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If a transponder has a blind altitude encoder how does ATC or flight following know the actual altitude?

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General Aviation

It`s my understanding that many GA aircraft`s transponders have blind altitude encoders (set to a barometer of 29.92) so they can of course be way off (not uncommon to be as much as 500 feet off).

That would be not a huge problem in separating and advising if everyone was flying with blind encoders, but (again IIRR) some aircraft have thier regular panel adjustable altimeters (that they periodically reset to reported current barometer) that transmitt that more realistic altitude into the transponder, and in turn to ATC radar, etc.  So they`re, so to speak, on a different page it would seem.

(And let`s not complicate the issue further with the pilot that might not have a transponder and reads/reports his altitude off the GPS because GPS altitude is frequently more accurate than reading off a altimeter corrected to a barometer reading from a distance station.)

So my question is along these lines:  Does ATC (or its presumably sophisticated radar system) know if a transponder has a blind encoder sending very likely inaccurate altitude reports (unless pressure happens to be close to 29.92 that moment)?

I`m imagining that if the transponder is known to have a blind encoder that it would be easy to take the reported altitude from it and have the system at ATC (which knows the actual barometric pressure ... at least close to the airspace) and automatically make a correction for the blind encoded pressure report.

Is that so?   Or is there something else mitigating this potential SNAFU?

Or are aircraft at the same altitude flying around with the two different types of transponder altitude encoders (blind vs non-blind) often substantually in disagreement about what altitude they are on?



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3 Answers

  1. Jonathan Seitz on Jul 01, 2014

    The computers in Air Traffic Centers compensate for the blind encoders by adjusting the input signal.

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  2. John D Collins on Jul 02, 2014

    You have a basic misconception. Blind encoders and encoding altimeters (the ones that include an adjustable barometric setting) both encode the altitude based on a barometric setting of 29.92 (pressure altitude). In the case of a blind encoder, there is no pilot adjustment. Although an encoding altimeter has both a indicator to the pilot that is adjustable to the local barometric pressure, it also has an encoder that uses 29.92 as its reference for the encoded altitude. So all encoders send exactly the same pressure altitude to the radar site. The software at the radar site corrects the pressure altitudes to an equivalent altitude.

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  3. Russ Roslewski on Jul 02, 2014

    And if you think about it, that’s what you want. You don’t want the transponder to send your indicated altitude to ATC – after all, what if you have it set incorrectly? ATC thinks you’re at the right altitude but you’re not, so they take no action. Instead, you want it to send a pressure altitude based off a standard altimeter setting to ATC so that everybody is working off the same reference. At the ATC facility they will enter the actual altimeter setting for the area they’re working and the display of aircraft altitude will be adjusted accordingly.

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