Firing laser pulses from a satellite is the best way to measure wind velocity. Thus the launch of the lidar-equipped Aeolus satellite last August marked a breakthrough in measuring wind speeds around the world from the ground to the stratosphere for use in weather forecasting and climate research.
Its first nine months proved its worth. "The quality of the wind data is fantastic," Josef Aschbacher, director of Earth observations at the European Space Agency, said in May. However, the high energy pulses from the Aladin ultraviolet laser used to make the measurements were growing dimmer.
This week ESA is switching on a backup laser, and expects to spend three to four weeks commissioning it. "The first data, available only to expert users, should be available around the end of July," Aeolus mission manager Tommaso Parrinello told Spectrum in an email. ESA expects to need another month to confirm data quality for continuing studies of weather and climate.
Weather and climate trends depend on wind velocities, so accurate measurements of their throughout the atmosphere are essential for weather forecasting and climate research. We can estimate wind velocity indirectly by watching the motion of wind-driven clouds and the turbulence of wind-blown ocean surfaces. However, the most accurate velocity measurements come from inserting probes directly into the moving air. Weather balloons are the most common probes, supplemented by ground-based and aircraft-mounted instruments, but the numbers of these probes are limited.
Firing laser pulses into the air is another way of measuring wind velocity. Putting the laser on a satellite allows its pulses to probe the atmosphere around the whole world. Air molecules scatter some laser light directly back at the laser, in the process shifting the wavelength slightly by an amount that depends on the velocity of the molecule. A sensor mounted on the satellite can detect the difference between the laser output and the reflected light, thus measuring the air speed. This kind of laser is called a laser radar or lidar, and it is best known for its short-range uses in police speed traps and self-driving cars.
Lidars have been flown in space before, but mostly for measuring distance to the ground. One famous example is the Mars Orbiter Laser Altimeter which mapped the red planet from the Mars Global Surveyor. That worked well because hard surfaces reflect much of the laser energy. Air, on the other hand, reflects only small fraction of the laser light, making it harder to measure wind velocity. However, air scatters more light at shorter wavelengths, so lidar returns can be increased by using lasers firing high energy pulses at short ultraviolet wavelengths.
ESA custom-built a high-power laser they called Aladin to operate very stably, firing 50 pulses per second in a precise band for measurement. The laser worked well, but when they tested the optical system in a vacuum in 2005 they found that the intense ultraviolet pulses destroyed fragile coatings on optical surfaces and clouded the optics. Fixing that took more than a decade and raised the total cost of Aeolus to $560 million when it launched last year.
The laser fired ultraviolet of 65 millijoules once it reached orbit, but that energy declined 20 to 30 percent in the first nine months, and Aschbacher said it was losing one millijoule per week in May. That was a big advance over earlier high-energy ultraviolet lasers in space, which failed within a few hours, Parrinello says. The European Center for Medium-Range Weather Forecasts found the resulting data was good enough to improve weather-forecast quality. However, at the rate power was dropping, Aeolus would not be able to collect the three years of data sought for research.
Analysis of satellite performance found the losses originated from multiple problems with the laser rather than a recurrence of the earlier optical difficulties. The laser beam seems to be drifting away from the target area, reducing power delivered. The semiconductor diode lasers that convert input electricity into the light that powers the ultraviolet laser appear to be dimming.
Fortunately, ESA's design included a redundant laser designed as a backup to the original. The backup laser's design is essentially the same as that of the fading laser, but the backup was not being used, and ground tests show that its pulse energy can be adjusted more readily. That extra margin of the backup laser should be good enough for Aeolus to collect the desired three years of data, Parrinello says, but warns that high-power laser missions, "are only one shot away from failure." Satellite lifetime also is limited by the limited fuel available to stabilize its orbit at 320 kilometers, an elevation so low it requires a boost every week.
Results from Aeolus have already raised interest in a follow-up operational satellite for meteorological observations, and Parrinello says ESA has already exchanged letters of interest with the European Organization for the Exploitation of Meteorological Satellites.