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ESA launches lidar wind measuring satellite

On 22 August, a powerful lidar instrument was launched into orbit by the European Space Agency as part of the first space mission to acquire profiles of the wind on a global scale. 

The near-real time observations will improve the accuracy of weather predictions and advance our understanding of climate change.

Highlighted by the World Meteorological Organisation, the lack of direct global wind measurements is one of the major deficits in the Global Observing System.

Not too long ago, wind speed and direction were measured just twice a day at selected locations around the globe using helium filled balloons that were assumed to move with the wind. The result is a patchy wind record that adds uncertainty to weather forecasts.

ESA's new mission, called Aeolus, will provide insight into how the wind influences the exchange of heat and moisture between Earth’s surface and the atmosphere – important aspects for understanding climate change. By filling this gap, Aeolus will give scientists the information they need to understand how wind, pressure, temperature and humidity are interlinked.

The wind satelitte flies in Sun-synchronous orbit at an altitude of 320km. It takes 90 minutes to complete one orbit around Earth and seven days to cover the globe. 

Credit: ESA/ATG medialab

'Aeolus carries the first instrument of its kind and uses a completely new approach to measuring the wind from space,' commented ESA’s director of earth observation programmes, Josef Aschbacher.

Jan Wörner, ESA director general, added: 'Aeolus... will fill a gap in our knowledge of how the planet functions and demonstrate how cutting-edge technology can be used in space,' 

Aeolus carries one of the most sophisticated instruments ever to be put into orbit. The first of its kind, the instrument, called Aladin, includes revolutionary laser technology to generate pulses of ultraviolet light that are beamed down into the atmosphere to profile the world’s winds – a completely new approach to measuring the wind from space.

The laser system generates a series of short light pulses in the ultraviolet spectrum at 355nm. The ultraviolet region is used because the backscatter from atmospheric molecules at this short wavelength is particularly strong.

The laser consists of a complex system of laser sources and amplifiers, which are all packaged closely together. There are two small lasers to fix the frequency of the emitted pulses, a laser oscillator to generate pulses, two amplifier stages that boost the energy of the light pulses to the required value and a frequency conversion crystals stage to produce the correct wavelength.

Aladin is dominated by a large telescope, which measures 1.5m across. It is used to collect the backscattered light from the atmosphere and then directs it to the receiver. Although large, it is made of lightweight ceramic material so it weighs only 55kg.

The telescope is pointed 35° away from the orbit plane 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 atmosphere.

The receiver analyses the Doppler shift of the backscattered signal with respect to the frequency of the transmitted laser pulse.

Two optical analysers measure the Doppler shift of the molecular scattering, ‘Rayleigh’, and scattering from aerosols and water droplets, ‘Mie’.

Highly sensitive photo-detectors then transform the light signals into electronic signals. The wind profiles are accumulated over at least 20 individual measurements before being downlinked to Earth for further averaging.

The Atmospheric Laser Doppler Instrument (Aladin) is one of the most sophisticated instruments ever to be put into orbitCredit: ESA/ATG medialab

While Aeolus is set to advance science, it will also benefit society. Although weather forecasts have advanced considerably in recent years, Aeolus will provide global wind profiles to improve the accuracy even further. In addition, its data will be used in air-quality models to improve forecasts of dust and other airborne particles that affect public health.

The satellite is being controlled from ESA’s European Space Operations Centre in Darmstadt, Germany. Controllers will spend the next few months carefully checking and calibrating the mission as part of its commissioning phase.

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