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    Interface ModeSMessageEnvelope

    A Mode-S message with its downlink format and CRC remainder extracted, prior to any per-type decoding. The envelope every other decoder in this package builds on.

    interface ModeSMessageEnvelope {
        bytes: Uint8Array;
        crcRemainder: number;
        downlinkFormat: number;
    }
    Index
    bytes crcRemainder downlinkFormat
    bytes: Uint8Array

    Raw message bytes - 7 for a short (56-bit) message, 14 for a long (112-bit) message.

    crcRemainder: number

    24-bit CRC remainder computed over the entire message. Interpretation depends on downlinkFormat:

    • DF11/17/18 (squitters): zero indicates an unmodified message. A non-zero value on DF11 (up to 127 - a 1-bit IC selector plus either a 4-bit legacy II code or a 6-bit SI code) identifies the interrogator the ground station used, not a validation failure.
    • DF0/4/5/16/20/21 (surveillance replies): this value IS the transmitting aircraft's 24-bit ICAO address, XORed into the parity field by design. There is no pass/fail check for these formats in isolation - callers confirm validity by matching this value against an ICAO address already known from squitter traffic.
    downlinkFormat: number

    The raw 5-bit downlink format value (0-31) from the top of byte 0. Values 24-31 all denote Comm-D (DF24) - the lower 2 bits of this field are the KE/ND subfields for that format, not distinct downlink formats. This package does not decode Comm-D; callers should treat any value

    = 24 as "DF24, unsupported" rather than as a distinguishable DF25-31.

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