October 9, 2018
Using lasers for global wind maps
Wind data is still based on weather balloons, wave monitoring, and measurements on the ground. The ESA satellite Aeolus now measures wind from space, using high power lasers.
Aeolus has been orbiting Earth at a height of 320 km since August 22. And one thing is already clear: the concept is bearing fruit. The first wind measurements have exceeded the expectations of the scientists who are involved. On a three-quarter orbit around the Earth from the North Pole via the South Pole to the equator, the high-tech LIDAR measuring system on board recorded vertical wind profiles between the stratosphere and Earth’s surface. These include subtropical jet streams on both sides of the equator, which reach the highest wind speeds at heights of 7 to 15 km and a strong polar vortex to a height of 30 km.
With the help of laser technology, the wind measurements from the orbit will close a large data gap in weather forecasting. In the past, forecasters had to rely on isolated measurements. Winds over the oceans, polar regions, and sparsely populated areas are seldom recorded. This makes mid-range weather patterns and storms difficult to forecast. Therefore, over the next three years, the European Space Agency (ESA) Aeolus mission will provide proof that it is feasible to profile the wind from space.
The centerpiece of the mission is Aladin, the Atmospheric Laser Doppler Instrument. For three years, it will send 50 UV laser pulses per second to Earth from two high-power lasers. Where they encounter air molecules, water droplets, dust, and aerosols, light is reflected. Depending on whether these particles are smaller or larger than one of the transmitted UV wavelengths (355 nm) this is described as Rayleigh or Mie scattering. Aladin captures this scattering with a high-tech reflecting telescope and measures it with a high-sensitivity receiver. The time between sending the light pulse and receiving the signal back determines the altitude above Earth. Because the light pulses are sent continuously, based on the respective Rayleigh or Mie scattering, it can also be seen at what speed and at which altitude the particles and molecules are moving in the wind. This requires use of the Doppler effect, in other words, the minimum wavelength shift that is based on light colliding with the particles that are moving in the wind. Finally, Aladin provides detailed data about the flow and wind conditions and the moisture distribution in all air layers from the Earth’s surface to an altitude of 30 km.
According to the ESA and the German Aerospace Center, Aladin is one of the most sophisticated and sensitive measuring instruments ever to have been put into orbit. The high thermal impact on the laser optics was one of the hurdles that had to be overcome during development. Specially designed coatings will ensure that the optics withstand the laser-induced surface temperature of 700°C for the three years of the mission. The solutions were provided by SMEs, such as Layertec and Laseroptik and research institutes such as Laser-Laboratorium Göttingen and Laser Zentrum Hannover. They helped Munich-based OHB System AG develop and test the central sending and receiving optics for the UV laser beam. Also involved were Airbus Defence and Space, Tesat Spacecom GmbH, as a supplier of the communication systems and Aladin reference laser, and the German Aerospace Center (DLR).
High-tech optics and heated reflecting telescope
In addition to the heat-resistant coatings of the laser optics, the developers also demanded the necessary precision of the reflecting telescope. The result is a silicon carbide-based telescope weighing just 55 kg with a diameter of 1.5 m. The low thermal expansion of this material and its perfect reflective surface will ensure long-term precision—in addition, the primary mirror and fastening struts can be heated for fine adjustment.
More information about the Aeolus Mission and the laser system as well as a host of information about weather observation and videos of the construction phase and the start on August 22 can be found on the ESA portal (www.esa.int).