SIL and invisibility

[PaulSS]

Wherever ADS-B technology is used it is worth highlighting that most ADS-B In traffic receivers will only display aircraft traffic that generate a non-zero quality indicator, (also referred to as SIL 1). In contrast a zero quality indicator (referred to as SIL 0) ADS-B Out, is ignored and not displayed on a traffic display – rendering an aircraft invisible.

Spurred into reading about the Trig TN72 by Giver's post, I came across the information above and I have to admit that I am now a bit confused.

Let's say I don't have an 'approved' GPS source, such as the TN72 but I wish to use the extended squitter capability of my Mode S transponder to transmit ADSB data. I might have something like the non-approved GPS output from an EFIS (MGL in my case) or it may be similar to the non-approved GPS output from a PAW unit (as in Giver's case). I understand that in both these scenarios I would set the SIL value to 0 in the transponder setup. But then I look at the information above from Trig!

They say that if I have a SIL of 0 then I'm not going to be displayed on a traffic display. Now, does this mean all traffic displays? Will a PAW user with the standard ADSB receiver and Sky Demon not be able to see me?

I don't think this is correct and I suspect that it may be a case of certified ADSB In systems not being able to see me because they only allow SIL 1 or better to be 'seen' and filter out SIL 0. Non-certified ADSB In systems, I think, will be able to see me (as they don't filter SIL 0) but I'd be very grateful if anyone that knows 'stuff' can categorically confirm or deny the information from Trig.

Could this also be linked to the appearance or non-appearance of aircraft in such cases as http://forum.pilotaware.com/index.php/topic,1229.0.html

[Ian Melville]

Yes they are refering to certified systems like TCAS or that used by Air Traffic Services. I also believe that on some systems you would still be displayed, but no alerts would be triggered.

[AlanG]

Hi Paul

Ian is correct.  It is the warning systems like TCAS that will "ignore" Sil=0 and will not issue a warning/instruction for avoiding action.  One of our club aircraft had an incident over the Forth and under the EGPH TMA when an Easyjet was suffering icing on approach to Edinburgh and asked for an immediate descent.  As our aircraft was on listening squawk and EGPH approach frequency the controller called him to ask his intentions and advised about the CAT at 6000'.  Our man advised he had heard the exchange and was immediately turning away to clear the path for the CAT.  The controller advised the Easyjet he could descend but to be aware that the microlight was in the area.  The Easyjet replied that he could see him on his screen.  The microlight ADS-B out is via a Trigg Xponder set to Sil=0.

Regards
Alan

[PaulSS]

Thanks guys, at least the muddy waters are clearing a little, regarding SIL 0 v SIL 1.

Alan, I'm surprised Easyjet couldn't see Peter. They don't have ADSB In but I cannot for the life of me see why they couldn't see his transponder on their TCAS. I understand he has a Trig Mode S (TT21 I believe), which will be the same set up as mine. His GPS, with a SIL 0 would obviously not be seen IF they had ADSB In but their TCAS should certainly have seen the Mode S. Hopefully he was just out of the bottom of their TCAS 'box', as opposed to some fault in the Mode S/ADSB system.

This is the sort of thing that is concerning me a bit and kind of goes hand-in-hand with the SkyEcho not being able to transmit on ADSB at the same time as having a Mode S transponder on, due to an aircraft shadow. I hate to say it but I wonder whether the guys over The Pond have got it right in insisting that certified kit is used. I really hope they're wrong because it is stupidly expensive and not inventive but when I hear stories of no Mode S being seen (possibly because of uncertified ADSB out), ADSB not being seen at all by the likes of Huntwing and the difficulties of trying to get PAW to 'see' Mode C/S, remember the signal strength and build a model of transponder presence then I do question whether we should just go to one standard and mandate fitment. I know, I know, sacrilege and I deserve burning at the stake. I'm still very much a believer but I would like to see some of these things cleared up.......not necessarily by the PAW Team but cleared up in general.

[exfirepro]

Hi Paul,

The issue is -as you are aware - around the System Integrity Level (SIL), which - up until last week HAD to be set to SIL 0 for all non-certified UK GA installations to comply with UKCAA CAP 1391 (to prevent the possibility of TCAS or ACAS alerts from aircraft inadvertently broadcasting incorrect position data).

UKCAA CAP 1391 was revised on 12 April 2018 and now allows installations using GPS sources which comply with recognised standard TSO-C199 (which includes both Trig’s TN72 and uAvionix SkyEcho) to transmit SIL 1. This means that they will now be seen and reported by Commercial TCAS (and other similar systems).

