Function parseModeSFrame

Parses the shared envelope (downlink format, CRC remainder) out of a raw Mode-S message. decodeModeSMessage uses this to decide how to route a message before calling the per-type decoders - those decoders take a raw ME field or an already-extracted field value, not the envelope itself.

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

const envelope = parseModeSFrame(rawMessageBytes);
if (envelope.downlinkFormat === 17 && envelope.crcRemainder === 0) {
  // unmodified DF17 extended squitter, safe to decode further
}
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