The Python Decoder for ADS-B (DF17) and Enhance Mode-S (DF20/21)
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The Python Mode-S Decoder (2.0-dev)

Python library for Mode-S message decoding. Support Downlink Formats (DF) are:

Automatic Dependent Surveillance - Broadcast (ADS-B) (DF 17/18)

  • TC=1-4 / BDS 0,8: Aircraft identification and category
  • TC=5-8 / BDS 0,6: Surface position
  • TC=9-18 / BDS 0,5: Airborne position
  • TC=19 / BDS 0,9: Airborne velocity
  • TC=28 / BDS 6,1: Airborne status [to be implemented]
  • TC=29 / BDS 6,2: Target state and status information [to be implemented]
  • TC=31 / BDS 6,5: Aircraft operational status [to be implemented]

Mode-S Comm-B replies (DF 20 / 21)

  • BDS 1,0: Data link capability report
  • BDS 1,7: Common usage GICB capability report
  • BDS 2,0: Aircraft identification
  • BDS 2,1: Aircraft and airline registration markings
  • BDS 3,0: ACAS active resolution advisory
  • 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

DF4 / DF20: Altitude code

DF4 / DF21: Identity code (squawk)

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

New features in v2.0

  • New structure of the libraries
  • ADS-B and Comm-B data streaming
  • Active aircraft viewing (terminal curses)
  • Improved BDS identification
  • Optimizing decoding speed

Source code

Checkout and contribute to this open source project at:

API documentation at: [To be updated]


To install latest development version (dev-2.0) from the GitHub:

pip install git+

Live view traffic (modeslive)

Supports Mode-S Beast and AVR raw stream

modeslive --server [server_address] --port [tcp_port] --rawtype [beast_or_avr] --latlon [lat] [lon]

  -h, --help         show this help message and exit
  --server SERVER    server address or IP
  --port PORT        raw data port
  --rawtype RAWTYPE  beast or avr
  --latlon LAT LON   receiver position
  --show-uncertainty  display uncertaint values, default off

If you have a RTL-SDR receiver or Mode-S Beast, use modesmixer2 ( to create raw beast TCP stream:

$ modesmixer2 --inSeriel port[:speed[:flow_control]] --outServer beast:[tcp_port]

Example screen shot:

Use the library

import pyModeS as pms

Common functions:

pms.df(msg)                 # Downlink Format
pms.icao(msg)               # Infer the ICAO address from the message
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.gray2int(str)     # Convert grey code to interger

Core functions for ADS-B decoding:


# typecode 1-4

# typecode 5-8 (surface), 9-18 (airborne, barometric height), and 9-18 (airborne, GNSS height)
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)


# typecode: 19
pms.adsb.velocity(msg)          # handles both surface & airborne messages
pms.adsb.speed_heading(msg)     # handles both surface & airborne messages

Note: 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.

Decode altitude replies in DF4 / DF20

pms.common.altcode(msg)   # Downlink format must be 4 or 20

Decode identity replies in DF5 / DF21

pms.common.idcode(msg)   # Downlink format must be 5 or 21

Common Mode-S functions

pms.icao(msg)           # Infer the ICAO address from the message
pms.bds.infer(msg)      # Infer the Modes-S BDS code

# check if BDS is 5,0 or 6,0, give reference spd, trk, alt (from ADS-B)
pms.bds.is50or60(msg, spd_ref, trk_ref, alt_ref)

# check each BDS explicitly

Mode-S elementary surveillance (ELS)

pms.commb.ovc10(msg)      # overlay capability, BDS 1,0
pms.commb.cap17(msg)      # GICB capability, BDS 1,7
pms.commb.cs20(msg)       # callsign, BDS 2,0

Mode-S enhanced surveillance (EHS)

# for BDS code 4,0
pms.commb.alt40mcp(msg)   # MCP/FCU selected altitude (ft)
pms.commb.alt40fms(msg)   # FMS selected altitude (ft)
pms.commb.p40baro(msg)    # Barometric pressure (mb)

# for BDS code 5,0
pms.commb.roll50(msg)     # roll angle (deg)
pms.commb.trk50(msg)      # track angle (deg)
pms.commb.gs50(msg)       # ground speed (kt)
pms.commb.rtrk50(msg)     # track angle rate (deg/sec)
pms.commb.tas50(msg)      # true airspeed (kt)

# for BDS code 6,0
pms.commb.hdg60(msg)      # heading (deg)
pms.commb.ias60(msg)      # indicated airspeed (kt)
pms.commb.mach60(msg)     # MACH number
pms.commb.vr60baro(msg)   # barometric altitude rate (ft/min)
pms.commb.vr60ins(msg)    # inertial vertical speed (ft/min)

Meteorological routine air report (MRAR) [Experimental]

# for BDS code 4,4
pms.commb.wind44(msg, rev=False)  # wind speed (kt) and heading (deg)
pms.commb.temp44(msg, rev=False)  # temperature (C)
pms.commb.p44(msg, rev=False)     # pressure (hPa)
pms.commb.hum44(msg, rev=False)   # humidity (%)


To perform unit tests. First install tox through pip, Then, run the following commands:

$ tox