This projects uses Software Defined Radios to view airplanes in the sky from the https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance%E2%80%93Broadcast protocol. This is rather inexpensive, and easy to setup on your own!

See https://github.com/rsadsb/adsb_deku and https://github.com/rsadsb/dump1090_rs for code building and running examples.