Connecting an Uncertified GPS to a Transponder for ADS-B Out

Disclaimer: The following is how I have set this up and although approved by the BMAA under TIL118 for our aircraft things maybe different in yours and therefore this guide is for reference only and action taken as a result of reading this are entirely at your own risk!

Adding an uncertified GPS input to Funke Or Trig Transponder


Why do this Modification?

At under £80.00 including the Mod certification fee (currently waived by the BMAA as it’s a safety mod) the question really is why would you not do this modification! At the flyer show 2016 NATs were very keen to encourage people to do this, the person I spoke to cited being visible to TCAS in military jets, FLARM and PilotAware users as a great safety bonus as well as ATC having your location too.

Software levels

There is a minimum software level required when connecting an uncertified GPS. An older version may result in incorrect parameters being broadcast. You can verify the software version installed in your transponder when you switch it on, this is briefly shown on the screen at power-up by both Funke and Trig devices.

Funke TRT800H transponder software level 5.2 or newer. depending on the age and version of your current software Funke may charge for an upgrade as they did us.

Trig TT21 and TT22 transponders the software version must be 2.4 or newer. In addition, the Transponder Controller software version must be 1.7 or newer. Trig provide, at the time of writing, a free and very quick upgrade service.

For Trig TT31 transponders the software version must be 3.4 or newer (I have not tested this with a Trig TT31)

Note re Trig software version numbering: 2.10 is a higher version than 2.4 (mathematically it isn’t!) so read their numbers as release x “.” as a separator version number Y

Note refitting a certified Position Source: If fitting the Trig TN72 and configuring it as the TN72 it will output the wrong values thus not complying with the BMAA TIL118. Having spoken to Trig they advise setting the Position Source in the Trig transponder to “C199 TABS” which will result in an SDA and SIL level of 1 and thus complying.


Parts used excluding sundry items:

GPS – GlobalSat BR-355  / BR-355 S4 (this is the 5V RS232 version, do not confuse this with the 5V TTL version)

Warning! there are now two speed versions of the BR-355 / BR-355 S4 it can be ordered in 4,800 and 112,500 baud. Please only order the 4,800 variant

Note: I was installing the latest S4 GPS today (30th March 2021) its speed was 115,200 Baud this needed changing as this speed is unsupported by the transponders. To change the speed send one of the below $PSRF100… however these changes are not permanent.

4800: $PSRF100,1,4800,8,1,0*0E
9600: $PSRF100,1,9600,8,1,0*0D
19200: $PSRF100,1,19200,8,1,0*38

5V Regulator – Regulator 12V to 5V 3A 15W Buck Power Supply
ordered from HK via auction site.



5V Regulator – CPT Regulator 12V to 5V 3A 15W Power Supply

the above power supply cause interference across the civil airband

Other parts

Inline (soldered) fuse holder, 500mA quick blow fuse (both from Maplin), wire, connectors and heat shrink insulation.

Weight of all components including wires and fuse for this mod is: 94 gm.


Preparing the GPS and regulator

  1. Remove the GPS magnetic base to avoid any interference with the compass.

Remove the sticker from the underside on the GPS to expose the magnet, slide a small screwdriver down the side of the magnet and gently lever the magnet out.  Backfill the hole with some hot glue and smooth flush. Lastly attach a piece of self-adhesive Velcro which will be used to fix the GPS mouse to the top of the dash later.

  1. Connecting the power

Cut the GPS mouse plug off (needles to say there goes the warranty) and strip back the cables.  Solder the +V out, red wire of the regulator to the red wire of the GPS. Solder the –V out, yellow wire to the black wire of the GPS. Insulating heat shrink should be used to cover both connections individually.

  1. Add an inline fuse to protect the aircraft

Solder an inline 20mm Radio fuse holder to the “in” +V line of the regulator and insert a 500mA quick blow fuse.

  1. Data connections (optional step for testing and optimising the GPS)

Connect the green wire (GPS TX) of the GPS to an RS232 port on a PC using pin 2 on the PC serial port.
Connect the white wire (GPS RX) of the GPS to the RS232 port on pin 3.

Out of aircraft testing

  1. Connect the +V via the fuse and –V of the regulator “in” to a 12V power source, I used a bench power supply.
  2. Check the Red LED illuminates, this will be solid while the GPS is acquiring its position. Within 2 mins it will start to flash to indicate a 3D fix has been acquired.
  3. Next run up a serial terminal program on a PC, I use the free “RealTerm” for Windows, set the terminal to the correct Com port and 4800 Baud, 8, 1, N and connect. If all is well a constant stream of data will be seen such as below.


