24 August 2018
The European Space Agency’s technically bold bid to measure wind speeds from space and so transform the accuracy of weather forecasting has survived its 22 August Vega launch from Kourou and three intensive days of early-orbit operations to enter a commissioning phase expected to last several months. That commissioning phase will test all onboard systems and ultimately see Aeolus – named after the keeper of winds in ancient Greek mythology – deploy its sole payload, a laser range finder being billed as one of the most sophisticated instruments ever orbited.
The Atmospheric Laser Dopler Instrument, or Aladin, will use lidar – laser detection and ranging – which, like radio-based radar, measures distance and direction by the time taken for a signal to be reflected back to the source. But, with the much smaller wavelengths of an ultraviolet laser, lidar is capable of detecting air molecules or airborne particles and so measure wind direction and speed. Lidar systems are used for measuring clouds or aerial mapping, and some autonomous vehicles use lidar to detect obstacles; aircraft-borne lidar has even been trialled as a means of detecting air turbulence that might upset an aircraft.
But Aeolus will be the first satellite to directly measure winds from space, and is designed to map winds as high as 30km altitude, hopefully filling in gaps in basic science, climate modelling and meteorological data that is today only available from weather balloons or by tracking clouds, measuring temperature and looking at sea waves to detect surface winds.
Perhaps with understatement, Aeolus programme director Pierre-Yves Bertin describes the mission as “challenging”. Some 16 years in development, Aladin must be both sensitive and robust. The laser will pulse 50 times per second, heating the instrument’s optics to 1,700°C. Temperature control has been a design priority for the Airbus Defence & Space-built spacecraft. Aeolus flies in a Sun-synchronous orbit maintaining a path above either dusk or dawn on the surface, and takes its measurements from the “night” side of the satellite. But when flying over whichever hemisphere is in winter, Aeolus is in the Earth’s shadow for some 20 minutes per orbit, so temperature variations are large.
The laser system is kept cool by a “complex” radiator on the dark side of the satellite.
Maintaining the instrument’s integrity was a challenge even before launch. This latest in ESA’s Earth Explorer series of science missions arrived in Kourou by ship, rather than taking the customary airlift from Europe, to avoid exposing the satellite to air pressure changes during with flight.
Aeolus is orbiting at a fuel-hungry 320km, a compromise between flying close to the particles Aladin is trying to measure but high enough to maintain station without excessive fuel consumption. Of the satellite’s 1,360kg launch mass, 266kg was fuel.
Speaking during the launch from ESA’s space centre in Kourou, French Guiana, Bertin said the planned mission life is just three years; the small size of the satellite restricted its fuel capacity, and Aeolus is a technology demonstrator.
One Aeolus partner hoping this technology demonstrator is a success which leads to further missions of its type is the European Centre for Medium Range Weather Forecasts (ECMWF) in Reading, UK. The ECMWF relies on in-situ and satellite measurements to make detailed two-day global weather predictions, less detailed forecasts for two weeks and indications of likely trends up to a year ahead. As one ECMWF spokesperson put it while watching Aeolus lift off, lives depend on accurate forecasts of extreme weather events: “The more accurate we can make our forecasts the earlier we can advice people to take action to avoid being involved in weather disasters.”