The key features of the unit are:
• Small physical size. The control head occupies less panel space than a 2¼ inch instrument.
• Light weight. The combined system weighs less than 500 grams (including connectors and control head, and an allowance for the required interconnect cable).
• Low power consumption. The system consumes less than 5 watts of aircraft power.
• ADS-B upgrade path. The system already includes the ADS-B software.
• Low cost.

The TT21 is a class 2 transponder, which means that it can be used on any aircraft flying below 15,000 feet and airspeed of less than 175 knots. With an ultra-compact front panel controller and a small transponder block the new TT21 will squeeze into any aircraft. Ideal for light sport and microlight aircraft, as well as conventional light aircraft where there is not much panel space, the TT21 low power design means that it can also run on batteries for aircraft with no electrical system, such as gliders and balloons.

The TT21 is a two part system – the front panel controller features an LCD display and conventional controls, and incorporates an altitude encoder, whilst the remote transponder box contains the power supply and the receiver/transmitter. The front panel controller fits in either a standard 57 mm (2¼ inch) instrument cut-out, or where space is really at a premium, it can fit in an ultra-compact 42 x 57 mm cut-out. The depth behind the panel is less than 70 mm, so the controller fits in the smallest of spaces. The rear box is not much larger than a pack of cards, and can be mounted anywhere convenient in the aircraft. A lightweight cable connects the controller to the rear box; aircraft power and the antenna cable also plug into the rear box.

Despite the small size the TT21 nominal power output for reply pulses is 130 Watts, and it even supports ADS-B Out using 1090MHz extended squitters, which means that it offers a path to future ADS-B based surveillance.

The TT21 is very light and uses very little power, which is particularly important for microlight aircraft that are restricted in weight.

The TT21 is TSO certified and will be delivered with EASA Form-I.