Primarily for general aviation discussion, but other aviation topics are also welcome.
Our EV97A Eurostar has been emitting ADS-B Out for a couple of years using an unapproved GPS source, via an RS232 connection to a PilotAware Classic unit with external antennae. To accomplish this our 2011 Becker BXP6401 Mode S ES transponder had to be returned to Germany for a hardware and firmware update to enable ADS-B functionality and to transmit SIL and SDA = 0 as required by CAA/LAA. At the time, I was interested in the TN72, but was unable to get a quote from Trig (last Flyer exhibition at Telford) as it didn’t seem to really be on sale at that time.

Now that the CAA rebate scheme is here, can anyone point out any compelling advantage to obtaining a TN72 for our case, i.e. for a Day/ VFR LAA aircraft?

My thoughts are ATC can still see our Mode S, I’m not at all bothered that they might not see our SIL/SDA = 0 ADS-B as I understand most units do not have ADS-B In and no plans for it. If they really wanted to see me they could adjust their receive settings rather than cause me extra expense! The main problem with SIL/ SDA =0 seems to be for reception by ACAS X, but assume that TCAS/ACAS/ ACAS X will still be able to detect our 1090 Mode S transmissions. Bearing in mind that the transponder would presumably have to go off to Germany once again to be adjusted to whatever SIL/SDA the TN72 and compatible antenna would justify, I just can’t see any advantage in changeover of GPS source. Does the panel agree?
Broadcasting SIL-1 will allow another aircraft that doesn’t have TAS/TCAS but does have a certified ADS-B IN system to see you. Transponders from L3 and the Garmin GTX345 offer this capability in conjunction with an approved traffic display (GNS, GTN etc).
However, whether a TN72 can work with a Becker transponder is a separate issue.
I have a Garmin GDL50 ADS-B receiver linked to a Aera 795. I am very pleased with the traffic display. It showed traffic data for an aircraft that was fitted a non TABS gps with SIL set to 0. (According to the owner).
I have asked Garmin support what the display logic is for the GDL 50 regarding SIL level. I am still waiting for a reply.
Seeing as GDL50 is not certified and is for VFR use only, then it is likely it will display SIL=0 SDA=0 aircraft. It’s normally just the certified kit that needs SIL=1 and SDA=1 to display ADS-B.

WARNING: TheattitudeinformationprovidedbytheGDL5Xisnotintended to be used as a source of attitude information for flight in instrument meteorological conditions

The GDL 50/52 traffic information is provided as an aid in visually acquiring traffic. Pilots must maneuver the aircraft based only upon ATC guidance or positive visual acquisition of conflicting traffic.
In the UK, ADS-B / SIL-1 is used by ATC in certain parts of the North Sea (low level) from what I understand. Not many GA aircraft spend time low level, over the sea, near the oil rigs. However I understand that the system can also detect Mode-C aircraft using Multi-lateration. I'm not sure what ATC do that with info ( ... -north-sea viewtopic.php?t=111583).

The chances of encountering an aircraft without TCAS (which as you say would use your Mode-S response) but with a certified traffic display (so showing ADS-B in) in a place where it matters (when not being watched by a radar controller) I suggest are fairly remote in the UK.
As other posters have commented, no real reason to upgrade your equipment at the moment.
Your transponder will alert TCAS equipped aircraft if you come close.
At the moment there are probably far more aircraft with no EC capability at all than those with the unique combination of no TCAS and SIL-1 (or higher) detection only.
I wondered whether to use the subsidy to upgrade my Trig transponder with a TN72. At present it gets its GPS signal from my Avmap Ultra so it is properly wired in, not a plug in cable.
In another long thread about EC - don't ask me to find it now! - I asked the same question, i.e. would SIL=1 confer any advantage in the foreseeable future for VFR flying, mostly OCAS, but in the crowded south east?
Nobody could offer anything definite. There was a bit of wishful thinking.

Upshot - I'm leaving it as is.
Thanks everyone for your thoughts on this. They mainly confirm that there is little to be gained in swapping over to a certified source in my circumstance, especially as the transponder will most likely need to be modified once again by the manufacturer to redefine the SIL and SDA. This is not a change that the user can accomplish through a menu selection.