Decodes raw Mode-S/ADS-B messages: downlink format and CRC extraction, CPR position, airborne velocity, aircraft identification, altitude (both the ADS-B position-message field and legacy Gillham-coded surveillance replies), squawk identity, alert/ident flags (from a reply's Flight Status or an airborne position's Surveillance Status), emergency status, ACAS/TCAS Resolution Advisories, target state and status, aircraft operational status, and Enhanced Surveillance Comm-B registers (selected vertical intention, track and turn, heading and speed). Transport-agnostic - it operates on already-framed message bytes and has no opinion about where they came from (a live Beast feed, a logged capture, or any other source). For a Beast binary parser and live TCP client built on top of this package, see @squawk/beast.

Part of the @squawk aviation library suite. See all packages on npm.

Installation

npm install @squawk/mode-s

Usage

Decoding a whole message

decodeModeSMessage is the main entry point - it reads the downlink format, validates the CRC, and routes to the right per-type decoder, returning a single discriminated result.

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

const decoded = decodeModeSMessage(rawMessageBytes); // 7 or 14 raw bytes
if (decoded?.kind === 'extendedSquitterPosition') {
  console.log(decoded.icaoHex, decoded.latCpr, decoded.lonCpr);
  console.log(decoded.baroAltitudeFt, decoded.geoAltitudeFt); // exactly one is populated, or neither for a type-code-0 no-fix / surface message
}

bytes' length must match what its downlink format implies (7 bytes for a short message, 14 for long) or the message is rejected outright - a length mismatch means the buffer is truncated or corrupted, not a genuine message of that format.

DF17/18 (extended squitter) messages are only decoded when their CRC checks out exactly - a corrupted squitter reports as undecodable rather than returning plausible-looking wrong data. DF18 is further gated on its control field: CF=0/1/2/5/6 share DF17's type-code-coded ME layout and decode the same way; CF=3 (TIS-B coarse format) uses a different field layout this package does not decode, and CF=4/7 carry no per-aircraft state. Every decoded DF17/18 message carries messageSource, so a consumer can tell a real, direct ICAO address from an anonymous or ground-derived one rather than losing that distinction after the gate:

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'extendedSquitterPosition' && decoded.messageSource !== 'icaoDirect') {
  // decoded.icaoHex may not be a registered ICAO address (anonymousDirect/anonymousTisb),
  // or the position may be ground-derived rather than heard directly (icaoTisb/adsr)
}

DF0/4/5/20/21 (Mode-S surveillance replies) are targeted responses whose CRC is XORed with the responding aircraft's ICAO address rather than being a plain checksum, so their candidateIcaoHex needs cross-checking against an address already known from squitter traffic before it can be trusted - this package doesn't perform that cross-check itself. DF20/21 ('commBAltitudeReply'/'commBIdentityReply') carry the same altitude/squawk payload as DF4/5 plus a 56-bit MB field, decoded into commBRegisters - see Enhanced Surveillance Comm-B registers. DF4/5/20/21 additionally carry identActive/squawkAlert - see Flight Status (alert/ident) - and airborne position messages carry the same conditions as surveillanceStatus - see Surveillance Status (alert/ident).

Resolving CPR position

Airborne and surface position messages carry raw CPR-encoded lat/lon fields, not a directly usable position - resolving them needs either a paired even/odd frame or a nearby reference position, and pairing state (tracking the most recent even and odd frame per aircraft) is the caller's responsibility, not this package's:

import { decodeAirborneCprPair, decodeAirborneCprWithReference } from '@squawk/mode-s';

// From a paired even + odd frame (global decode, no reference needed)
const position = decodeAirborneCprPair(evenFrame, oddFrame, 'even');

// From a single frame plus a known-nearby reference (e.g. the aircraft's
// last known position, or the receiver's own location for a new aircraft)
const position2 = decodeAirborneCprWithReference('even', frame, referencePosition);

decodeSurfaceCprPair / decodeSurfaceCprWithReference are the equivalents for on-ground traffic (type codes 5-8), which additionally need a reference position to resolve the correct hemisphere and longitude quadrant.

An airborne position message with type code 0 signals that no position fix is currently available - latCpr/lonCpr are undefined in that case even though baroAltitudeFt may still be populated (type code 0 shares its altitude field layout with the barometric type codes 9-18, never GNSS-height).

