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Python Mode-S Decoder

Project Description

Python library for Mode-S message decoding. Two separate methods are implemented to decode the following messages:

  • Automatic Dependent Surveillance - Broadcast (ADS-B) (DF17)
    • aircraft information that contains: ICAO address, position, altitude, velocity (ground speed), callsign, etc.
  • Mode-S Elementary Surveillance (ELS) (DF4 and DF5).
    • DF4: Altitude
    • DF5: Squawk code
  • Mode-S Enhanced Surveillance (EHS) (DF20 and DF21). Additional information in response to SSR interrogation, such as: true airspeed, indicated airspeed, mach number, wind, temperature, etc.
    • DF20: Altitude
    • DF21: Squawk code
    • BDS 2,0 Aircraft identification
    • BDS 2,1 Aircraft and airline registration markings
    • BDS 4,0 Selected vertical intention
    • BDS 4,4 Meteorological routine air report
    • BDS 5,0 Track and turn report
    • BDS 5,3 Air-referenced state vector
    • BDS 6,0 Heading and speed report

A detailed manual on Mode-S decoding is published by the author, at:

Source code

Checkout and contribute to this open source project at:

API documentation at:


Checkout source code, or install using pip:

pip install pyModeS


import pyModeS as pms

Common functions:

pms.df(msg)                 # Downlink Format
pms.crc(msg, encode=False)  # Perform CRC or generate parity bit

pms.hex2bin(str)    # Convert hexadecimal string to binary string
pms.bin2int(str)    # Convert binary string to integer
pms.hex2int(str)    # Convert hexadecimal string to integer

pms.bin2gray(str)    # Convert binary string to grey code
pms.gray2bin(str)    # Convert grey code to binary string

Core functions for ADS-B decoding:


pms.adsb.position(msg_even, msg_odd, t_even, t_odd, lat_ref=None, lon_ref=None)
pms.adsb.airborne_position(msg_even, msg_odd, t_even, t_odd)
pms.adsb.surface_position(msg_even, msg_odd, t_even, t_odd, lat_ref, lon_ref)

pms.adsb.position_with_ref(msg, lat_ref, lon_ref)
pms.adsb.airborne_position_with_ref(msg, lat_ref, lon_ref)
pms.adsb.surface_position_with_ref(msg, lat_ref, lon_ref)


pms.adsb.velocity(msg)          # handles both surface & airborne messages
pms.adsb.speed_heading(msg)     # handles both surface & airborne messages

Hint: When you have a fix position of the aircraft, it is convenient to use position_with_ref() method to decode with only one position message (either odd or even). This works with both airborne and surface position messages. But the reference position shall be with in 180NM (airborne) or 45NM (surface) of the true position.

Core functions for ELS decoding:

pms.els.icao(msg)       # ICAO address
pms.els.df4alt(msg)     # Altitude from any DF4 message
pms.ehs.df5id(msg)     # Squawk code from any DF5 message

Core functions for EHS decoding:

pms.ehs.icao(msg)       # ICAO address
pms.ehs.df20alt(msg)    # Altitude from any DF20 message
pms.ehs.df21id(msg)     # Squawk code from any DF21 message

pms.ehs.BDS(msg)        # Comm-B Data Selector Version

# for BDS version 2,0
pms.ehs.isBDS20(msg)    # Check if message is BDS 2,0
pms.ehs.callsign(msg)   # Aircraft callsign

# for BDS version 4,0
pms.ehs.isBDS40(msg)    # Check if message is BDS 4,0
pms.ehs.alt40mcp(msg)   # MCP/FCU selected altitude (ft)
pms.ehs.alt40fms(msg)   # FMS selected altitude (ft)
pms.ehs.p40baro(msg)    # Barometric pressure (mb)

# for BDS version 4,4
pms.ehs.isBDS44(msg, rev=False) # Check if message is BDS 4,4
pms.ehs.wind44(msg, rev=False)  # wind speed (kt) and heading (deg)
pms.ehs.temp44(msg, rev=False)  # temperature (C)
pms.ehs.p44(msg, rev=False)     # pressure (hPa)
pms.ehs.hum44(msg, rev=False)   # humidity (%)

# for BDS version 5,0
pms.ehs.isBDS50(msg)    # Check if message is BDS 5,0
pms.ehs.roll50(msg)     # roll angle (deg)
pms.ehs.trk50(msg)      # track angle (deg)
pms.ehs.gs50(msg)       # ground speed (kt)
pms.ehs.rtrk50(msg)     # track angle rate (deg/sec)
pms.ehs.tas50(msg)      # true airspeed (kt)

# for BDS version 5,3
pms.ehs.isBDS53(msg)    # Check if message is BDS 5,3
pms.ehs.hdg53(msg)      # magnetic heading (deg)
pms.ehs.ias53(msg)      # indicated airspeed (kt)
pms.ehs.mach53(msg)     # MACH number
pms.ehs.tas53(msg)      # true airspeed (kt)
pms.ehs.vr53(msg)       # vertical rate (fpm)

# for BDS version 6,0
pms.ehs.isBDS60(msg)    # Check if message is BDS 6,0
pms.ehs.hdg60(msg)      # heading (deg)
pms.ehs.ias60(msg)      # indicated airspeed (kt)
pms.ehs.mach60(msg)     # MACH number
pms.ehs.vr60baro(msg)   # barometric altitude rate (ft/min)
pms.ehs.vr60ins(msg)    # inertial vertical speed (ft/min)


To run tests, run the following commands: ` $ tox `

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