Secondary Surveillance Radar

[steve181]

I'm interested to know how the controller is able to see an aircraft's altitude on the radar screen when the aircraft's transponder is set to mode Charlie. It's not like the transponder has a built in altimeter or anything.

[43Inches]

An encoder attached to the altimeter sends basic pressure data to the transponder, which then transmits it to the SSR receiver. The data is then interpreted by the controllers software and displayed as an altitude readout. I think the system can still generate close to the correct altitude even if operating on an incorrect pressure setting.

[steve181]

Oh ok so the altimeter & transponder are physically linked together.

Any chance you know a little more about that encoder that's attached to the altimeter? The transponder is a digital system where as a traditional altimeter is analog based as far as I'm aware so the encoder would need to make that conversion. Is the encoder like a computer chip?

[43Inches]

The encoder may or may not be attached to the primary altimeter. It is a pressure transducer (aneroid or solid state) not related to the information displayed on the altimeter. The encoder just senses and converts the actual static air pressure to digital format and sends its data referenced to 1013, aka pressure altitude. As stated earlier this information is converted by the controllers system to an actual altitude using QNH. If you operate on a different or incorrect QNH the error may be queried by the controller as they may see you off altitude even though yours indicates spot on.

[Jabawocky]

Tha vast majority (if not all) of altitude encoders in GA aircraft are a thermal barometrics pressure device. Some send out digital data via an RS232 interface to the Transponder this is then transmitted in Mode C when your transponder is pinged. The majority of encoders transmit a value in a greyscale format, they have a 10 wire cable (plus power) and a 1 or 0 on each wire makes up the greyscale output.

Some of the manuals from the Microair Transponders explain all this in plain English and with pictures :) if you wish to know more!

The encoder is calibrated for 1013 millibars and stays that way! Any variation in local/area qnh is accounted for at the ATC end of things.

Hope that helps!

J:ok:

[steve181]

That information does clear things up! Thanks guys. :)