WSPR
From charlesreid1
WSPR is a weak signal propagation report network. It consists of radios that are listening for very weak signals reporting a callsign and a transmit power. Reverse beacons then monitor specific frequencies and post callsigns that they hear. This enables you to, say, determine that you can reach Europe with 10 mW.
What exactly is WSPR? More info on wikipedia: https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)
Page on using Raspberry Pi's internal clock and GPIO pins connected directly to an antenna to turn it into a WSPR beacon: https://gerolfziegenhain.wordpress.com/2013/04/13/raspi-as-wspr-transmitter/
More Github code to do the same: https://github.com/JamesP6000/WsprryPi