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Why does my AXP 322 transponder show 300' low?
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warrenc View Drop Down
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    Posted: 28 Mar 2016 at 9:08pm
I have noticed that my AXP322 transponder shows 300' low when ADSB out is interegated, but right on altitude when with center/approach.  The following flight from Tallahassee to Sarasota is an example.  Cruising at 9,000', ADSB shows 8,700' the whole way, except for one hit from Jacksonville, about half way down, has me at 9,000'.  Sorry the following tracklog isn't a URL.  Is this just an adjustment somewhere?  (Fighting a 20K headwind)

http://flightaware.com/live/flight/N777YM/history/20160308/1400Z/KTLH/KSRQ/tracklog
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pburger View Drop Down
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Points: 406 		Post Options Post Options   Thanks (0) Thanks(0)   Quote pburger Quote  Post ReplyReply Direct Link To This Post Posted: 29 Mar 2016 at 9:30am
Your transponder is reports and shows the "pressure altitude" from your encoder.  The encoder altitude is based on a 29.92 altimeter setting.  The only time your pressure altitude will match your actual altitude is when the altimeter setting is 29.92.

What was your altimeter setting at the time?

I assume that "Flightaware ADSB" sources don't correct for the local altimeter settings along the route. (It looks like they just pass the pressure altitude as received from the ADSB data packet.)  ATC facilities (like a Center or Approach facility) do correct for the local altimeter setting, and should match what you see on your altimeter.




Edited by pburger - 29 Mar 2016 at 9:40am
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AviSimpson View Drop Down
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Points: 765 		Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSimpson Quote  Post ReplyReply Direct Link To This Post Posted: 29 Mar 2016 at 9:34am
Originally posted by warrenc warrenc wrote:

I have noticed that my AXP322 transponder shows 300' low when ADSB out is interegated, but right on altitude when with center/approach.  The following flight from Tallahassee to Sarasota is an example.  Cruising at 9,000', ADSB shows 8,700' the whole way, except for one hit from Jacksonville, about half way down, has me at 9,000'.  Sorry the following tracklog isn't a URL.  Is this just an adjustment somewhere?  (Fighting a 20K headwind)

http://flightaware.com/live/flight/N777YM/history/20160308/1400Z/KTLH/KSRQ/tracklog

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