We have just test flown a new 1090 ADSB installation, using Apareo Stratus ESG Transponder, which has internal GPS.

The PAPR for the test flight show both Baro Alt and Geo Alt errors in the Missing Elements Table. The MCF count is identical for both, and the %Fail percentage is 8.27%.

All ground testing has been successful, both before and after the test flight.

I have studied the FAA PAPR User's guide and understand what the

I would appreciate any thoughts on the possible reasons for these errors.

  • 2
    $\begingroup$ Try to spell out less common abbreviations at least once in the question or comment. What error do you want feedback about? You only say there is an altitude error but you do not tell us exactly what that error is. $\endgroup$
    – Max Power
    Commented Oct 11, 2023 at 18:26

1 Answer 1


In the ADS-B velocity message (Field Type Code 19, IIRC), the geometric altitude is encoded as difference from the barometric altitude. If barometric altitude is not available, the difference cannot be computed.

Based on this, I deduct that the problem lies in the barometric altitude source connected to the transponder. In 8.7% of the messages that did not provide a valid altitude to the transponder, resulting in both pressure altitude and geometric altitude becoming unavailable.

I would have the connection between the altitude encoder and the transponder and the encoder itself checked out.


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