Function decodeModeSMessage

Decodes a raw Mode-S message into its typed, discriminated result.

A downlink format value of 16 or higher always denotes a 112-bit (14 byte) long message; below 16 always denotes a 56-bit (7 byte) short message - this holds for every downlink format, decoded or not. Any bytes whose length does not match what its own downlink format implies is rejected before further decoding, since a mismatch means the buffer is truncated or corrupted (e.g. a bit-flipped DF field misreading a short reply as a long one) rather than a genuine message of that format.

DF17/18 (extended squitter) messages are only decoded when their CRC is exactly zero - a non-zero CRC on a squitter means the message was corrupted in transit, and this package would rather report "not decodable" than return plausible-looking but wrong data. DF18 messages are further gated on the control field: only CF=0/1/2/5/6 (which share DF17's type-code-coded ME layout) are decoded - see MessageSource, which every decoded DF17/18 message carries so callers can tell a real, direct ICAO address from an anonymous or ground-derived one rather than losing that distinction after the gate. DF24 (Comm-D) is recognized but not decoded - see DecodedModeSMessage. DF19 (military extended squitter) and DF22 (military use) are reserved formats with no publicly documented payload and are not decoded, for the same reason as DF24.

import { decodeModeSMessage } from '@squawk/mode-s';

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'extendedSquitterPosition') {
  console.log(decoded.icaoHex, decoded.latCpr, decoded.lonCpr);
}
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