@squawk
    Preparing search index...

    Interface SurveillanceAltitudeReply

    A DF4/20 Mode-S surveillance altitude reply. Unlike a squitter, this is a targeted response to an interrogation - its CRC is XORed with the responding aircraft's ICAO address rather than being a plain checksum, so candidateIcaoHex is only trustworthy once cross-checked against an address already known from squitter traffic (this package does not perform that cross-check itself; see ModeSMessageEnvelope.crcRemainder).

    interface SurveillanceAltitudeReply {
        altitudeFt: number | undefined;
        candidateIcaoHex: string;
        identActive: boolean | undefined;
        kind: "surveillanceAltitudeReply";
        squawkAlert: boolean | undefined;
    }
    Index
    altitudeFt candidateIcaoHex identActive kind squawkAlert
    altitudeFt: number | undefined

    Decoded altitude in feet, or undefined if the AC field is empty or invalid.

    candidateIcaoHex: string

    Candidate 24-bit ICAO hex address, recovered from the CRC field - verify against known traffic before trusting it.

    identActive: boolean | undefined

    True when the transponder's Ident (SPI) pulse is active, from the Flight Status field. Undefined for a reserved Flight Status value.

    kind: "surveillanceAltitudeReply"

    Discriminant tag identifying this as a decoded DF4 surveillance altitude reply.

    squawkAlert: boolean | undefined

    True when the transponder is flagging a recent squawk code change, from the Flight Status field. Undefined for a reserved Flight Status value.

    Settings
    Member Visibility

    On This Page
    Properties