If you were to find yourself on the dual-carriageway in Stevenage driving past a nondescript, industrial-looking Airbus Defence and Space building you’d probably never know that sitting inside is a world-leading satellite that has been more than a decade in the making.
Airbus engineers have finished putting the Aeolus satellite through functional testing on the ground and are preparing it for final operational tests before it is launched into orbit in December.
Aeolus is part of a European Space Agency mission to measure the wind speed in the earth’s atmosphere and will be the first satellite to do so across the whole planet.
The data it provides should result in much more accurate weather forecasting. The Aeolus wind profiles will also be used to advance our understanding of atmospheric dynamics and how the air streams work at varying altitudes in the lowermost 30km of the earth’s atmosphere. This will give scientists unprecedented access to data in regions that have previously been incredibly hard to gather information on, particularly around the tropics where more traditional methods such as ground sensors or airships have been unable to be used.
The Aeolus demonstrator project will run for three years and aims to prove the Atmospheric Laser Doppler Instrument (Aladin) technology, which will be the first space-borne wind lidar offering global coverage of the earth.
Aladin works on the principle of measuring the Doppler effect – the change in frequency or wavelength of a wave for an observer moving relative to its source – throughout the atmosphere.
Aeolus will orbit the globe every 90 minutes and project an ultraviolet laser beam through the earth’s atmosphere. The beam will be reflected against molecules in the atmosphere that are moving with the wind speed. The satellite will then measure the back scatter from the molecules from varying altitudes, which will be collected by the large silicon carbide telescope. The telescope will determine through the Doppler effect what the shift in speed is of the molecules, and the relative position of those molecules, by comparing the signal transmitted to the return signal it receives.
Measurements will be taken every 0.01 seconds and averaged over seven-second periods, during which time the satellite will have travelled 50km, to obtain wind profiles from altitudes from 0 to 16km and, with less accuracy, up to 30km.
The laser power, at 55MW/cm2, is about 5,000 times the power of the central heating in an average house directed onto an area the size of a little fingernail.
Richard Wimmer, project manager at Airbus in Stevenage, says that most of the challenges with Aeolus have been around developing this powerful UV laser, which must be stable, uniform and not damage the 80 optical components it passes through while operating in the harsh environment of space. These challenges have seen the project go far beyond its original intended launch date of 2007.
The Airbus team has explored various coating technologies to protect the optics, and, in some cases, is using the third generation of coatings.
Phil McGoldrick, engineering manager at Airbus Defence and Space, explains that frequency stability is another challenge for the Aladin system. “A lot of equipment on spacecraft generates micro-vibration, from fire thrusters to momentum wheels used for altitude control,” he says. The team introduced dampers to limit the impact of the micro-vibrations and managed to stay below 10mg of acceleration induced onto the laser equipment itself.
Airbus has also developed systems to protect the laser optics from degradation by contamination while in orbit, which could lead to loss of transmission. McGoldrick explains: “When you’re in a vacuum, outgassing of materials, typically from hydrocarbons such as insulation and plastics, is unavoidable.”
To avoid damage from hydrocarbons reacting with the optics, the engineers created a system that provides a slow flow of oxygen over the critical optics which are exposed to high laser energy. The cleaning system carries 13.4kg of oxygen to last for the life of the mission.
Aeolus is also advanced in terms of its operational autonomy, to reduce costs. It is capable of monitoring and adjusting its own systems while in orbit, to sustain itself with very little communication needed with the ground. McGoldrick says: “We want to demonstrate that you can set a spacecraft up and only have to talk to it once a week to maintain its operation.”
Before Aeolus reaches space, it will go through a fair amount of travelling here on earth. Having completed integration and functional testing in Stevenage, Aeolus is set to begin its mechanical test phase at Intespace in Toulouse, France. This will verify the integrity of the satellite and its capability to survive the launch.
The launch will be simulated in a vibration environment, an acoustic environment and in shock testing to ensure that the sensitive equipment on board the satellite can withstand the harsh conditions. From France, the satellite will move to Belgium to undergo thermal tests at extreme temperatures. Aeolus will then move back to Toulouse for the final stages of integration. It will then be shipped to French Guiana and launched into orbit on a Vega rocket.
Meteorologists use a global observing system to monitor weather patterns. This encompasses satellite observations, including polar orbiting and geostationary satellites; surface observations, including data from ships and ocean buoys; and land observations, including weather radar and surface sensors. Airborne observations come from aircraft and weather balloons.
But there is room for improvement. Gemma Halloran, a research scientist at the Met Office, says: “The World Meteorological Organisation has stated that there is a gap in this wind observing system and that providing wind profiles at all levels outside the main populated areas is the number-one priority for global weather prediction. The best way to achieve this is by using satellites.”
The Met Office expects Aeolus to increase the number of wind observations it receives by 10%. It will provide wind observations of the stratosphere, above the level at which aircraft fly, which is a region of the atmosphere where wind is poorly observed. “Changes of wind in the stratosphere can have quite a significant effect on our weather,” says Halloran.
A study by the European Centre for Medium-Range Forecasting in Reading has shown that Aeolus could improve wind forecasts in the tropics by as much as 15% in the upper troposphere, which could improve forecasts at the UK’s latitude too.
John Remedios, director of the National Centre for Earth Observation, says: “If you look at several flooding events in Europe, their genesis is from events that start in the tropics days earlier, bringing humid air which is then dumped on Europe.”
Remedios is also looking forward to receiving data that Aladin will pick up on the movement of aerosols, particles that can cause pollution and contribute to human health problems.
Andy Stroomer, director of earth observation, navigation and science at Airbus, says: “Aeolus is a great example of bringing together the Met Office and science community, helping to create a UK presence in European space science.”
Aeolus specifications
• The Aeolus satellite has a mass of 1,400kg, including 266kg of fuel
• The Aladin instrument weighs 500kg
• Aeolus’ structural model was subjected to vibration tests that equate to 8.9 on the Richter scale
• Two solar panels will provide 2,000W to power Aeolus
• Aeolus will travel at 27,000km/h
• The 1.5m Aladin telescope is made of silicon carbide ceramic to keep the device as light as possible at 70kg