Plane Finder is another option and it comes in three different forms: Plane Finder Lite, Plane Finder, and Plane Finder AR.
If you want the best experience you'll need to upgrade to the full version, Plane Finder for iOS and Android, which comes with a full suite of features including alerts, route data, and airport departure boards. You'll also get access to the app's Augmented Reality features.
With the full app, or the standalone Augmented Reality app for iOS, you can literally point your phone at a plane in the sky and find out pertinent information about the flight.
You'll see the airline, flight number, route, aircraft registration, speed, altitude, and how far it is from you.
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