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300' Difference In Traffic Altitudes Between Two Units

ArVeeNiner

Well Known Member
Last weekend I decided to cross check ADS-B traffic reports between two different displays and saw some surprising differences.

First of all, this is what I?m running:

-ADS-B In/Out: Echo UAT + SkyFYX GPS

-Transponder: Garmin GTX-327

-Display #1: iFly 740b Moving Map GPS

-Display #2: Apple iPad Mini 4 running FlyQ

Regarding relative traffic altitudes, the displays were not agreeing with each other by about 300 feet. I found this to be a pretty consistent delta. That?s a huge difference! They are both getting their information from the Echo UAT so I?m thinking the issue lies within either the iFly 740b software or FlyQ and not with the Echo UAT. I didn?t get enough data to figure out which one was lying to me but I?ll have to figure that out on a future flight.

Any thoughts?
 
Dave:

I will surely contact iFly.

Jim:

Hmmm. Well, I know the iPad was getting traffic information from the Echo so I would imagine that would be altitude info as well. If so, then that should be pressure altitude and not GPS altitude right?
 
Could it be the difference between pressure altitude and GPS altitude?

I assume what Jim is saying and I am thinking is the ADSB is reporting altitude difference from your flight and the traffic flight position. But what are the two devices using for your altitude? GPS or Pressure?
 
iFly uses Pressure Altitude to calc the difference with ADSB reported traffic altitude since that's what is being broadcast. I don't know what FlyQ uses.
 
I assume what Jim is saying and I am thinking is the ADSB is reporting altitude difference from your flight and the traffic flight position. But what are the two devices using for your altitude? GPS or Pressure?

I believe that the Echo receives altitude information from the transponder. This information is sent out to the ADS-B tower on the ground as well as sent via WiFi to any device connected to it. In my case both the iFly and iPad/FlyQ get this same information. I'm no expert here though.

What is interesting is that I've played back the flight on FlyQ on my iPad. It's showing my altitude to be about 115-150 feet lower than I was flying. This doesn't account for the 300' difference I was seeing but perhaps when it is receiving data from the Echo it displays pressure altitude, not GPS altitude. Of course pressure altitude would not be the same as the indicated altitude I was seeing.

I can't remember the exact altimeter setting that day but it was 30.XX so my indicated altitude would have shown higher than pressure altitude.

The more I think about it the more my head spins.

Bottom line is I believe that anything that receives the Echo data via WiFi should display the same altitude deltas for traffic right?
 
I?ve got the same ADSB arrangement in my RV7. My iFly seems to use the baro setting from the nearest tower. I had a friend in the plane with Foreflight linked to my ADSB. His iPad was reporting the baro for the destination and the reported traffic alt. was different by the baro diff. YMMV. Bob
 
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