What Americans see in the sky, are they drones, helicopters or airplanes?
Experts who study unmanned flying objects - otherwise known as drones - say that from several kilometers away it can be difficult to tell the difference. However, there are some signs that can help with this. '
A light in the night sky can be easily misinterpreted, according to the director of the Center for Unmanned Aircraft Systems Research and Operations at the University of Maryland, John Slaughter.
"You can't see (a flying object) and immediately say this is not a drone, or it is one," he says.
All you can really say is, "I saw a light in the sky," says Mr. Slaughter.
Dozens of mysterious flying objects seen at night in New Jersey last month and now across the United States have raised concerns among residents and officials.
Part of the concern stemmed from flying objects that were first spotted near Arsenal Picatinny, a military facility used for weapons research and production, and over President-elect Donald Trump's golf course in Bedminster.
Are objects with flashing lights planes?
Both airplanes and helicopters have flashing lights and at least one red light that helps anti-collision by improving visibility, often two such lights.
They also have lights that stay on all the time, red and green, which provide information on the position, direction or status of the aircraft.
Many also have flashing lights on the tips of their wings, as well as powerful lights when they land.
Drones flying at night are required to be equipped with only one bright anti-collision light, which must be visible from 5 kilometers away.
But the people who own the drone can add other lights to it, so some have more than one.
Drones are smaller than airplanes and helicopters, so when they have some lights, they are close together, and at night it is not possible to tell exactly how far apart they are.
"A light is just a bright spot," said Mr. Slaughter. "It could be 100 meters or 65 kilometers away and it would still look the same."
What noise does a drone make?
Drones seem to make less noise than airplane engines or helicopter propellers. But larger drones can make a louder noise, and the sound can be hard to tell from a distance whether it's a drone or another flying vehicle.
Drones heavier than about 0.2 kilograms may have an identification number displayed on their exterior.
"Of course it can't be seen at night, and you have to be close to see it, it has to be daytime," says Paul R. Snyder, director of the Unmanned Aircraft Systems Program at the University of North Dakota.
Airplanes and helicopters tend to move steadily through the air, but multi-propeller drones can stop instantly, turn 90 degrees and change course, Mr. Slaughter said.
"That kind of movement can be a clue that you're looking at a drone and not an aircraft," he said.
The vast majority of drones operate at an altitude lower than 122 meters, based on federal regulations. / Voice of America (A2 Televizion)
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