Message types

gr-adsb decodes ten Mode S Downlink Formats (DFs). Each DF maps to a fixed message length and a specific set of decoded fields. This page documents every supported DF and the ADS-B Type Codes (TC) found inside Extended Squitter messages.

Downlink Format reference

The DF is encoded in the first 5 bits of every Mode S message. The table below lists all supported formats.

DF	Name	Length	Key fields
0	Short Air-Air Surveillance (ACAS)	56 bits	VS, RI, Altitude, CC
4	Surveillance Altitude Reply	56 bits	FS, DR, IIS, IDS, Altitude
5	Surveillance Identity Reply	56 bits	FS, DR, IIS, IDS, Identity Code
11	All-Call Reply	56 bits	CA, AA (ICAO address)
16	Long Air-Air Surveillance (ACAS)	112 bits	VS, RI, Altitude, VDS1, VDS2
17	Extended Squitter	112 bits	CA, AA, ME (Message Extended Squitter)
18	Extended Squitter/Non-Transponder	112 bits	CF, AA, ME
19	Military Extended Squitter	112 bits	AF, AA, ME
20	Comm-B Altitude Reply	112 bits	FS, DR, IIS, IDS, Altitude, MB
21	Comm-B Identity Reply	112 bits	FS, DR, IIS, IDS, Identity, MB

Formats not listed above (e.g., DF 2, 3, 6–10, 12–15) are decoded as “Reserved” and are not processed. DF 22–31 are partially named in the lookup table but are not fully decoded.

Short messages (56 bits)

DF 0 — Short Air-Air Surveillance (ACAS)

Transmitted by ACAS-equipped aircraft in response to an air-to-air interrogation (UF 0). Carries basic surveillance data.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `0`
Vertical Status (VS)	1	`0` = In Air, `1` = On Ground
Crosslink Capability (CC)	1	Whether the transponder supports crosslink
Reply Information (RI)	4	ACAS capability and max speed category
Altitude Code	13	Encoded barometric altitude in feet
Address/Parity (AP)	24	CRC XOR’d with the ICAO address

CRC is verified by XOR-ing the AP field with the computed CRC; the result must match a known ICAO address in the plane dictionary.

DF 4 — Surveillance Altitude Reply

Reply to a Mode S altitude interrogation. Provides flight status and barometric altitude.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `4`
Flight Status (FS)	3	Alert, SPI, and on-ground/in-air status
Downlink Request (DR)	5	Whether the aircraft has data to downlink
IIS	4	Interrogator Identifier Subfield
IDS	2	Identifier Designator Subfield
Altitude Code	13	Encoded barometric altitude in feet
Address/Parity (AP)	24	CRC XOR’d with the ICAO address

DF 5 — Surveillance Identity Reply

Reply to a Mode S identity interrogation. Provides flight status and the Mode A squawk code.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `5`
Flight Status (FS)	3	Alert, SPI, and on-ground/in-air status
Downlink Request (DR)	5	Whether the aircraft has data to downlink
IIS	4	Interrogator Identifier Subfield
IDS	2	Identifier Designator Subfield
Identity Code	13	Mode A squawk code (4-digit octal)
Address/Parity (AP)	24	CRC XOR’d with the ICAO address

DF 11 — All-Call Reply

Broadcast reply to an all-call interrogation. This is one of the two message types (along with DF 17) that directly reveals an aircraft’s ICAO 24-bit address in plain text. gr-adsb uses DF 11 to populate the plane dictionary, enabling parity checks on subsequent DF 0/4/5 messages.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `11`
Capability (CA)	3	Transponder level and flight status capability
Address Announced (AA)	24	ICAO 24-bit aircraft address
Parity/Interrogator ID (PI)	24	CRC check field

CRC is a direct comparison: PI must equal the computed CRC over bits 0–31.

Long messages (112 bits)

DF 16 — Long Air-Air Surveillance (ACAS)

Extended version of DF 0. Used by ACAS for coordination between aircraft. The MV (Message, Version) field carries ACAS coordination data.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `16`
Vertical Status (VS)	1	`0` = In Air, `1` = On Ground
Reply Information (RI)	4	ACAS capability and max speed
Altitude Code	13	Encoded barometric altitude in feet
VDS1	4	First nibble of the Version/Data Subfield
VDS2	4	Second nibble of the Version/Data Subfield
Address/Parity (AP)	24	CRC XOR’d with the ICAO address

Full MV field decoding is noted as not yet implemented in the source.

