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Transponder Issues

Pfatt2

Member
I have a 6A with the EchoUAT connected to my Garmin GTX327. Also Dynon D100 with IFD440.
ATC continues to show me at the wrong altitude. An FAA check shows my ADSB reporting is correct. When I recycle the transx, it usually comes back and shows the correct altitude.
I know these ADSB products work in very mysterious ways... How can I tell if it is my transponder (is the 327 an encoding transponder or would I have a separate encoder?) or if it is the Echo?
Does anyone have a similar setup who might have some insight?
Thank you.
 
How far off is ATC seeing you. Are you sure that it is not your altimeter that is incorrect? My understanding is that if you are getting a good ADS-b performance report that your Transponder is reporting you in the same position that ATC's radar is showing you - I could be way off on that, but that is how I currently understand it.
 
ADSB Altitude

As it is ADSB, the position (GPS) most likely comes from the IFD440.
There will be a difference between GPS altitude and the not so accurate pressure altimeter.

Good luck
 
I have a 6A with the EchoUAT connected to my Garmin GTX327. Also Dynon D100 with IFD440.
ATC continues to show me at the wrong altitude. An FAA check shows my ADSB reporting is correct. When I recycle the transx, it usually comes back and shows the correct altitude.
I know these ADSB products work in very mysterious ways... How can I tell if it is my transponder (is the 327 an encoding transponder or would I have a separate encoder?) or if it is the Echo?
Does anyone have a similar setup who might have some insight?
Thank you.

The 327 has to get it's mode C altitude data from somewhere, which would be either the Dynon or a separate encoder (the echo has no encoder).
My guess would be a separate encoder is getting flakey or needs adjustment. Part of a 91.411 cert is to verify accuracy of the encoder as it relates to the barometric altimeter.
I've tested a zillion 327's and don't recall ever seeing one report incorrect altitude unless it was receiving bad data or had a external wiring fault.
(Older 327's do suffer from frequency drift though and need that adjusted occasionally)
 
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As it is ADSB, the position (GPS) most likely comes from the IFD440.
There will be a difference between GPS altitude and the not so accurate pressure altimeter.

Good luck

All aircraft altitudes are based on barometric which will be different than GPS altitudes, with the correct barometric setting altimeters are quite accurate.
 
Remember the altitude you see is adjusted to local barometer pressure, what ATC sees and reports back to you is pressure altitude, same as if you rolled the baro input to 29.92. Next time ATC says they see you at x altitude, change your baro setting to 29.92, at that point you can know for sure if something is askew.

Your Dynon is likely your encoder, feeding altitude data via rs232 or gray code to the GTX327. Your position data is coming from the ifd440. You could have an separate encoder as well. The GTX config page shows how your unit is receiving altitude data. Download the manual and determine how to view, and how it is receiving altitude info, rs232 #1 or rs242 #2 or gray code.

https://chapters.eaa.org/eaa245/-/media/79DDFC29FE204136896E96F90A075FC0.ashx


I have GTX327 and EchoUAT in my 6a.
 
So much misinformation!

1. Your altimeter displays indicated altitude. But your encoder sends pressure altitude to your transponder and adsb-out.
2. ATC computers know local altimeter settings, and re-convert pressure altitude to indicated altitude. What ATC tells you should match your altimeter.
3. Exception to rule 2: I’ve seen a few times when over the central valley I changed ‘early’ to LVK’s altimeter setting, but ATC’s computer still had me on another altimeter setting, and the controller noted the difference.
4. The rules for adsb require you to use the same encoder for both your transponder and adsb-out, so this split should not happen.
 
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