Somewhat related. For those who are new to SDRs, you can easily view airplane altitude, heading, speed, and other stats of passing aircraft with a cheap SDR. Aircraft transmit this using ADS-B and it's totally open and unencrypted.

FlightAware (among other services) work by aggregating data from users around the world. You can even get a free premium subscription to their service by contributing. Easiest way is to use a Raspberry Pi, install their software (much easier now than back in 2013!), and you're good to go!