ACAS / TCAS Resolution Advisories

An active Resolution Advisory reaches decodeModeSMessage two ways: DF16 (a targeted air-air surveillance reply, replacing DF0 while an RA is active) and a DF17/18 type-code-28 subtype-2 message (the same report broadcast over ADS-B so aircraft without interrogation capability can still see it).

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'longAirAirSurveillanceReply' && decoded.resolutionAdvisory?.active) {
  console.log(decoded.resolutionAdvisory.advisoryType); // e.g. 'climb', 'crossingDescend', 'increaseClimb'
}

advisoryType names the RA per RTCA DO-185B Table 2-16 (climb, descend, crossingClimb, crossingDescend, increaseClimb, increaseDescent, reduceClimb, reduceDescent, doNotClimb, doNotDescend, reversalToClimb, reversalToDescend), derived from the report's corrective/sense/rate/crossing/reversal flags - all of which are also exposed individually on AcasResolutionAdvisoryReport, along with the Resolution Advisory Complement flags (doNotPassBelow/doNotPassAbove; doNotTurnLeft/doNotTurnRight are decoded for completeness but TCAS II issues vertical RAs only, so they are expected to always be false). advisoryType is undefined when no RA is currently active, or for the one flag combination DO-185B leaves undefined (positive + preventive).

threat identifies what the RA is responding to, discriminated by threat.threatType:

if (decoded.resolutionAdvisory?.threat.threatType === 'icaoAddress') {
  console.log(decoded.resolutionAdvisory.threat.threatIcaoHex);
}

'none' carries no further fields; 'icaoAddress' carries threatIcaoHex; 'altitudeRangeBearing' carries threatAltitudeFt/threatRangeNm/threatBearingDeg (each independently undefined if that specific field is unavailable).

Target state and status

A type-code-29 ADS-B message reports what the aircraft's flight management system is currently targeting - selected altitude, selected heading, and which autopilot modes are engaged - not the aircraft's actual state:

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'extendedSquitterTargetStateAndStatus') {
  console.log(
    decoded.targetStateAndStatus.selectedAltitudeFt,
    decoded.targetStateAndStatus.selectedHeadingDeg,
  );
}

Each field is independently undefined when its source data isn't available - selectedAltitudeSource further distinguishes an MCP/FCU-selected altitude from an FMS-selected one. autopilotEngaged/vnavModeActive/altitudeHoldModeActive/approachModeActive/lnavModeActive are reported together (all defined or all undefined) since they share a single status bit; tcasOperational has its own independent status.

Aircraft operational status

A type-code-31 ADS-B message reports the transmitting aircraft's ADS-B version, surveillance integrity/accuracy figures, and heading reference - metadata about the quality of the aircraft's other broadcasts rather than its position or state:

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'extendedSquitterOperationalStatus') {
  console.log(
    decoded.operationalStatus.adsbVersion,
    decoded.operationalStatus.navAccuracyCategoryPosition,
  );
}

capabilityClassCode/operationalModeCode are exposed as their raw 16-bit values rather than individually decoded sub-fields. nicBaro is only populated for an airborne report on ADS-B version 1+; silSupplementPerHour is only populated on version 2, where SIL changes from "per sample" to a choice between per-sample and per-hour.

Enhanced Surveillance Comm-B registers

DF20/21's 56-bit MB field can carry any of several "Enhanced Surveillance" Comm-B registers (BDS 4,0 selected vertical intention, BDS 5,0 track and turn report, BDS 6,0 heading and speed report), but unlike a DF17/18 ME field's type code, nothing in the message itself declares which one. This package validates the MB field's bytes against each register's expected structure (status-bit consistency, reserved bits, physically-plausible ranges) and returns every register that plausibly matches - usually exactly one, occasionally none, and occasionally more than one when the bytes are genuinely ambiguous (BDS 5,0 and 6,0 are the pair most likely to overlap):

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'commBAltitudeReply') {
  for (const register of decoded.commBRegisters) {
    if (register.bdsCode === '5,0') {
      console.log(register.rollAngleDeg, register.trueTrackDeg, register.groundSpeedKt);
    }
  }
}

inferCommBRegisters(mb) is also exported directly for callers working with a raw MB field outside of decodeModeSMessage's DF20/21 dispatch.