Mode S-ES (ADSB) from a non-certified GPS source transmitting SIL 0 or SIL 1 will continue to be seen and displayed via PilotAware. 

While the ‘upgrade’ to SIL 1 means that anyone using such a system will be ‘safer’ in that they are more visible to commercial and other air traffic using certified systems, it also of course brings with it an additional responsibility not to ‘intrude’ into a position where they could trigger a Resolution ‘Alert’ in a TCAS equipped aircraft.

Best Regards

Peter

[Moffrestorer]

I looked into recommending the TN72 to our flying Group, as a GPS source for our Becker Mode S, when it was first announced at the last Flyer Live show, because it’s SIL=1. I was concerned TCAS equipped aircraft would not see us (with SIL=0) if I connected the standard PAW GPS Mouse. What I hadn’t appreciated is the current generation of TCAS relies solely on Mode C or S (which we have) to generate an alert and ADS-B doesn’t yet enter the equation, though the next generation currently under development ultimately (early 2020’s onward) will employ ADS-B assistance as well.

The UAvionix built in GPS sources are apparently SIL=1, presumably necessary for the USA home market where it’s likely there is no concession from the FAA for use of non-certification GPS. These devices are not transponders and therefore do not transmit Mode C or S, and will not currently trigger TCAS alerts.

As far as I can see, the CAA/ NATS/ EASA really needs to get their act in order to determine what they really want from all of this. CAP1391 encourages use of EC with a SIL=0, though that’s being rewritten I understand to included SIL=1 to encompass the uAvionix offerings that currently have to be “written down” as SIL=0 to comply with the spec. Certified ADS-B receivers that NATS etc. are presumably intending to eventually source are likely to be operated so they ignore uncertified SIL=0 traffic. Well I say that that would be sheer lunacy on their part and is “their loss” if they make no use of the information that will increasingly be out there as non-certified ADS-B gains take-up.

[PaulSS]

Certified ADS-B receivers that NATS etc. are presumably intending to eventually source are likely to be operated so they ignore uncertified SIL=0 traffic. Well I say that that would be sheer lunacy on their part and is “their loss” if they make no use of the information that will increasingly be out there as non-certified ADS-B gains take-up.

I think this may very well be the reason we may see the rules changed to SIL 1 kit being required as a minimum.

[Moffrestorer]

I can’t think there is any likelihood that the rules will be changed Paul. NATS did a study that concluded that uncertified GPS was adequate for GA ADS-B EC, and convinced the CAA of the need to trial it and hence CAP1391 was issued. If and when air traffic units equip with ADS-B it will no doubt be within their power to operate it so that SIL=0 Traffic is visible. If for some reason they don’t want to see it that is their choice but on safety grounds somewhat questionable IMHO.

I had to have the Transponder modified at the factory to achieve SIL=0 (it was set to 2 if I recall correctly). Would not be a happy bunny if this was required to be changed once again!

In the meantime as long as I can see ADS-B traffic of whichever flavour via PAW, as well as P3i, Flarm and Mode C/S, and people with PowerFlarm can see my ADS-B transmissions, I am ecstatic.

Regards,

Chris

[PaulSS]

I sincerely hope you're right, Chris and that SIL 0 continues unchanged for non certified equipment. It does, indeed, make more sense for ATC, or whomever, to alter their gear to see SIL 0 instead of all non certified having to upgrade.......which many would not bother doing and, thereby, negate the whole purpose of ADSB for all.

I'm optimistically happy about some of the information I have gleaned today. From the genius who owns MGL (and I mean that genuinely) I have discovered that their EFIS GPS meet the SIL 1 criteria, even though they have never used them as such. The new bits of kit they're going to fit actually comply with FAR blah, blah, blah and will be suitable for certified equipment. However, if the current kit complies with SIL 1 (which I plan on using for the GPS data for ADSB from my TT21 transponder) and I'm able to use it as such, then I'll be a happy bunny.

I have no idea if one is able to change the SIL figure in the TT21 (and I have read all the manuals I can on the subject) but I have asked the Trig guys and they are normally excellent at replying and telling you exactly what you need to know. So, hopefully, I'll just be able to use the EFIS GPS data, adjust something to SIL 1 and let ATC (in the future) berate me for infringing their airspace.

The TN72 looks a good bit of kit and a good way to output SIL 1 GPS info but I really would prefer not to have to fit another bit of kit and have another antenna. What with my external antennas for PAW, transponder, radio etc and a heap of GPS aerials I'd look like something out of Star Trek.