Lasers from space support the daily life on Earth
ESA’s Aeolus mission data are now being distributed publicly to forecasting services and scientific users in less than three hours of measurements being made from space.
Aeolus is one of ESA’s Earth Explorer missions, which all set out to demonstrate how new ways of observing Earth can advance our understanding of how the planet works as a system. Carrying one of the most sophisticated instruments ever to be put into orbit, Aeolus is the first satellite mission to directly profile Earth’s winds from space by using a Doppler Wind Lidar (DWL).
A powerful combination the phenomenon of light scattering and the Doppler Effect allows the acquisition of data and the reconstruction of wind-profile. A lidar works by emitting a short, but powerful, pulses of light from a laser through the atmosphere and then collects light that is backscattered from particles of gas and dust and droplets of water in the atmosphere. The time between sending the light pulse and receiving the signal back gives the distance to the ‘scattering element’ (and thus the altitude above Earth). As the scattering particles are also moving in the wind, the wavelength of the scattered light is slightly shifted as a function of speed. The DWL measures precisely this change determining the velocity of the wind and providing the vertical profile.
The Aeolus instrument, named Aladin from Atmospheric Laser Doppler Instrument, comprises a powerful laser system, a large telescope and a very sensitive receiver.
The laser system generates a series of short light pulses in the ultraviolet spectrum at 355 nm, which is invisible and safe to the naked eye. The laser is actually a complex system of laser sources and amplifiers: two small lasers “fix” the frequency of the pulses generated by the main laser oscillator and further boosted by two amplifier stages to the required power level. Moreover, a frequency conversion crystals stage convert the laser wavelength from the laser into the correct one.
Aladin is dominated by a large but lightweight telescope, which measures 1.5 m across. It is used to collect the backscattered light from the atmosphere and then directs it to the receiver. The telescope is pointed 35° away from the orbit plane in order to transmit and receive light perpendicular to the speed of the satellite allowing to determine the east-west horizontal component of the atmosphere.
The receiver analyses the Doppler shift of the backscattered signal with respect to the frequency of the transmitted laser pulse using two optical analyzers for the Doppler-shift of Rayleigh scattering (molecular) and Mie scattering (from aerosols and water droplets).
These vertical slices through the atmosphere, along with information on aerosols and clouds, will advance our knowledge of atmospheric dynamics and contribute to climate research. Moreover, providing measurements almost in real time, it is a extremely powerful tool to improve weather forecasts.