How does JetDex identify a plane?

Almost every airliner broadcasts its position, altitude, speed and identity several times a second over a radio system called ADS-B. Volunteers around the world receive those signals and share them openly. JetDex gets the live positions of every aircraft within about 80 km of you.

When you point your phone at the sky, JetDex knows three things:

• Where you are, from GPS.
• Where the camera is looking, from the compass, gyroscope and accelerometer.
• Where every plane is right now, from ADS-B, moved forward to the current second using its speed and heading.

It works out where each plane should appear in your camera view and picks the one closest to where you are aiming. When you take the photo, it also finds the plane or contrail in the picture and follows how it moves across the sky for a moment. That movement separates two planes that look close together.

If JetDex is not sure, it keeps tracking for another second instead of guessing. You get a card with the flight number, airline, route, aircraft type, altitude and speed.