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ATC altitude question

pa38112

Well Known Member
The altitude we see on our altimeters is corrected for atmospheric pressure prior to takeoff. The encoder that transmits our altitude to ATC is not. Is the encoder set to standard conditions and then the ATC computer corrects the altitude for planes below 18,000 ft?
How are they handling the new GPS altitude information VS the encoder information?
 
The encoder is set to standard atm pressure/conditions and the computer corrects it below Transition Level. GPS altitude info? Dont know
 
The altitude we see on our altimeters is corrected for atmospheric pressure prior to takeoff.
…….
How are they handling the new GPS altitude information VS the encoder information?

Strictly speaking, your barometric altimeter is corrected for non-standard pressure AND temperature prior to takeoff….

I think you’re asking about adsb-out data? It is supposed to send out the same pressure altitude that the transponder uses. For some specific adsb boxes - ones that passively listen for the transponder - the faa is allowing the box to send gps altitude instead, if no transponder signal is available. Presumably if the transponder isn’t being ‘pinged’ the airplane must be low, in which case there are only small differences between pressure (corrected by ATC to local altimeter setting) and gps altitude.
 
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