Aeolus carries a laser Doppler lidar, called Aladin, that will probe down through the atmosphere to see which way the wind is blowing and how fast.
Today, we have multiple ways of measuring the wind, from whirling anemometers and balloons to satellites that infer wind behaviour by tracking cloud movement or sensing the choppiness of the seas.
But these are somewhat limited indications, telling us what is happening in particular places or at particular heights.
Aeolus, on the other hand, will attempt to build a global, 3D view of the way the wind blows on Earth, from the surface of the planet all the way up through the troposphere into the stratosphere (from 0km to 30km).
The models used to forecast tomorrow’s weather will clearly benefit from this, but so too will the simulations that investigate future climate scenarios.
Circling the globe, the satellite’s ultraviolet beam will pulse the air below it.
The time taken for the light to scatter back off molecules, dust and moisture particles will reveal where the big wind streams are in the atmosphere.
Small shifts in the frequency of the light will betray the speed at which those various markers - and the winds that carry them - are moving.
The concept is well established. Lasers like this are routinely fired into the sky from the ground to retrieve similar data at a single location.
The first problem was in finding diodes to generate a laser source with a long enough lifetime to make the mission worthwhile.
With that fixed, the mission looked in great shape until engineers discovered their laser system could not work in a vacuum - a significant barrier for a space mission. Tests revealed that in the absence of air, the laser was degrading its own optics.
“Inside the instrument there are over 100 optical surfaces - lenses and mirrors to prepare the laser beam - and they were becoming contaminated,” explained Aeolus Airbus project manager Richard Wimmer.
“There were two sources of contamination. If there were particles on the optical surfaces, the laser would burn them and blacken the surfaces. But the laser was also dragging particles on to the surfaces that were outgassing from the spacecraft.”
The solution was to introduce a means to flush Aladin with oxygen at a very gentle rate. Its implementation has, of course, elongated the programme and added significant extra cost.
What was supposed to have been a €300m mission is now estimated at €450m.
'We'll be ready'
For many years, I would pass through the Airbus cleanroom in Stevenage and see the spacecraft bus - that part of the satellite which holds its computers, avionics, fuel tanks, and the like - sitting idly in the corner, waiting on experts in Italy (Leonardo) and France (Airbus) to solve the instrument issues.
But on Thursday, the British engineers were finally able to lower a completed Aladin laser instrument - together with the telescope it will use to spy the scattered light signal - on to the rest of the satellite.
The spacecraft must now undergo a series of tests prior to riding its Vega rocket into orbit next year.
Richard Wimmer has stayed with Aeolus throughout its trials and tribulations and was clearly delighted to see the full satellite come together.
“It’s one of those bizarre things where you wait and wait and wait, and then it comes and it seems like just another event. But it’s a major milestone for sure because now we’re on a more standard assembly, integration and testing sequence.
“We’ve got one very important and complex procedure to do in Liège in Belgium, where as well as putting the satellite in a thermal vacuum chamber we’ll also operate the whole instrument system and measure its performance. Then we’ll be ready.”
For Europe, Aeolus is an important step. The hard lessons learned are being applied to the next Esa laser mission called Earthcare, which will study the role clouds and atmospheric particles play in a changing climate.
But having this technology opens up other possibilities as well, such as making very precise surface height measurements. The Americans, for example, have done this with ice sheets and forest canopies.