OpenTX is an open-source firmware for RC transmitters used in RC airplanes, helicopters, drones, and other RC models. It is a free and community-driven software that provides a customizable and powerful interface for controlling RC models. It is designed to work with a wide range of transmitters from various manufacturers, including FrSky, Turnigy, and Spektrum, among others. It offers a wide range of features and customization options, including support for telemetry, mixing, scripting, and model templates, as well as customizable menu layouts and voice prompts.
The photo shows a transmitter which is OpenTX compatible.
OpenTX is intended to be highly configurable and compatible. Mode 1 radio has the throttle located on the right transmitter stick, while Mode 2 has the throttle stock on the left. Japanese and Australian pilots predominantly fly Mode 1 while in the US and China it is predominantly Mode 2. Mode 3 and Mode 4 are some special variations not really popular in the market.
OpenTX allows you to map and change controls and modes easily due to its ease of output channel reassignment. Such flexibility can be highly valuable.