Flight Status (alert/ident)

DF4/5/20/21 carry a 3-bit Flight Status (FS) field with the transponder's Alert (recent squawk change) and Ident (SPI) flags:

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'surveillanceAltitudeReply') {
  console.log(decoded.identActive, decoded.squawkAlert);
}

Both fields are undefined for the two reserved FS values (6, 7), where alert/ident status is not defined - decodeFlightStatus(fsField) is also exported directly for callers working with a raw FS field outside of decodeModeSMessage's dispatch.

Surveillance Status (alert/ident)

Airborne position messages (type codes 0, 9-18, and 20-22) carry the same conditions in a different shape: a 2-bit Surveillance Status field holding the one condition the transponder is flagging, decoded as surveillanceStatus:

const decoded = decodeModeSMessage(rawMessageBytes);
if (decoded?.kind === 'extendedSquitterPosition' && decoded.surveillanceStatus === 'ident') {
  // the pilot has pressed Ident
}

The values are none, permanentAlert (an emergency squawk: 7500, 7600, or 7700), temporaryAlert (a recent change to any other squawk), and ident. The field holds a single condition and an alert outranks an ident, so an alert says nothing about whether an ident is active at the same time. It is undefined for a surface position, which has no such field. Position messages are broadcast twice a second whether or not the aircraft is being interrogated, so they report these conditions far more often than DF4/5/20/21 replies do. decodeSurveillanceStatus(field) is also exported directly for callers working with a raw ME field.

Mode A/C

Mode A/C predates Mode-S and carries no ICAO address, so it has no natural key in an ICAO-hex-indexed tracking model - it's decoded here, but deliberately not part of decodeModeSMessage's dispatch:

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

const reply = decodeModeAc(twoRawBytes);
console.log(reply.squawk, reply.identActive, reply.altitudeFt);

API

• decodeModeSMessage(bytes) - decodes a raw 7 or 14 byte Mode-S message into a discriminated DecodedModeSMessage.
• decodeModeAc(bytes) - decodes a raw 2-byte Mode A/C reply.
• parseModeSFrame(bytes), extractDownlinkFormat(bytes), computeCrc24(bytes) - lower-level envelope parsing, used internally by decodeModeSMessage.
• decodeAirborneCprPair, decodeAirborneCprWithReference, decodeSurfaceCprPair, decodeSurfaceCprWithReference, cprNumLongitudeZones - CPR position resolution.
• decodeAirborneVelocity(me) - airborne velocity/heading from a type-19 ME field.
• decodeIdentification(me) - callsign and category from a type 1-4 ME field.
• decodeIdentityCode(idField) - 4-digit octal squawk from a 13-bit identity field (shared by DF5/21 and ADS-B emergency status).
• decodeAltitudeCode(altitudeCode), decodeAdsbPositionAltitude(field), decodeAdsbGnssAltitude(field) - altitude decoding.
• decodeSurfaceMovement(field) - ground speed from a surface position message's movement field.
• decodeEmergencyState(rawState) - emergency/priority state from an ADS-B aircraft status message.
• decodeFlightStatus(fsField) - Alert/Ident flags from a DF4/5/20/21 Flight Status field.
• decodeSurveillanceStatus(field) - the alert/ident condition from an airborne position message's Surveillance Status field.
• decodeAcasResolutionAdvisory(payload) - ACAS/TCAS Resolution Advisory report from a DF16 MV field or a type-code-28 subtype-2 ME field.
• decodeTargetStateAndStatus(me) - target state and status from a type-29 ME field.
• decodeAircraftOperationalStatus(me) - operational status from a type-31 ME field.
• inferCommBRegisters(mb) - every Enhanced Surveillance Comm-B register (BDS 4,0/5,0/6,0) a DF20/21 MB field plausibly holds.
• decodeSelectedVerticalIntention(mb), decodeTrackAndTurnReport(mb), decodeHeadingAndSpeedReport(mb) - decode a single Comm-B register directly, given its BDS code is already known.

Every decoder that can fail to produce a result returns undefined rather than throwing - a message that doesn't decode cleanly (unsupported type, failed CRC, reserved value) is an expected outcome, not an exceptional one.