On Tuesday, April 7, 2015 at 10:00:36 AM UTC-7, Andy Blackburn wrote:
On Tuesday, April 7, 2015 at 9:43:26 AM UTC-7, wrote:

Problem: The rest of the world isn't concerned about frequency congestion and airlines are less than enthusiastic about having to have both mod existing 1090 transponders and ALSO install a UAT ADS-B box onboard. Solution: Keep 1090 for aircraft that fly above 18,000 since they are the ones that will encounter the overseas environment.

Isn't it true that most of the RF congestion comes from the bandwidth-intensive interaction of TCAS with Mode C (and earlier) transponders? I've been told that the reason Europe didn't have to go down the UAT (or equivalent) path was because they mandated Mode S. It's possible the US could have done the same, but apparently the FAA didn't want to force a transponder upgrade (which seems silly given all the other mandated equipment and duplicate antennae). And so we get the mess we have that, as you pointed out, the FAA isn't so concerned about because our concerns have little to do with the problems they were solving for.

I remain somewhat skeptical that GA will go en masse to UAT. If Trig comes out with a Mode S/1090ES Out single box that hooks up to an existing Mode C antenna I think it could be quite popular.

Andy

That would be my summary of much of this (and BTW TCAS only interrogates Mode C or S, it won't ever interrogate Mode A and just won't see a Mode A transponder at all). I think a significant part of the issue was the FAA was under pressure from lobbyists like AOPA who wanted to keep Mode C transponders in use. Doing that can cause significant radio frequency congestion issues because of TCAS interrogation of those Mode C transponders, and also makes SSR a little less wonderful than it could be in a pure Mode-S environment. I was never impressed by the technical depth of the arguments used that dual-link was required because of 1090MHz/1030MHz congestion. This at least in part relied on some Monte-Carlo simulation of RF congestion in the LA Basin, but there just never really seemed to be as much academic research and debate as I would have hoped with this. I would have preferred to see wider research, with more broad assumptions/scenarios allowed, all the other mitigation options really well explored, and more academically peer-reviewed studies made before all the cost of a complex dual-link ADS-B system was inflicted on owner/pilots and the US taxpayer.

But anyhow layering complexity on complexity the FAA went dual-link over 109ES and UAT. Then you fold in other agendas like trying to sell UAT to GA aircraft by providing FIS-B data over UAT. Although you only need a cheap UAT-receiver to use FIS-B so it does not really encourage adoption of ADS-Out on either UAT or 1090ES link-layer. And a cynical view was some folks in the FAA just wanted to resurrect the failed FAA FIS (airborne VHF weather data) service. Did we get dual-link becasue of FIS-B or did we get FIS-B because of dual-link? The reality is maybe parts of both. And WX Satellite weather is a better product with better features and wider coverage, especially on the ground pre-flight). But WX Weather requires a paid subscription unlike FIS-B (although you may see paid parts of FIS-B in future from ITT/Excelis). Possible revenue from commercial use of FIS-B is maybe another decision factor in the whole dual-link mess.

The FAA's reasoning about inducements and expectations around using UAT for GA aircraft never made sense to me. Newer aircraft have Mode-S transponders that can be upgraded to 1090ES Out, older aircraft with transponders may be due a transponder upgrade anyhow. So maybe that leaves some in the middle, but that just never seemed like a reason to prioritize "we must keep Mode C" like AOPA pushed for. And the FAA tried to make UAT more widely used by initially allowing use up to FL 240, but that just seemed long-term crazy for things like direct link compatibility with all the big guys flying around in Class A. Once it was clear you were going to needed 1090ES in Class A airspace, then the high-performance singles/light turboprops etc. were all going to go that way, and those owners were more likely to be ADS-B early adopters in the GA space. And once you have some of those folks with 1090ES Out, more of the GA community will follow, both becasue they have GA oriented product available and for direct compatibility with 1090ES Out used by those early adopters.

What the GA lobby should have been pushing for IMNSHO was a usable 1090ES-Only system that would support a usable GA-oriented traffic awareness system.. Or at a minimum some more extensive academic research to validate the claims for need of a dual-link system. End-of-life of Mode-C could have been done over a decade+ time frame. Relying on ground infrastructure to provide ADS-R link-layer translation makes the current ADS-B system a mess for GA traffic awareness. There are too many places of interest to GA where there is not ADS-R coverage, including close to many GA airports and gliderport traffic patterns. And the system is just too complex, still many pilot's don't see to understand they need ADS-B Out to receive ADS-R or TIS-B traffic service.

ADS-B is one of those things that is trying to serve many masters, and as well described in the earlier post, it's really a aircraft surveillance system for the FAA Nextgen project, everything else is really a secondary add-on to try to make it more appealing to other constituents. Hoping I expect from the FAA's view point that those other constituents will then support the adoption of ADS-B, but the only real significant reason the FAA want it is for NextGen surveillance. The poor FAA is stretched thin between powerful constituents and masters, that is largely the US Congress, Airlines, and the occasional whack of a stick from the NTSB. GA gets a small mention every now and again, but the lobbying from various owner/pilot organizations for things related to ADS-B seems to have been pretty confused and/or ineffective. And some of the folks lobbying for those organizations I've spoken to clearly did not understand the technology at a useful level. And its arguable that the FAA was in a tough situation, with NextGen very underfunded, but either way, that is certainly what is really driving ADS-B.

Meanwhile Europe looked at the 1090MHz/1030MHz congestion issue, they have airspace like over Paris where this was a concern (but yes still less ongestion than some US locations). Europe went Mode-S only, which largely solved the TCAS bandwidth problem as TCAS can do selective interrogation of one transponder at a time. And TCAS-II can also save bandwidth by reducing interrogation of a target aircraft if it can be tracked via 1090ES.... but it still hammers away at it with transponder interrogations if it thinks it is a real threat... (oh the irony that that 1090/1030MHz bandwidth saving trick does not work against a target with Mode C/UAT-Out). And their are other options for future like tweaking TCAS power levels to trade off range/congestion. I certainly don't think it is absolutely clear that the FAA *had* to go dual-link with ADS-B becasue of bandwidth.

Heck, if lots of folks don't go Mode-S/1090ES out and stick instead with Mode-C and TCAS use increases it might even be possible that at some point in the future the FAA has to come back and end of life Mode C anyhow. But maybe that possibility is so long off that natural aircraft and Mode C transponder attrition/replacement will prevent that happening.

Head hurting yet? Go crack a beer and dream about your next glider flight.....