tubes, which are positioned towards the direction of flight, detail the pressure created by the forward motion of the aircraft. The difference between the two observed pressures is used to indicate airspeed.

Pitot tubes usually have heating apparatus built in, but they're still prone to icing up under really cold conditions. By their very nature, they're also vulnerable to collisions with birds, and they're not particularly accurate at low speeds.

The new system, known as the Laser Air Speed Sensing Instrument (LASSI), was developed by BAE Systems scientists working in Chelmsford in the UK. Instead of relying on air pressure, the new technology makes use of an ultraviolet laser.

The laser is used to bounce light off the surrounding air molecules, and the change in the color of the reflected beam, as caused by the Doppler Effect, is measured.

This can be thought of in a similar way to the classic police siren example of the Doppler Effect. Just as the sound of the approaching object is altered as the frequency of the wave changes when the object moves closer and then further away, the frequency of the laser light (and therefore its color) is also altered depending on the relative velocity of the air molecules reflecting the light back to the detector.

The ultraviolet light is invisible to the human eye, but tiny changes in its color can be picked up by the system's detector. Essentially, the larger the degree of color change in the reflected light, the faster air molecules are moving relative to the craft, and therefore the faster the vehicle is moving.

BAE has already conducted ground vehicle and wind tunnel testing of LASSI, and the team is now looking to scale down the system, targeting use in aircraft within the next five years. Overall, the it could provide a big upgrade over current methods.

"LASSI can be located completely inside the aircraft and is accurate at low airspeeds." said BAE Systems' Dr Leslie Laycock. "These features should ensure that the equipment is robust against damage, requires less maintenance and be easier to operate at lower airspeeds."

Source: BAE Systems

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