An Optical Air Data System for in-Flight Measurement

We present experimental results from a novel velocimeter system that employs a laser transmitter-receiver concept to observe aerosol density fluctuations in order to determine the 3D air velocity. The system is designed to measure the airplane velocity, the angle of attack, and the side slippage angle. Additional built-in capability includes the determination of the local turbulence strength (Cn², the optical index of refraction structure parameter) during flight. The system consists of two independent channels, each providing a 2D velocity vector projection perpendicular to its line of sight. The two channels are constructed with a 30 degree angular separation to yield a complete 3D velocity vector. Each channel comprises of a diode laser transmitter and a quad APD detector constructed in a bi-static configuration that allows for light collection from ~2 meters beyond the airplane, i.e., from an unperturbed volume of air. The laser is pulsed at a high frequency to yield the intensity fluctuations resulting from aerosol density variations. These fluctuations are analyzed using a spatial-temporal cross correlation function between the different detectors and a zero-crossings technique in order to calculate the typical movement frequency across the field of view of the system and reconstruct the 2D velocity vector. The various noise factors affecting the system were analyzed and the system sensitivity was optimized accordingly.