The ASTERIX (version 2.1) items of ADS-B GS cat 21 contain True air speed but when we check the Asterix data that we recieved by GS we did not find the True air speed value, why ?

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1 Answer 1


If you scroll down in the document to the description of true airspeed, it says "This item is optional."

Many aircraft do not have the capability to measure their true airspeed, and it makes no sense to transmit anything if the value is unknown.

  • $\begingroup$ Thanks for your reply.but isn't the optional mean it is optional to decoding this items?.also the whole aircraft that I checked transmitted the true Air speed via EHS register ( Radar interrogation) $\endgroup$
    – SkyMaven
    May 10, 2023 at 18:38
  • $\begingroup$ @Uniform No, it says that under "encoding rules," not "decoding rules." An aircraft does not need to include true airspeed in an ADS-B message. Even if it is available, an aircraft is not required to transmit it. $\endgroup$
    – Chris
    May 10, 2023 at 19:05

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