Up in the sky, is that a drone, a plane or a helicopter?

Experts who study unmanned aircraft systems — better known as drones — say it can be tough to tell from miles away. But there are clues.

A light in the sky at night can easily be misinterpreted, according to John Slaughter, director of the Unmanned Aircraft Systems Research and Operations Center at the University of Maryland.

“You can’t just walk outside and say, ‘Oh, that’s not a drone,’ or ‘That is one.’ All you can really factually say is, ‘I saw a light in the sky,'” Slaughter said.

Dozens of mysterious nighttime flights first spotted in New Jersey last month and now being seen across the........

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