Tracking the Skies with a Software Defined Radio #SDR
60 Chequers Avenue discusses using an inexpensive software defined radio to look for aircraft.
One of the fun and cheap projects you can do is track all the aircraft that are flying around you. All commercial and some light aircraft are required to have an ADS-B transponder that sends out a radio signal on 1090MHz that contains their location, speed, etc. These signals can be received with one of these SDR dongles and decoded to display this information on your computer.
There is even a ready made piece of software called dump1090 (and many, many forks of it) that will also display this information on top of a google map.
Check out this inexpensive and fun build here.