The Atmospheric Laser Doppler Instrument (ALADIN), onboard the Aeolus mission, is a Direct Detection Doppler Wind Lidar operating in the ultra-violet spectral region (355 nm).
The lidar is aimed 35° from nadir and 90° to the satellite track (on the side away from the Sun), in order to transmit and receive light perpendicular to the speed of the satellite. This allows ALADIN to determine the east-west horizontal component of the wind along most of its orbit around the Earth.
The receiver analyses the Doppler shift of the backscattered signal with respect to the frequency of the transmitted laser pulse and as a function of time. In this way, it is possible to determine the wind at various altitudes above the surface.
Two optical receivers measure the frequency of the light backscattered along the instrument line of sight by molecules (‘Rayleigh signal’), and particles, ice crystals and water droplets (‘Mie signal’). This is then compared with the frequency of the emitted light pulses.