BAE Systems is leading a combined industry and academic team in the development of the next generation of air speed sensors. The Laser Air Speed Sensor Instrument (LASSI) programme is being undertaken by BAE Systems Advanced Technology Centre (ATC) in collaboration with Advanced Optical Technology Ltd, Spectrum Technologies and the Department of Physics at Hull University.
The programme, sponsored by the UK Department of Trade & industry (DTI), will see the development of an air speed sensor using an ultra-violet laser designed to increase accuracy of airspeed measurement over a much wider range of speeds than traditional methods.
The current method for measuring air speed uses the pitot tube, which is a cylindrical tube attached to the aircraft that points into the airflow. A device attached to the base of the tube measures the forward air pressure and the air pressure at right angles to the direction of motion or static air pressure. The difference in pressures is used to calculate the velocity of the aircraft.
Whilst accurate at high speeds, the ability of the pitot tube to resolve the differences in forward and static pressure at low speeds is said to be very limited. This makes the measurement of airspeed at low velocities difficult and is a serious problem for helicopters, particularly during low speed manoeuvres close to the ground. Additionally, on fixed-wing aircraft, the pitot tube increases the drag on an aircraft, increasing fuel consumption.
Under the DTI programme, the team will develop a laser based sensor system and construct a portable system demonstrator. The development activity will include amongst the overall concept, the research and design of a compact, short pulse laser and a fibre optic system. By firing the laser into the atmosphere, the nature of the light reflected from the air molecules will change with speed. This variation is measured by the LASSI system and converted to airspeed.
The LASSI programme will see the creation of a non-intrusive, all altitude, high accuracy measurement sensor for use at high and low speeds which, according to BAE Systems, will provide significant benefits for all forms of aircraft.
For helicopters, LASSI will improve the information provided to pilots during low speed activities, leading to a potential increase in safety. For fixed-wing passenger and transport aircraft, the reduction in drag achieved with the removal of pitot tubes will see savings in fuel over long journeys.
The ability to ‘steer’ the sensor will enable accurate vector measurements to be taken improving the control of flight. For military aircraft, LASSI will enable a reduction in the radar cross section of the aircraft and hence improve its survivability in enemy territory. LASSI could also be used for alternative applications such as mapping the airflow around buildings and structures.