Malaysian authorities revealed later that military radar had tracked the plane as it turned back to the west and flew across the Malaysian Peninsula, up the Strait of Malacca, before flying out of radar range at 2:14 a.m. and vanishing once again.

CNN: Pilot spots large object off Reunion Island

Each Global Hawk has infra-red and radar sensors, and a surveillance range of 100 miles.

ECONOMIST: Unmanned aircraft

Controlling planes over oceans presents all kinds of challenges because aircraft are out of normal radio and radar range.

WSJ: How New Air-Traffic Control Systems Operate Over Oceans

These systems use camera images or near-range radar to determine the direction and position of the vehicle in relation to lane markings.

ECONOMIST: Motoring

Utilizing four short-range radar sensors (two at the front, two at the rear), one long-range radar and a stereo camera, the vehicle is capable of cruising down an open freeway as well as negotiating heavy rush-hour traffic.

ENGADGET: Continental gets automated vehicle approved for Nevada roads

But radar can be blinded by bad weather, and not all volcanoes are within range of suitable equipment.

ECONOMIST: A new way of protecting planes from the ash from volcanoes

There is a second advantage to using lasers in preference to radar: while they have a similar range, laser sensors have a significantly wider field of vision.

ECONOMIST: Motoring

In the cases they present in their report, all conventional explanations of something natural or man-made had been eliminated by the authors and their associated teams of experts, and yet these objects were observed at close range by pilots, tracked on radar, and officially photographed.

FORBES: U.S. Government Denies That Aliens Have Come To Earth

The Bush-era proposal called for the United States to set up a radar site in the Czech Republic and 10 missile interceptors in Poland to counter the threat of Iran launching long-range missiles at America's allies in Europe.

CNN: Obama scraps Bush-era missile defense for new plan