[SafetyThird]

I'm a lapsed PPL returning to flying after 15 years away, a lot's changed in that time, particularly with electronic conspicuity, and I'm hitting the books and forums to get my head around it all while getting my flying recurrent.

I've just bought a Skyranger and had the option to fit a lightly used Trig 21 transponder when I bought it, which I did. It will, as I understand it, output ads-b if connected to a suitable gps box for position info. Now, for a few hundred quid I can get trig's box or I could buy a pilot aware and get all the extra abilities that brings and connect the gps output from the pilotaware to the transponder.

The trig box will give the ads-b output a '1' code whereas the pilotaware will give a '0'. I'm trying to understand what difference that means in real terms, ie what would someone receiving the ads-b signal see?

Does having the '1' code mean I can then access different types of controlled airspace than if I just have the regular pilotaware signal with the '0' code.

Thanks.

[PaulSS]

At the moment you're not able to use ADSB to access any controlled airspace but obviously the plan is for that to be possible in the future. Your Mode S transponder will permit you to be able to do that in the meantime.

Very basically SIL 0 is uncertified, SIL 1 (or above) is certified. Because the uncertified GPS sources don't necessarily do all the self-checking etc that certified kit does, your reported position can't be guaranteed enough for the likes of ATC. Clearly they are issuing clearances based on known positions and if your uncertified GPS could be incorrect then they're unable to do that job (or perhaps they can but the required spacing would be too great to make it feasible). The reality is that most of today's uncertified gear gives as good information as the certified but no bureaucrat is ever going to believe that.

The receivers at ATC centres, for instance, will be able to see your SIL 0 report but they'll have kit to ignore it.....or the controllers simply won't be able to use your ADSB position if they see a 0 tag. It is the same with the few airliners that have ADSB In; they'll only have SIL 1 or above displayed. Again, not a problem at the moment because TCAS uses transponders but in the future a similar system will use ADSB and the positions need to be guaranteed. Only piles of paperwork can guarantee this and the certification rigmarole is why the kit costs so much more; not because it is necessarily more clever and does a better job than uncertified.

So, at the moment it makes really no difference to you if you have 0 or 1. Your access to controlled airspace is unaffected and mixing with airliners is unaffected. By the time we are using ADSB for accessing airspace they may have more confidence that uncertified kit is actually far better than originally thought and we may be able to keep 0, who knows?

Your Trig transponder is very clever and will automatically transmit a 0 or 1 depending on how it gets its position, so there is no need to reconfigure if you change your mind or add a certified source later. My EFIS chucks out SIL 1, so I use that as a GPS position to my Trig TT21. I didn't need to do anything and SIL 1 appears on the Pilot Aware ADSB report that I submitted for approval of may installation.

My recommendation would be to go down the Pilot Aware route now. I say this not to start the age-old argument about SkyEcho versus Pilot Aware (again) but because you will be using your transponder's extended squatter for ADSB Out and, at the same time, can use the greater 'In' functionality of Pilot Aware. This, as you know, will give you SIL 0 and that will be sufficient in the foreseeable future. As for looking further ahead your guess is just as good as mine. If we did require SIL 1 then you could always retrofit Trig's certified receiver or maybe Pilot Aware will have a certified chip option by then (NO, that is not the start of a rumour, just a guess of what may be available in time to come) but, for now, you can get ADSB out (plus PAW of course) and lots of information in, using PAW, for significantly less money than just SIL 1 out using Trig's certified GPS.

[SafetyThird]

Paul, thanks very much for such a thorough reply, that makes everything much clearer now. I think I'll do as you suggest as I was looking to get pilotaware anyway so that I have more awareness of what's around me and can always retrofit a suitable gps to the trig later if it becomes required.

[MikeW]

One minor point, on my TT21 transponder I did have to tell it that the GPS input was SIL 0. It doesn't actually use that term, I think you choose uncertified or something, it's in the manual. I think its factory setting is for SIL =1 rather than detecting it. Otherwise agree entirely.
You'll find more than one big thread in the general aviation forum about PAW vs alternatives, have fun reading, before or after you get your PAW!