Don Maxwell wrote:
FlyingMonkey wrote:
Sigh...
Hi again, FflyingMonkey. Sorry for not responding right away. I got distracted by car trouble and Life, and now the conversation has gone on.
I've been wondering which ADS-B Out equipment you have. Or more specifically, which frequency you're transmitting ADS-B data on. The international standard ADS-B is on 1090 mHz. But inside the USA and below 18,000 feet, the FAA also allows "UAT" transmissions on 978 mHz. The differences are complicated--but one that doesn't get much mention on these forums is that UAT is not recognized by many of the ADS-B tracking apps. For example, FlightRadar24 and Plane Finder seem to display UAT-equipped aircraft--if at all!--only when their transmissions have been relayed by a 1090-equipped aircraft. Of all the trackers I've tried, only FlightAware appars to "see" UAT Out transmissions, and then not reliably. In my home area in Virginia, even the FAA's do-it-yourself ADS-B test site doesn't track my UAT Out transmissions unless I get above 1500 or 2000 feet at least once during a flight. I don't find that the least bit comforting.
As for whether iFly's rendering of ADS-B is accurate and complete--well, I don't know, but I'm certainly interested! I've been puzzled by seeing some flying buddies planes' displayed on my iFly, but not others--when I could see them all in flight at the same time with my own eyes. And sometimes their equipment sees my plane and sometimes it doesn't. I suspect the FAA, but really, it's a Great Mystery.
Hey Don...
That's an interesting point. My equipment list:
Garmin GDL-82 ADS-B OUT
Garmin GTX-327 XPDR
Stratux ADS-B IN
iFly 740b Display
The GDL-82 transmits on UAT (978Mhz). I did some research on this, and my understanding is that whatever frequency you have, you get direct transmission via TIS-B between aircraft, and get retransmits via ADS-R from ground stations for freqs you don't have. If you have both Frequencies OUT, then you'll get all your traffic via TIS-B.
If you have dual IN capability (most units are dual freq IN), you'll see a complete traffic picture via the combination of TIS-B and ADS-R. There is of course some lag on ADS-R, but it should be on the order of milliseconds rather than seconds. Assuming you have OUT capability on 978, 1090, or both, your IN capability is really what determines what traffic you see. Of course if you are not OUT equipped you are not getting ADS-R at all and are completely reliant on what TIS-B traffic you can "sniff" out from other airplanes' TIS-B broadcasts...which is why ADS-B IN only provides some but not a complete traffic picture.
EDIT: The above is incorrect and I misstated it. You *will* still receive ADS-R traffic, but only from *other* aircraft receiving from the ground stations instead of rebroadcasts directed at your aircraft. As a result the traffic received may or may not be relevant to you based on position, so again your traffic picture is still incomplete.
There is nothing inherently worse or better between 978 and 1090 devices are far as I can tell. 1090 devices get UAT traffic via ADS-R, just as 978 devices get 1090 traffic via ADS-R. Only dual freq OUT devices don't use any ADS-R, and those tend to be very expensive (and usually require multiple antennae) compared to single freq devices. 978 devices are far more common than 1090 at the low altitudes I generally fly at, so I get traffic that is more relevent to me via TIS-B traffic on 978 than I would get on 1090. Sure anybody who flies at 18,000ft+ or has deep pockets will be on 1090, but they are usually well above me and I'm okay with getting their signals via rebroadcast.
At least that is how I understand it. 
Back to the issue at hand...on the long flight I mentioned previously where I didn't see my wingman for about four hours, *both* aircraft were equipped with the Garmin GDL-82 for ADS-B OUT, and both had dual-freq ADS-B IN. He saw me the entire time, but I never saw him (but I did see other traffic). The only equipment difference was in the IN units and the displays (I was using the 740b, he was using Foreflight on an iPad). That's why this concerns me -- We were close enough we both should have been getting the same TIS-B and ADS-R signals, but yet one aircraft was "invisible" to the other. In my mind the obvious potential reasons for that is that either the Stratux is not relaying all traffic to iFly, or iFly is filtering the traffic based on some unknown criteria (or a bug).