The idea of a balloon that floats high up above Earth indefinitely is a tantalizing one. Solar power would allow such stratospheric balloons to operate like low-cost satellites at the edge of space, where they could provide communication in remote or disaster-hit area, follow hurricanes, or monitor pollution at sea. One day, they could even take tourists on near-space trips to see the curvature of the planet.

It’s not a new idea. Indeed, the original stratospheric balloons were flown by NASA in the 1950s, and the agency still uses them for science missions. And Project Loon, owned by Google’s parent company Alphabet, successfully deployed such balloons to provide mobile communications in the aftermath of Hurricane Maria in Puerto Rico.

There’s a major snag, though: current balloons shift with the wind and can only stay in one area for a few days at a time. At the height of the stratosphere, some 60,000 feet (18,300 meters) up, winds blow in different directions at different altitudes. In theory it should be possible to find a wind blowing in any desired direction simply by changing altitude. But while machine learning and better data are improving navigation, the progress is gradual.

DARPA, the US military’s research arm, thinks it may have cracked the problem. It is currently testing a wind sensor that could allow devices in its Adaptable Lighter-Than-Air (ALTA) balloon program to spot wind speed and direction from a great distance and then make the necessary adjustments to stay in one spot. DARPA has been working on ALTA for some time, but its existence was only revealed in September.

Ball Aerospace

“By flying higher we hope to take advantage of a larger range of winds,” says ALTA project manager Alex Walan. ALTA will operate even higher than Loon at 75,000 to 90,000 feet (22,900 to 27,400 meters or 14 to 17 miles), where the winds are less predictable. That shouldn’t be a problem if the balloon can see exactly where the favorable winds are.

The wind sensor, called Strat-OAWL (short for “stratospheric optical autocovariance wind lidar”), is a new version of one originally designed for NASA satellites. Made by Ball Aerospace, OAWL shines pulses of laser light into the air. A small fraction of the beam is reflected back, and the reflected laser light is gathered by a telescope. The wavelength of the reflected light is changed slightly depending on how fast the air it bounced back from is moving, a change known as doppler shift. By analyzing this shift, OAWL can determine the speed and direction of the wind.

Unlike other wind sensors, OAWL looks in two directions at once, giving a better indication of wind speed and direction.

“It’s like looking out with two eyes open instead of one,” says Sara Tucker, a lidar systems engineer at Ball Aerospace.