DF 17 — Extended Squitter

The primary ADS-B message type. Broadcast spontaneously by Mode S transponders with ADS-B capability. Contains the ICAO address and a 56-bit ME field whose content is identified by the Type Code (TC).Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `17`
Capability (CA)	3	Transponder level and flight status capability
Address Announced (AA)	24	ICAO 24-bit aircraft address
Message Extended Squitter (ME)	56	ADS-B payload; first 5 bits are the Type Code
Parity/Interrogator ID (PI)	24	CRC check field

CRC is a direct comparison: PI must equal the computed CRC over bits 0–87. See Type Codes below for ME field details.

DF 18 — Extended Squitter/Non-Transponder

ADS-B messages from non-Mode-S sources, including TIS-B ground stations and ADS-R rebroadcasts. The 3-bit CF (Control Field) determines the source and address type.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `18`
CF	3	Control Field — identifies message source
Address Announced (AA)	24	ICAO address (CF=0) or anonymous address (CF=1)
Message Extended Squitter (ME)	56	Payload; decoded like DF 17 when CF is 0, 1, or 6
Parity/Interrogator ID (PI)	24	CRC check field

CF values:

CF	Meaning
0	AA field is the ICAO address
1	AA field is an anonymous address
2	Fine TIS-B message using ICAO address
3	Coarse TIS-B airborne position/velocity message
4	TIS-B and ADS-R management message
5	Fine TIS-B using non-ICAO address
6	ADS-B rebroadcast
7	Reserved

DF 19 — Military Extended Squitter

Extended Squitter used by military aircraft. The 3-bit AF (Application Field) identifies the message type. Only AF=0 (ADS-B Message) is decoded; all other AF values are reserved for military use.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `19`
AF	3	Application Field — `0` = ADS-B message
Address Announced (AA)	24	ICAO 24-bit aircraft address
Message Extended Squitter (ME)	56	ADS-B payload decoded like DF 17 when AF=0
Parity/Interrogator ID (PI)	24	CRC check field

DF 20 — Comm-B Altitude Reply

Extended version of DF 4. Carries altitude plus a 56-bit Comm-B message (MB field) used for Mode S data link services.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `20`
Flight Status (FS)	3	Alert, SPI, and on-ground/in-air status
Downlink Request (DR)	5	Downlink request indicator
IIS	4	Interrogator Identifier Subfield
IDS	2	Identifier Designator Subfield
Altitude Code	13	Encoded barometric altitude in feet
Message Comm-B (MB)	56	Comm-B data link payload
Address/Parity (AP)	24	CRC XOR’d with the ICAO address

MB field decoding is noted as not yet implemented in the source.

DF 21 — Comm-B Identity Reply

Extended version of DF 5. Carries the Mode A squawk code plus a 56-bit Comm-B message.Decoded fields:

Field	Bits	Description
Downlink Format (DF)	5	Always `21`
Flight Status (FS)	3	Alert, SPI, and on-ground/in-air status
Downlink Request (DR)	5	Downlink request indicator
IIS	4	Interrogator Identifier Subfield
IDS	2	Identifier Designator Subfield
Identity Code	13	Mode A squawk code
Message Comm-B (MB)	56	Comm-B data link payload
Address/Parity (AP)	24	CRC XOR’d with the ICAO address

MB field decoding is noted as not yet implemented in the source.

Type Codes

For DF 17, 18 (CF=0/1/6), and 19 (AF=0), the first 5 bits of the ME field carry the Type Code (TC), which identifies the ADS-B information that follows.

TC 1–4 — Aircraft Identification

Broadcasts the aircraft’s callsign (flight number or registration). The callsign is 8 characters, each encoded using 6 bits and mapped through a 64-character lookup table (CALLSIGN_CHAR_LUT).

