On Wednesday, March 28, 2018 at 4:09:09 PM UTC-4, Darryl Ramm wrote:

Just one point: TABS per-se is likely not visible to ATC, it is not in current deployments of ATC approach/TRACON systems I'm aware of and it's best to assume its not visible.

I asked Trig tech support this question:

"Will a TT21 + TN72 transmit a non-zero quality indicator, aka SIL = 1 ?"

They replied:

"The TT21 is fully certified to the latest ADS-B Out cert TSO C166b – so it can output all the required data. It is true that if you wish to meet ADS-B Out compliance FAR 91.227, you should have a higher power TT22. However, there is no way for the radar to tell if you’re using a TT21 or TT22.
Inspection of your aircraft would be the only way to tell. As such I believe a number of experimental pilots have opted to ignore the requirement , but of course I cannot recommend this as it would be in violation of FAR 91.227 – the requirements for 2020 compliance."

"The TN72 offers both a SIL =1 output, and also a SIL = 3 output. So this means that you can meet the FAA compliance check using your TN72 plus TT21. https://adsbperformance.faa.gov/PAPRRequest.aspx"

I interpreted this to mean that, a TT21 + TN72 (TABS)
looks exactly the same to ATC as a TT22 + TN70

I understand that TT21 + TN72 (TABS) does not satisfy 2020 ADS-B compliance, but I can be flying in Class E and be visible to ATC and other aircraft equipped with ADS-B In, exactly the same as an aircraft with 2020 ADS-B compliance.

I apologize if this muddies the water.