Restricting data output.

The GPS outputs a lot of different “sentences” however the TRT800H only uses $GPRMC and Funke recommend only this message type is passed to the transponder. Trig has no such recommendation.
Unused message types are removed by sending the following 3 commands:




Each command is sent in turn using the “Send” tab in Realterm and “Send ASCII”, ensure the +CR and +LF options are selected and then send each command in turn.

The output should now only display the $GPRMC sentences required by the Funke TRT800H.

At this point I bench ran the regulator and GPS for 24 hours to ensure no issues, both units should and did run cold.

The assembly is now tested and ready to connect to the transponder. The GPS’s RX (white wire) is no longer used and should be insulated using heat shrink.


Configuring the TRT800H transponder to accept the uncertified GPS

  1. Set transponder on STBY
  2. Hold FID until it says ’47’ in upper right corner
  3. Press MODE until “Comm-A/B” appears
  4. Change the option to NMEA-format using the lower left rotary control.
  5. Press MODE until exited
  6. Turn transponder off and back on


Note : For Ver 5.3 the following proceduer has been fed back by Ken (thank you):

Push and hold ID until display shows “enter setup “ 3 presses of mode button takes you to the RS232 settings. 


Configuring the Trig Transponder

Use Function to move through the options, the right-hand knob to select and enter to save.

  1. Switch transponder off
  2. Hold the FN and switch the unit on with the FN button held in. Release once the unit has started up.
  3. Step through to GPS Input and select NMEA 0183.
  4. Step through to GPS speed /Baud rate and set 4800
  5. Step through to GPS Certification Level and set uncertified 
  6. Step through to GPS NAC velocity GPS NAC velocity (NACv) and set unknown
  7. Step through to the end
  8. Power off

Connecting the assembly to the aircraft, Ensure the aircraft power is isolated for safety before proceeding.

  1. Connect the fused +V and -V to a suitable power source (I connected this to the auxiliary power that feeds the 12V socket via male and female spade terminals).


  2. Connect the GPS’s TX Green wire to the Funke transponder’s RX pin 12 on the 15 way D connector or the RS232 RX lead on the RS232 breakout lead of the transponder if fitted.
    Note if your aircraft is a certified aircraft i.e not permit, you are not allowed to open the 15 way connector as it forms part of the certification.


  3. Connect the GPS’s TX Green wire to the Trig transponder’s RX pin 5 on the 25 way D connector of the transponder box.


  4. Reconnect the aircraft power.
  5. Switch on the auxiliary circuit and ensure the red led illuminates or flashes on the GPS unit before proceeding.
  6. Switch on the transponder and proceed with testing.
  7. After testing secure all parts as is appropriate in your aircraft. Ours is mounted using Velcro and the moulded cable is cable tied, so acting as a lanyard.

The regulator is cabled tied to an appropriate point behind the instrument panel.


Testing the TRT800H Transponder

  1. Set transponder to STBY
  2. Hold FID until it says ’20’ in upper right corner
  3. TM displayed confirms you are now in testing mode
  4. Set transponder to mode ACS
  5. If any GPS data is available in the upper corner will appear a small dot, and a big ‘P’ appears in the middle-left of the screen. This ‘P’ only appears when a valid RMC sentence is available. It maybe that the ‘P’ will blink
  6. Power off the transponder to exit test mode
  7. Power the transponder back on
  8. Optionally I used ModesDeco2 to ensure the Geocode is correctly being sent by the transponder.
  9. Check that no interference is generated in any of the aircraft systems e.g. hiss on the Radio, Compass direction…


Testing the Trig

  1. Switch on the transponder
  2. Select ALT
  3. Press FN twice
  4. Check the Latitude and Longitude are showing GPS data.
  5. Check that no interference is generated in any of the aircraft systems e.g. hiss on the Radio, Compass direction…



Validating the connection for the Mod approval TIL118

For this step you will need access to a PilotAware unit or you can download my free application for windows from this website here

  1. In PilotAware go to the traffic menu and check the aircraft is shown, it may take a few minutes to show up.
  2. Check that SDA and SIL are both zero, in the current version of PilotAware (November 2016) the fields are no longer labelled.

    The comma delimited field “ADSB Status” is in the order: vers, nacp, sda, nica, sil, sils
  3. Take a screenshot of the screen including the date and time.
  4. Have your BMAA inspector sign your TIL118 form and email it including the picture as a single PDF to the BMAA technical Office.