ForeFlight and GTN750 ADS-B altitude discrepancy

[DesertNomad]

I have a GTN750 with a GDL88 for ADS-B in. I also have a FlightStream 210 and ForeFlight (latest version). Today on a flight, traffic was reported on my GTN750 at +18 (1800' above) but the same traffic on the iPad was reported at +15 (1500' above).

I have never seen this. There was no other traffic to compare (rural Nevada), but on the way back there was some traffic and the GTN and FF agreed.

Any idea how FF could report a different altitude since it should be receiving the same data as the GTN? Cellular was turned off so the traffic should not have been from ForeFlight's rendition of FlightAware-stlye traffic.

 

[Jesse Saint]

I see this regularly. The GTN would likely be more accurate because it has a better Gps altitude and pressure altitude information. I’m not sure how much information makes it to the iPad, but the difference is likely a pressure altitude versus true altitude versus gps altitude issue. When in doubt, believe the GTN. Again, the traffic sends out its position and altitude information. It’s your equipment that converts that to a relative position and altitude, so differences are based on your equipment and what basis it uses to make that conversion.

 

[Jim_R]

The GTN would likely be more accurate because it has a better Gps altitude and pressure altitude information.

Where does a GTN navigator get pressure altitude? I thought that was a pretty rare setup, requiring a separate air data computer device of some sort.

 

[Jesse Saint]

It usually gets it from the transponder or directly from the altitude encoder.

 

[John Collins]

I already responded on another forum.

Go to More>Devices>Garmin Connext> and find the setting "Use as pressure altitude" and turn it on. If it is off, there is a bug that prevents the pressure altitude from the ownship message being used to compare its pressure altitude with the target pressure altitude and can result in what you are seeing. This will be fixed in a future update.

Did you already report this to team@foreflight.com? If not, I would encourage you to do so in the future. Hopefully you will get faster response going that route.