The US Military is Making Balloons That Hover at the Edge of Space, Indefinitely

A sensor that can spot the wind direction from miles away will let DARPA's surveillance balloons hover at the very edge of space in one spot indefinitely. An anonymous reader writes: DARPA, the US military's research arm, 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. '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.

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