Mode C Transponder Error Tolerance

[Colinmtbmx]

Could anyone point me towards the source information for the maximum permissible error between your altitude (as reported by your altimeter) & your transponder's reporting altitude (as reported by ATC?) Can't seem to find where this information is, Thanks.

[nbinont]

There isn't such a spec, but there are related specs.

In the altimeter setting region (probably where you are asking about), your altimeter is meant to display your altitude AMSL, using the nearest altimeter setting. This is handy, as it allows you to determine how high you are relative to the ground AMSL. But there are many errors due to calibration and the atmosphere (primarily low/high pressure changes, temperature) and how accurate of a altimeter setting you are using. Near the ground around the ground station, the calibration error is typically the largest error, and actual errors are usually limited to 100ft range or so. (This is the primary reason CAT I ILS and RNAV APV approaches have the minimums they do.) Further away, errors can increase to around 1000ft (but weather isn't usually much more).

BUT your transponder doesn't send the altitude displayed on your altimeter to ATC. Your transponder sends your pressure altitude to ATC. That is, the altitude your altimeter would read if you set the altimeter setting of 29.92. Then the radar/adsb receiver applies what they think the altimeter setting should be.

If we assume everyone is using exactly the same altimeter setting and we just want to compare what ATC sees to what you see (let's ignore weather effects, as weather effects will be the same for your altimeter and transponder altitude encoder), then the question comes down to how accurately the altitude encoder is calibrated. Here's the spec:

https://tc.canada.ca/en/corporate-servi ... tions-cars (read Appendix B)

Then the last piece of information you need is the precision by which your transponder reports your pressure altitude. For Mode A/C It is reported to the nearest 100ft: http://www.aeroelectric.com/articles/Al ... cascii.txt Mode S/ADSB has a scaled format that can report down to 25ft increments: https://mode-s.org/decode/content/ads-b ... e-decoding .

In summary, for most GA aircraft, outside the flight levels, they should match within about 100ft.

PS: If you have a ADSB/Mode S transponder, it will also report your GNSS height (which is entirely independent of the pressure altitude!)

[16SidedOffice]

Check the AIM, section 8.5.

[ahramin]

During the biennial check it's 125'. CAR 571 Appendix F.

[photofly]

Could anyone point me towards the source information for the maximum permissible error between your altitude (as reported by your altimeter) & your transponder's reporting altitude (as reported by ATC?) Can't seem to find where this information is, Thanks.

I think if your altimeter shows 300' different from what ATC receives, they must ask you to "stop squawk altitude" and you have to proceed without Mode C. So the operationally permitted maximum difference is 200'. Is that what you mean?

[Colinmtbmx]

Could anyone point me towards the source information for the maximum permissible error between your altitude (as reported by your altimeter) & your transponder's reporting altitude (as reported by ATC?) Can't seem to find where this information is, Thanks.

I think if your altimeter shows 300' different from what ATC receives, they must ask you to "stop squawk altitude" and you have to proceed without Mode C. So the operationally permitted maximum difference is 200'. Is that what you mean?

That is what I meant, yes. Was able to find it in the AIM. Thank you.