Commercial aircraft constantly broadcast details of their location, altitude, speed and other statistics to air traffic controllers. This data can also be picked up by anyone with the necessary receiver and decoded.

Flight Radar 24 is a web site operated by a group of volunteers from around the world, who pick up the signals and feed them back into a central system that displays the data in a friendly form overlaid on a map.

You can thus browse the details of thousands of commercial flights, in real time. In the screen shot below, I've selected part of North America and clicked on a flight, and you can see the relevant data on the left hand panel. The various shades of blue on the plane's trail shows its height during the flight.

You can access the system at You must login or register to view this content. and it's totally free. It's also fascinating to sit watching the main screen, and see the positions of the plane icons gradually move as they continue on their journey.

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