MODE C ALT and Baro corrected altitue

WannFly

Final Approach
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Priyo
Last Saturday i asked for a VFR Practice Approach to ILS 36 and was getting vectored to FAF; and was asked by ATC to maintain 2800. i noticed my transponder was reporting 2700, when i was holding 2800, which as i understand is within limits [91.217(b)]. I have a blind encoder

there was another aircraft talking to ATC and ATC informed them about my location saying Archer is indicating 2700.

as i understand, ATC has some kind of Gizmo that corrects the Pressure Altitude and Baro and shows them the correct Altitude the Mode C is reporting. ... not that its a big deal with 100 feet difference, i am wondering why ATC was getting 2700 instead of the baro corrected 2800 that i was flying. the reason i ask is, when it gets too cold, i have noticed over 300 feet difference between what my baro is saying and what the transponder reports.

thoughts?
 
Two different measuring instruments, with different error over the whole scale of measuring will never be perfectly matched, nor are they required to be.

Tune your altimeter to 29.92 and it should be close to what you see on the transponder... Obviously it won't be identical.
 
Last Saturday i asked for a VFR Practice Approach to ILS 36 and was getting vectored to FAF; and was asked by ATC to maintain 2800. i noticed my transponder was reporting 2700, when i was holding 2800, which as i understand is within limits [91.217(b)]. I have a blind encoder

there was another aircraft talking to ATC and ATC informed them about my location saying Archer is indicating 2700.

as i understand, ATC has some kind of Gizmo that corrects the Pressure Altitude and Baro and shows them the correct Altitude the Mode C is reporting. ... not that its a big deal with 100 feet difference, i am wondering why ATC was getting 2700 instead of the baro corrected 2800 that i was flying. the reason i ask is, when it gets too cold, i have noticed over 300 feet difference between what my baro is saying and what the transponder reports.

thoughts?

Just acceptable instrument error. Mode C readouts on the Controllers scope are considered valid if they are within 200 feet. Yes there is that Gizmo. Local pressure is inputed into the Radar Computer either by the Controller manually doing it or automatically. Encoders are fixed at 29.92. The Radar computer does the math and puts an altitude on the Scope
 
Your encoder and, or your altimeter is slightly off. Need to dial in the Kollsman window 2992 for a sec and see how closely it matches the transponder. 100 ft off is pretty common though.

But yes, depending on the system, the scope either gets a auto feed (29.73) from the AWOS / ASOS or it’s one that the controller types it in manually.

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That is how I discovered a pito static system leak once...ATC said I was over 300' off in Bravo vs what I was indicating.

...but to your question as I understand it there is an acceptable margin of error as the altimeter setting for your sector that you are flying through may vary from your actual point in space at that time...but the goal is more have everyone on the same setting so all aircraft in proximity of each other may be "off" at the same margin.
 
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