TC	Category set
1	Category Set D
2	Category Set C
3	Category Set B
4	Category Set A

The callsign is extracted from bits 40–87 of the full 112-bit message (8 characters × 6 bits = 48 bits):

CALLSIGN_CHAR_LUT = "_ABCDEFGHIJKLMNOPQRSTUVWXYZ_____ _______________0123456789______"

callsign = ""
for ii in range(0, 8):
    callsign += CALLSIGN_CHAR_LUT[self.bin2dec(self.bits[40 + ii*6 : 40 + (ii+1)*6])]
callsign = callsign.replace("_", "")

TC 5–8 — Surface Position

Ground position and movement data for taxiing aircraft. Decoding is not yet implemented in gr-adsb. Messages are forwarded on the unknown output port.

TC	Description
5–8	Surface Position

TC 9–18 — Airborne Position (Barometric Altitude)

The most common position message type. Carries barometric altitude and CPR-encoded latitude/longitude.Decoded fields (bits relative to the 112-bit message):

Field	Bits	Description
Surveillance Status (SS)	bits 37–38	Emergency, alert, or SPI condition
NIC Supplement-B	bit 39	Navigation Integrity Category supplement
Altitude Code (AC-12)	bits 40–51	Barometric altitude (12-bit encoding)
Time (T)	bit 52	Whether timestamp is synced to 0.2 s UTC epoch
CPR Frame Flag	bit 53	`0` = even frame, `1` = odd frame
CPR Latitude	bits 54–70	17-bit CPR-encoded latitude
CPR Longitude	bits 71–87	17-bit CPR-encoded longitude

Both even and odd frames must be received within CPR_TIMEOUT_S = 30 seconds for a position fix to be computed.

TC	Description
9–18	Airborne Position (Baro Altitude)

TC 19 — Airborne Velocity

Speed, heading, and vertical rate. Two subtypes are defined:

ST 1/2 (Ground velocity): East-west and north-south velocity components are decoded separately and combined to produce speed (knots) and heading (degrees).
ST 3/4 (Air velocity): Airspeed and heading relative to the air mass.

Ground velocity fields (ST 1/2):

Field	Bits	Description
Intent Change (IC)	bit 40	Whether the aircraft intent has changed
NAC	bits 42–44	Navigation Accuracy Category
S-EW	bit 45	East-west velocity sign
V-EW	bits 46–55	East-west velocity magnitude (knots + 1)
S-NS	bit 56	North-south velocity sign
V-NS	bits 57–66	North-south velocity magnitude (knots + 1)
VR Source	bit 67	`0` = GNSS/INS, `1` = barometric
S-VR	bit 68	Vertical rate sign
VR	bits 69–77	Vertical rate magnitude (64 ft/min per LSB)

Speed and heading are computed as:

velocity_we = (v_ew - 1) * (-1 if s_ew == 1 else 1)
velocity_sn = (v_ns - 1) * (-1 if s_ns == 1 else 1)
speed   = np.sqrt(velocity_sn**2 + velocity_we**2)
heading = np.arctan2(velocity_sn, velocity_we) * 360.0 / (2.0 * np.pi)
vertical_rate = (vr - 1) * 64 * (-1 if s_vr == 1 else 1)

TC 20–22 — Airborne Position (GNSS Height)

Like TC 9–18 but uses GNSS geometric height instead of barometric altitude. Decoding is not yet implemented in gr-adsb. Messages are forwarded on the unknown output port.

TC	Description
20–22	Airborne Position (GNSS Height)

TC 28 — Extended Squitter A/C Status

Reports emergency and priority status. Decoding is not yet implemented in gr-adsb.

TC 29 — Target State and Status

Reports autopilot target altitude and heading (Version 2 format). Decoding is not yet implemented in gr-adsb.

TC 31 — Aircraft Operational Status

Reports operational capabilities and version information. Decoding is not yet implemented in gr-adsb.

Unimplemented Type Codes are published on the unknown message port of the Decoder block. You can connect a PDU sink or custom block to that port to inspect or handle them independently.

Get Started

Guides

Concepts

Downlink Format reference

Short messages (56 bits)

Long messages (112 bits)

Type Codes

Build docs developers (and LLMs) love

Get Started

Guides

Concepts

​Downlink Format reference

​Short messages (56 bits)

​Long messages (112 bits)

​Type Codes

Build docs developers (and LLMs) love

Downlink Format reference

Short messages (56 bits)

Long messages (112 bits)

Type Codes