T16 Transponder

[rainier]

It's time for an update - probably my longest development yet.
The first prototype was done in 2016 - pretty much a traditional receiver and using a TX transistor made for the purpose (rather expensive little bugger that). Did not have much in the way of suitable RF test equipment at the time but the basics where OK.
I then did a second prototype ditching the old fashioned superhet receiver and experimenting with a satellite receiver chip - that was interesting from a mode-s perspective as decoding the data uplink from secondary radar would be much more robust (I/Q receiver).
To reduce cost the entire pulse decoding and receiver (including mode-s phase decoder) was moved into the processor. This handled the TX as well. No FPGA in sight. It's quite a challenge - you have to receive data at 4MBits/s on the Mode-S link and measure pulse amplitudes and times to high levels of accuracy. Usually this is done with dedicated hardware - but that costs money and needs more parts. Anyway, got that working (interestingly managing to do 8 million arctan calculations per second with precise timing). Transmitter starting to produce a little power- but not enough. My idea to further save costs - the whole transponder, like the V16 radio must be on a single PCB and further to that no exotic PCB materials are to be used (you normally need impedance controlled ceramic substrates for high frequencies - costly stuff).
The third prototype cleaned things up a little and introduced a small amount of additional hardware to assist with pulse edge detection as the previous version could not get down to the required level of resolution (nano-seconds). Transmitter still struggled to get enough power out but it was just at the level needed for a Class 1 transponder. I wanted nearly double that.
So I relented and did a forth prototype which moved the final stage of the transmitter to its own PCB - but still without the need for expensive materials.

Then Covid hit.

The transponder was put on hold, some of the components we already bought for a first production batch where used up to keep other product lines going - component shortages and supply issues forced a redesign of nearly all our products to use alternative parts. It was a nightmare. Then I needed to spend around a year to do the G4 CPU which will be phased into future EFIS systems - in addition to a rather involved time spent on porting the Linux operating system to it - and porting the EFIS so it would run in Linux.

Now that is done and I have dug out the transponder again - switched the forth prototype on for the first time and fiddled with the transmitter. This now works great - I'm getting about 230W of power and everything is properly within limits (spectral masks, harmonics etc). Nice.
It works really well as Mode-A/C at the moment and the test results are quite a bit better compared to an expensive transponder I am using as reference for comparison. The Mode-S side of things is implemented and I'm going through that at the moment. It all looks good - should be done by end of the week.

The official classification of this transponder is a Class 1, level 2el to the latest DO-181F standard (equivalent to Class 1, level 2els in previous versions - the "s" now being mandatory and no longer part of the label).

So far so good - but of course there is a road block - there always is. Now for the red tape. Sadly our local CAA has lost the plot and is no longer able to certify anything and has not renewed bilateral agreements with the FAA. This means a manufacturer in our country can no longer produce a certified part for an aircraft. It's not the end of the World - but I am now looking for options on how to get this thing to you.

[rainier]

After some more tweaking the transmitter is now sending out around 270W-280W of power with a clean signal compliant with FCC.
The various certification tests as described in DO-181 are very laborious and require very expensive kit. Some are quite arcane - the method they propose to measure the carrier frequency for example was last used by Noa on his ark if I am not mistaken. They propose a test setup using some very old microwave plumbing such as airlines and slot tuners - you can't even buy that anymore - you would have to borrow this from a museum.

But you are allowed to use suitable alternatives. Since I made my own ATC mode A/C/S radar simulator to help with the transponder development I'm going to take that one step further - it needs a second RF source to create interference of various descriptions and levels and effectively a receiver that is able to do accurate measurements on the transponders replies (both physically as well as data content) - so I'm going to add all that to arrive at a nice transponder certification tool that can quickly step through the many tests in semi-automated fashion without needing time consuming setup and validation of the test setup for every test and manual execution of testing. With the modern chips available now this is not that difficult anymore. Might even make that available as low cost but fully compliant ramp test set for annual transponder checkouts (the ramp check only tests the basics).

We may have to partner with a company oversees due to our local CAA's catastrophic failings. The transponder will have to attain certification in another country and production also needs to happen there (as the facility itself has to be certified and audited). But that can be as little as inserting a fully built sub-assembly into a housing. As long as acceptable QC is in place that's fine.

The T16 roughly resembles the V16 (same housing) and is also single PCB that just slides in. The design, in usual MGL fashion is very unusual and dispenses with tradition to use current technology. This actually reduces cost and complexity. There is not even a FPGA in sight - everything including the transmission and receiving is done in software on a very fast CPU. There is still one very expensive part which cannot easily be replaced - the final TX transistor pretty much has to be something the FAA knows and has relevant MTBF data. It's NXP's AFIC10275 which is made for the purpose. It costs more than everything else the transponder is made of put together - but that's unavoidable. It's quite an investment for a manufacturer too - you have to buy 500 minimum quantity - that's roughly $100.000 USD.

