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I know primary radar of ASR can determine only slant range, so by using Mode C transponder, ATC can know the altitude of aircraft.

And I found that if altimeter setting of radar is changed, displayed altitude also be changed.

So I guess that Mode C transponder can deliver only pressure information not altitude information. So Radar convert this pressure information to altitide information referring to radar altimiter setting. And it is not important what altimeter seeting pilot set.

Is it correct?

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  • $\begingroup$ Mode C transponders transmit QNE altitude (flight level). The FL can be provided by a blind encoder or an encoding altimeter. They are built to provide, in the US, the FL with an accuracy of 125 ft, the pilot has no action on the setting. $\endgroup$
    – mins
    Commented Nov 19, 2017 at 12:32

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Correct. Mode C responses include the pressure altitude. This means that many installations use a separate altitude encoder that has no barometric settings.

Whether it is integrated or a separate unit, the kollsman setting does not change the pressure altitude reported.

From http://www.faraim.org/aim/aim-4-03-14-133.html

  1. Mode C Veil. [...] Unless otherwise authorized by ATC, aircraft operating within this airspace must be equipped with automatic pressure altitude reporting equipment having Mode C capability.
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    $\begingroup$ Thanks. Does 'pressure altitude' mean the altitude in 2992inch? I mean, no matter what pilot sets altimeter setting, Mode C sends altitude information by converting as standard atimosphere. $\endgroup$
    – Min
    Commented Nov 19, 2017 at 6:15
  • $\begingroup$ Yes. aviation.stackexchange.com/questions/8168/… $\endgroup$
    – BowlOfRed
    Commented Nov 19, 2017 at 6:17

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