ADSB Traffic Alert

Went on a short trip last week with a friend in his Bonanza to show off his Garmin 750 and ADSB receiver, comparing features to the Dynon Skyview in my RV. Leveled off at 3000, watched him put autopilot on, then looked directly ahead and saw an oncoming airplane. I said "Traffic, straight ahead" and pointed. He said "Don't see 'em" then maybe two seconds later "I don't" and I grabbed the wheel, turned hard and pulled. As I turned the wheel, I heard from the Garmin, "Traffic Alert, Same Altitude." Within a five or six seconds a Cessna Caravan passed just behind us, maybe a quarter mile, same altitude.

This was a worst case scenario, two fairly fast airplanes approaching head on, closing at 300 knots or so. I did some reading later and found that Dynon will give a Traffic Advisory warning if a 1/4 mile conflict is projected to occur in the next 30 seconds. Dynon also states there may be a lag of 2-12 seconds due to radar (did they say lag) and other technical issues. I suspect the Caravan did not have ADSB out and we were getting his position from TIS. Uh, let me think.... say 20 seconds plus a 1/4 mile would have put the advisory about two miles apart. Don't know how that compares to the Garmin but that is not much time and certainly not the warning we received.

For several weeks I have been oohing and aahing about seeing all that traffic on my display. Yes, it really is neat technology. With all that it will do, there are limitations. I've been flying 38 years and seen a few airplanes close up. Made a couple of turns to add some distance. This is the only time I have seen another plane when pointed directly at each other. There is little time to react.

Dave, you are so correct when you see them dead on or there about they are really harder to see. I too since my RV has all the ABS.B am amazed eapecially when you use the Ipad running foreflight also.

Jack

Keep head out the window

The technology can cause complacency. For a long time I would get a traffic warning as soon as I departed my local field. It turns out it was just picking me up in the system. So, after a while I got used to it, and kinda used it as a "system is working" check. One day as I took off and was making a climbing right turn, Igot the usual TRAFFIC aural warning and didn't oay attention. As I lowered the wings to level out of the turn there was a King Air at my 9 0'clock on a collision course. REAL CLOSE. I pushed hard, and I don't think he/she saw me at all.

Vic

It is a GDL88. If I read it correctly, the pilot users guide has a chart showing a ratcheting scale with <2350 height above terrain getting a 25 second alert if projected to conflict within .20 NM. The guide warns that TIS position is broadcast every 3-15 seconds. Do the math, could be real close. That is why Garmin repeatedly warns about using as traffic avoidance.

I don't know if this traffic ever showed on the display in advance. I was looking out the windows, then watched the autopilot being set up and a trim button being pushed a couple times, then asked a question about it's use. Then looked out again. I don't think the Caravan saw us, he appeared not to maneuver at all.

SFO runway 28 departures require us to stay below 1500 ft along the coast line. I was giving a first ride to someone, and she asked, "Why isn't that plane showing up as traffic." I had explained how the traffic shows up on the screen. Sure enough a big B747 had just taken off. It was probably at 2 or 3,000 feet at our 1 or 2 o'clock. No traffic signal. I ranged in and out and saw traffic landing at SFO and elsewhere. No idea why a huge B747 wouldn't show up.

My newbie passenger was thoroughly unimpressed with Nextgen technology!

Update: actually, i realize now it might have been a setting with Skyview that only shows traffic when it is within a certain altitude threshold. B747 could have been high enough to be above the range.

jchang10 said:

No idea why a huge B747 wouldn't show up.

My newbie passenger was thoroughly unimpressed with Nextgen technology!

Click to expand...

All of us who work in the IT field are (or bloody well should be) well acquainted with the often wide gap between theory of operation, and actual results. The more complex the system, the more ways it can fail, and Nextgen is very complex.

You can build anything you want, but you should never -- ever -- trust it completely.

IIRC ATC radar has a 4-8 sec update rate for traffic it is observing via a transponder (MODE C or S). I am not sure what the delay is in receiving the data from the ATC Radar system and "repackaging" it for retransmission via TIS to an ADSB-B receiver but it isn't zero.

Bottom line is I believe the latency built in to the traffic data displayed via ADS-B acquired in any manner but via air-to-air ADSB out transmission seriously compromises the usefulness for ADS-B for collision avoidance.

It is great to provide greater situational awareness ("There is a plane at 5 miles converging at my 7 O'clock") but not good to avoid collision (as in " TURN RIGHT DIVE NOW").