A second variant of the T16 is the T16D which will be released if we can get the T16 off the ground. The T16D is the diversity version of this transponder.

[JimJab]

Thanks Rainer for the T16 update, I am waiting, but it's starting to look like there still is a journey to travel,
with the extra work, thanks to your Government and its CAA.

Cheers
JimJab

[rainier]

Well, in the meantime I have managed to get some certainty around the certification issue.
Transponders designed to TSO-C112F do not need to be formally certified is installed in experimentals or similar or can be fitted via the STC route in other aircraft. It's actually the same as for the VHF transceiver - you need FAA approval and that's it. This is obtained once the FCC has certified the device.
So - that's the route I am going. It does make things easier and I can now also do a more complete implementation (with formal certification every additional (but optional) thing your transponder does increases the certification effort so in effect is "punishment"). There is little point in doing a transponder above a level 2 but within a level 2 there are now many options as well. I'll now add everything that can be of some use.

There was a lot of confusion related to the need to formally certify the transponder and indeed - in early release of the MOPS that was the case but EAA apparently intervened and got the text changed - from "Certified to..." to "Designed to...". That's a small change but makes a huge difference.
Of course, it makes no difference in the end - the device has to meet all of the performance requirements and the manufacturer has to be able to demonstrate that at any time - the manufacturer cannot hide behind a TSO grant.

The T16 is now ready for production - will do a small batch mostly for testing and FCC certification and as soon as I have the FAA approval letter we go for it.

As an aside, checking my documents - the transponder development started in 2012. It's been nearly 12 years to get to this point.

[manolo.lezl]

Great news!

As I'm building a Savannah, which cables will be needed for that unit? CAN bus and power?

I'll install those cables now.

Thank you. Manolo.

[rainier]

It's a fully featured transponder - at minimum and if used with our EFIS systems you need the CAN bus and power/ground. That's it.

It will in this case use the EFIS GPS for the ADS-B out (or the SP-12 GPS if one is connected). In other words - it's the same as if you were to use the Trig with our little interface.

The transponder also offers inputs for a panel or stick mounted ident switch and ground mode detection switch (squat switch). This transponder conforms to TSO-C112F, the latest issue. This means flight detect is no longer permitted to be manual - it has to be an automatic source. Either you have some kind of switch to detect you are on the ground or you need to use the EFIS automatic flight detect.
The T16 can also be enabled to transmit ADS-B out when on the ground (ADS-B has a special ground mode for this with high resolution of position and velocity for operation around airports).

The T16 is planned to be available with some options - the base version (all altitude/GPS info from the CAN bus), option with built in GPS qualified for SIL/SDA of 2/3 (effectively a built in SP-12) and a optional ADS-B in receiver (1090ES only, no 978Mhz UAT (might be added later if there is demand)).

The T16 can be used controlled from the EFIS via CAN or as stand-alone transponder with a Vega control head (which in this case also contains altitude encoder for the transponder as well as RS232 input for GPS). Currently supported are SP-12, Any TABS GPS including Dynon, Trig Avionics, Avidyne as well as the GPS receivers from uAvionics. Standard NMEA GPS can also be used but SIL/SDA will be broadcast as 0/0. Garmin GPS receivers using their ADS-B+ protocol CANNOT be used (Garmin does not answer our requests). The Vega head can be used parallel to an EFIS as well if desired.

[manolo.lezl]

The transponder also offers inputs for a panel or stick mounted ident switch and ground mode detection switch (squat switch).

This is great!

Could this ident pushbutton be connected to an auxiliary input in the extender module? I'm almost finishing the electric installation in my plane and would like to install everything needed now.

Thank you Rainier.

[rainier]

The T16 provides an input for a squat switch (switch to ground - closed when on the ground).
You can also do this via iBOX or iEFIS Extender (and then choose the selected input as squat switch) but I would assume in the T16 case you would simply wire the switch into the T16.

Note: Transponder checks are done while on the ground and the transponder will only transmit the extended squitter or answer secondary radar when airborne - this means you will need a means of temporary overriding your squat switch to be able to perform these checks.

If you use the EFIS automatic flight detect as source you can simply temporary select manual flight detect and start a flight in order to activate the transponder.

Note that you can use the T16 in ground mode - if you allow this mode it switches to a different style of extended squitter for airport ground operations and will operate in ground mode. Note that usually transponders for small aircraft simply remain silent when on the ground. However in particular if you are operating in large airports ATC will see your location and velocity information on the ground which can be a valuable aid for safety.