Aeolus_separation

The technology behind Aeolus, ESA’s weather forecasting satellite – and why it was delayed for 10 years

Image credit: esa

The European Space Agency (ESA) blasted its new Aeolus satellite into orbit in August. The satellite uses an ultraviolet laser to measure wind speeds around the globe.

Assuming the technology works as anticipated, the satellite will pick up the reflection of the laser off the Earth’s atmosphere and use the data to give weather forecasters an unprecedented look at global weather patterns.

The data could be especially useful in forecasting major storms, which are often difficult to predict and cause the most devastation.

Teledyne e2v, the company behind the satellite’s ultraviolet laser detector, has been working on the project for almost 20 years. E&T sat down with Paul Jerram, Teledyne’s chief engineer, to get the inside scoop on how and why the technology works.

“The instrument sends laser pulses and looks for the reflection of those from particles within the atmosphere, such as aerosols and other things,” Jerram said.

“They bounce back and what we provide is the detector that receives those pulses. It’s a very special form of detector that can map the height at which the pulse has come back.”

Aeolus_fairing_opening

Artist depiction of Aelous exiting its capsule

Image credit: esa

The detector is effectively a form of charge-coupled device, and the time taken for the laser to bounce back gives information as to how high the particles in the atmosphere are at any given time.

The returned signal is also very slightly bluer or redder (depending on its wavelength), due to an effect known as the Doppler shift. Where the signal lies on the spectrum depends on whether the atmosphere at the point of reflection is moving towards or away from the detector.

“This is what changes the position of the returned signal across the detector and gives the wind speed,” Jerram explained.

In this way, the mission will provide scientists with data on wind speeds in remote areas, such as over oceans, that they have not been able to get from weather balloons, ground stations and aeroplanes in the past.

“If it gets really cloudy it will reflect too much from a single layer,” Jerram said. “This relies on the reflections being fairly well spread throughout the atmosphere.

“This covers the whole Earth in a small period of time, so although you will get less information from the bits that happen to be cloudy, you will get more information from the parts that aren’t cloudy. It’s really mapping the entire Earth.”

Profiling_the_world_s_winds

Image credit: esa

While the satellite’s laser is relatively powerful, once it hits the atmosphere the light will scatter in all directions, making the amount of return light very small and difficult to detect. To mitigate this issue, Aeolus beams out a rapid succession of laser pulses at 100Hz in order to boost the signal.

“Effectively, you get a long trace until it reaches the ground, reflecting back at every position and you add them together,” he said. “Although it’s not mapping the height in incredible detail, it maps about 30km of atmosphere and puts that into 25 different bins. It gives you information relevant to 1km blocks through the atmosphere.

“Each measurement of a slice of atmosphere takes about a tenth of a second (because it adds together 10 pulses). Then they’ll move the satellite along a bit and do another slice at a different position.”

While Teledyne’s original design for the laser was completed in 1999 and it was fabricated in 2003, the project was put on hold for years due to issues with delivering a powerful enough ultraviolet laser that would operate effectively in space.

Jerram explained that the laser had been destroying the glass it was shining through, as even tiny imperfections would result in internal reflections that would ultimately blacken the panel.

The quest to make perfect glass without any flaws that could disrupt the operation of the laser took a whole decade to overcome.

“There aren’t that many people here now that worked on these detectors originally,” Jerram reflected. “In fact, the designer is still here, but quite a lot of people have left in the intervening years.”

The Aeolus satellite is unique and it is the first time the technology has been used in this capacity. It will also be the first time that a map of global wind speeds will become available.

While wind speeds today are known at specific positions, weather scientists do not know how they vary through the atmosphere. “That’s obviously really important for trying to make better weather forecasts: you need to know how the wind is moving around the globe,” Jerram pointed out.

The satellite is equipped with solar panels to generate electricity, but due to its low flying altitude (around 320km) it also includes a fuel tank and engine in order to constantly right itself and maintain its height. Without such a system, small amounts of atmospheric drag would ground it pretty quickly.

This requirement also puts a limit on how long it can stay in orbit. When the fuel runs out in around three years' time, it will inevitably come crashing back down to Earth.

“If it’s successful, then we very much the hope is that there will be follow-on missions,” Jerram said. “At the moment, the European Space Agency is viewing this as almost a development mission to prove that it works.

“If it works, then hopefully they’re going to look at more continuous coverage, they’re going to have satellites working all the time to provide this data.

“In the future there might be a few satellites that provide a more regular update, but I think they’ll always be quite low altitude so we’ll have to launch them on a regular basis because they won’t last for that long.”

Aeolus_in_the_launch_tower

Aeolus on the launchapd

Image credit: esa

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