Trig Avionics TT21 Mode S Transponder

Trig Avionics TT21 Mode S ES Transponder

FASVIG is highly supportive of initiatives to increase the voluntary adoption of ADS-B in General Aviation and is working to help make the implementation of ADS-B Out as quick, simple and cost effective for users as possible.

See FASVIP VEE.5 ADS-B Out Implementation

FASVIG applauds the announcement by the LAA (previously reported here) to open up on a permanent basis the use in LAA Permit-to-Fly aircraft of ADS-B Out with an uncertified GPS position source.

One of the challenges for GA pilots is understanding which transponder makes and models and even which specific firmware levels support the ADS-B technical requirements for use with an uncertified GPS. Transponder manufacturer support for this is patchy to say the least and authoritative information has been lacking.

FASVIG is keen to encourage transponder manufacturers to support ADS-B configurations that incorporate an uncertified GPS position source and to make supporting information readily available.

Transponder manufacturers are invited to provide details to FASVIG of their product support for ADS-B Out with uncertified GPS position source and in return FASVIG will publish the information here on our website. This will greatly simplify the task for pilots wishing to select products at the purchasing stage and then to make use of ADS-B technology to enhance the safety of themselves and those sharing airspace with them.

Trig Avionics have provided the following statement covering their transponders.

“For those with Trig Mode S ES transponders wanting to enable ADS-B Out (using an uncertified GPS position source) the following information should be observed, ensuring you have the correct setup on your transponder.

For Trig TT21 and TT22 transponders the software version must be 2.4 or higher. In addition the Transponder Controller software version must be 1.7 or higher.

For Trig TT31 transponders the software version must be 3.4 or higher.

You can verify the software version installed in your Trig transponder when you switch it on, this is briefly shown on the controller screen at power up. Trig’s web site contains information on all software versions –

For LAA Permit to Fly aircraft owners with a Trig transponder using an uncertified GPS ADS-B Out, you must ensure that the correct software is installed before submitting your LAA MOD 14 forms. To perform a software upgrade you will need to remove your transponder back box hardware and controller and send your units to an Approved Trig Dealer who is qualified to install software (approved Part 145 facilities).

Finally, with the correct software installed, the GPS Certification Level needs to be set as  “Uncertified” in the transponder’s setup menu. Trig user manuals and installation manuals provide information on how to change these setting and can be found on (in the TT31 resources box) and (in the TT21 resources box).”


19th Jan 2016 Update
Adrian Price of NATS has provided this addition tip:
“There is a fairly easy way to check if the GPS connection is correct, turn your TT21 fully on (ALT) and press the FUNC button two times to get the ADS-B position page. You should get a Lat & Long displayed within a few minutes.

The most important part of the modification is to ensure the GPS data is tagged as being of low/no integrity. For the near future aircraft owners will still need to contact and tell NATS when they next intend to fly. The NATS’ current ADS-B coverage at 4000’ runs approximately from Portland Bill – Telford – Brough. If the flight is conducted south east of these boundaries, NATS will confirm the Surveillance Integrity Level (SIL) and System Design Assurance (SDA) are set correctly.”

There may well be similar features on other makes of transponder. Check your Operator’s Manual.