The TT21 is a two part transponder system, which includes an integrated altitude encoder. In design and technology it is equivalent to the TT22, however, the transmitted output power of the TT21 is lower, qualifying the unit as a Class 2 transponder. Class 2 transponders are suitable for aircraft cruising slower than 175 knots, and operating below 15,000 feet, VFR or IFR.

The TT22 system is our smallest, lightest, Mode S general aviation transponder. It can fit into the smallest of aircraft – and yet it is fully certified and compliant with all the transponder requirements. The TT22 is a two part transponder system, which includes an integrated altitude encoder. The TT22 is a Class 1 transponder.