ADSB traffic displays wrong altitude

[tdyer]

I seem to have a ADSB altitude discrepancy on my Challenger Lite. The traffic shows up 400ft lower on the EFIS then it does on my IPAD. I have had the system installed for several years and the two units used to agree. I am using the Uavionix EchoUAT and the skyFYX via serial port to the Challenger Lite and WiFi to the IPAD. Has anyone had a similar issue with their EFIS?

Thanks

[rainier]

Traffic on the screen is shown relative to your altitude. I would assume your iPAD and the EFIS are using a different altitude source ?

[tdyer]

My previous post wasn't very clear, It appears that my Challenger is reporting traffic 400ft low. I have flown near other aircraft and verified that ForeFlight is correct and the efis is reporting low. I have 2 altimeters in the plane and they are reading within 10ft of each other.
I have talked to tech support at uAvionix and they suggested reading the pressure altitude they are decoding from the transponder and verifying it matches the display on the transponder. Is there any way to diagnose this on the efis side?

Does the Challenger use the onboard altimeter and the adsb pressure altitude of the other aircraft to calculate the altitude offset?

I believe Foreflight is using the local barometric pressure it receives and GPS to determine traffic altitude.

Thanks for your help,

Terry

[andynoiseboy]

Isn't this a QNH versus ISA (1013.25hPa) thing?