Light Detection and Ranging (LIDAR)
LIDAR allows a precise and remote measurement of the wind velocity. It is based on Doppler shift determination of a light wave obtained from a single frequency laser that is reflected on natural atmospheric aerosols (Mie scattering). The aerosols are the wind field tracers to be analysed. The frequency shift is proportional to the air velocity and is detected via an interferometer measuring the beat between the backscattered wave from aerosols and a reference wave (local oscillator). The coherent mixing enables recovery of the backscattered wave phase. This phase contains the radial velocity information (along the laser line of sight). It also enhances the detection sensitivity thanks to the optical product of the signal beam with the reference beam which enables small target signal amplification. If required, the true air speed in three axes can be derived from multi axis sensing. This can be performed using 3 beams or more or a scanning device. LIDAR is able to give the velocity with no in-flight calibration. It is primary information without bias. Due to recent progress in fibre lasers and amplifiers, the present fibre LIDAR technology is a serious candidate for lightweight, compact, eye safe airborne anemometer probes. Experience is available on both continuous wave and pulsed fibre LIDAR for anemometry and vortex detection. Such anemometer probes for true air speed measurements with high velocity precision have already been flight tested. In 2003, DALHEC, a 3 axes wind LIDAR was built and taken on board of a Dauphin helicopter (see Figure 1). It is dedicated to TAS measurements at a medium range (30-100 m), away from rotor flow perturbations. The volume of the optical head is 1 litre, the processing rack around 200 litres. The original architecture has brought consistent progress in all fibre LIDAR technology. This device has been proven up to a 9000 ft altitude.
In 2007, the AIM work led to the specification of the 1.5 µm LIDAR sensor and was aimed at a simulation of the 1.5 µm LIDAR performance. The year 2008 was devoted to the 1.5 µm LIDAR sensor implementation, LIDAR measurements tests on a helicopter in hover flight at the airport in Braunschweig and their evaluation. Outdoor ground-based LIDAR measurements have been performed on a MBB 105 helicopter in hover flight above ground (schematically shown in Figure 2). Even with imperfect seeding, experimental results of the LIDAR tests prove the capability of the LIDAR technique to detect helicopter blade tip vortices.
Air data calibration using LIDAR systems is thought as an innovative tool which may yield an easier calibration procedure. Multiple particles Mie scattering LIDAR are dependant upon natural aerosol seeding conditions which are variable from day to day. However, variable availability of wind speed measurement is still compatible with the function of calibration. Moreover, angular positioning of the sensor inside the aircraft is a major concern, but Piaggio has experience in positioning sensors inside their aircraft. Hence, the further research and development of such an air data calibration system based on non-intrusive methods can be forward-looking towards future certification procedures and will therefore be a part of AIM².