I may be wrong but a lot can happen in 8 seconds on a head on heading as the OP documents.

I'm hoping to be proven wrong by those more knowledgable than I.

Floor is open.

All true but I have yet to have a false positive with my system, on the other hand plenty of actual traffic that did not show up in my system. The complacency can occurred in a number of ways including being on flight following and following the ATC instructions.

About a month or longer ago, coming back from Monterey and on a VFR with FF I was asked to make a 20 degree right turn for traffic, which I complied with but that put me directly in path of the incoming traffic 400? above us. I saw the traffic from ADSB and kept thinking that I will get a warning from ATC but never warned us about it. As we got closer, I made a diversion to avoid it and soon we were able to see the traffic visually. It would have been directly above us within 400 had we not made the direction change. I was glad to have the system to see it well in advance.

DaleB said:

All of us who work in the IT field are (or bloody well should be) well acquainted with the often wide gap between theory of operation, and actual results. The more complex the system, the more ways it can fail, and Nextgen is very complex.

You can build anything you want, but you should never -- ever -- trust it completely.

Click to expand...

EXACTLY! ADS-B is an aid, not a substitute, for looking out the airplane for traffic.

rbibb said:

...
Bottom line is I believe the latency built in to the traffic data displayed via ADS-B acquired in any manner but via air-to-air ADSB out transmission seriously compromises the usefulness for ADS-B for collision avoidance.
...

Click to expand...

Hence why I have a passive traffic monitor in my aircraft. Anyone pinging with mode A/C/S will be detected. Not dependent on ground equipment working perfectly. I wish all ADS-B In devices incorporated a transponder based passive monitor rather than just listening to ground transmissions.

Inn a 2.8 hr Phase 1 floght today I had 3 traffic alerts from the Skyview with ADS-B. All 3 were shown on the screen well before there was an alert. One was a Boeing that passed about 500ft above and maybe 1/4 mile awa. The second was a Cessna that I likely would have at least gotten close enough to see the color of the pilot's eyes if I hadn't turned. The third was a twin a couple of hundred feet right above me. All 3 times the ADS-B had them on e screen long before I got the alert, and there were 4 progressions of notifications that they were there. The sky is so big and planes are so hard to see until they are very close. Knowing that they are coming is priceless, IMHO.

vic syracuse said:

The technology can cause complacency. For a long time I would get a traffic warning as soon as I departed my local field. It turns out it was just picking me up in the system.

Click to expand...

I got a false target matching my coarse 100 ft directly below me while transitioning below a class B last week. Displayed directly below me for 5 minutes. Very disconcerting. False data is worse than no data. I just turned off the traffic display and used my eyes looking out the window full time. Not perfect but so far has proven to be better than ADS-B.

I had a similar ghosting issue over foristell into SUS this last Saturday. I suspect it's an FAA system issue. It's the first I have seen ghosting on my system

Actually, i realize now it might have been a setting with Skyview. There is a setting to see ALL traffic or just those within 1 or 2000 ft of your altitude. I do recall turning on the feature. The problem with showing ALL traffic in my area is that it gets quite busy with commercial traffic. The screen just fills with traffic diamonds if you zoom out enough.

This brings up another point. I think they need to work on the User Interface for traffic. For instance, I can zoom out enough where the size of the traffic diamond is literally touching my airplane icon. It "looks" like we are on top of each other.

However, you realize the zoom scale is set to like 50 miles. You zoom in and realize that you are actually like 5 miles apart now. I hate that part of the traffic display. Not sure what a good workaround is.

This ghosting and latency is just odd. GPS has the most precise clocks, so doesn't the location metadata have the time so it should show up precisely in the correct spot on a display? Or is it's acquisition delayed and it is just suddenly there?

Do any software guys know why this occurs?

Maybe some patent opportunity for projection of real time location to make this more consistent and usable.

The latency issue is for targets displayed where their location is determined from ATC Radar (Mode A, C, S) transponder returns. If the Radar only updates, say every 5 seconds (based on radar dish rotation rate) it only knows where the target was potentially 5 seconds ago, etc. The system takes this data, repackages it into the proper format, and broadcasts it via TIS over the ADS-B system. System in your plane decodes and displays traffic where it was 5 seconds or more before...

That is why when I am flying with my ADS-B IN receiver and start seeing traffic because I am near an ADS-B out aircraft that triggers the broadcasts I see my own craft fling in behind my actual location. First time I saw that I started maneuvering to see who was behind me....