I'm wondering if there's a relationship between Altimeter, Mode C and Transponder. I'm asking because I understand that the Mode C transmits pressure altitude and aircrafts identification code to ATC and nearby aircrafts. If this is correct may I know how these components are connected to each other.
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2$\begingroup$ This question is similar to: Where does the transponder read altitude, or does ATC get it from radar?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. -- see in particular my answer on that question. $\endgroup$– randomheadCommented Oct 18 at 4:25
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This is really multiple questions in one, so while I suggested that the "what is Mode C" question is is a duplicate of another (really several) on this site, the others may not be... but they're all relatively quick and relatively connected, so here's a brief answer. I don't have the time to make it more brief.
You asked about:
- Altimeter
- Mode C
- Transponder
The altimeter and the transponder are physical devices installed in an aircraft. Mode C is a communications/encoding protocol used by the transponder.
- The altimeter is a device which indicates to the pilot what their altitude is. The altitude indicated is referenced to a certain altimeter setting. If the pilot has not set the altimeter to the proper setting, the indicated altitude will not be accurate.
- The transponder can send two distinct pieces of information to ATC (and possibly other surrounding transponders), depending on which one the ATC radar has requested: Either a specific identifying 12-bit number, or the aircraft's pressure altitude encoded into Gillham code (a modified Gray code). The identifying number is called a "Mode A" response and the encoded pressure altitude is a "Mode C" response.
The transponder uses an "encoder" to translate the pressure altitude into the proper Gillham code. This encoder is distinct from the altimeter used by the pilot: The pilot can spin the altimeter setting up or down and that will change what they see on their instrument, but it will NOT affect the Mode C and so will NOT change what ATC sees on the radar scope.
