OpenTX

OpenTX is an open-source firmware for remote controllers used in RC hobby. It supports radios from such manufacturers as FrSky, RadioMaster, JumperRC, Team BlackSheep, etc.

OpenTX was forked by Bertrand Songis from er9x, which is in turn a fork of th9x.

It is distributed under the the GNU GPL v2.0 license.

Over the years, OpenTX has been criticised for being not user friendly and having a "huge learning curve".