+----------+----------+-------------+------------------------+-----------+
| DF (5) | CA (3) | ICAO (24) | ME (56) | PI (24) |
+----------+----------+-------------+------------------------+-----------+
Bit No. Bits Abbreviation Information
1–5 5 DF Downlink Format
6–8 3 CA Transponder capability
9–32 24 ICAO ICAO aircraft address
33–88 56 ME Message, extended squitter
(33–37) (5) (TC) (Type code)
89–112 24 PI Parity/Interrogator ID
CA Definition
0 Level 1 transponder
1–3 Reserved
4 Level 2+ transponder, with ability to set CA to 7, on-ground
5 Level 2+ transponder, with ability to set CA to 7, airborne
6 Level 2+ transponder, with ability to set CA to 7, either on-ground or airborne
7 Signifies the Downlink Request value is 0, or the Flight Status is 2, 3, 4, or 5, either airborne or on the ground
Type Code Dataframe content
1–4 Aircraft identification
5–8 Surface position
9–18 Airborne position (w/Baro Altitude)
19 Airborne velocities
20–22 Airborne position (w/GNSS Height)
23–27 Reserved
28 Aircraft status
29 Target state and status information
31 Aircraft operation status
Messages TC Ground (still) Ground (moving) Airborne
Aircraft identification 1–4 0.1 Hz 0.2 Hz 0.2 Hz
Surface position 5–8 0.2 Hz 2 Hz -
Airborne position 9–18, 20–22 - - 2 Hz
Airborne velocity 19 - - 2 Hz
Aircraft status 28 0.2 Hz (no TCAS RA and Squawk Code change) / 1.25 Hz (change in TCAS RA or Squawk Code)
Target states and status 29 - - 0.8 Hz
Operational status 31 0.2 Hz 0.4 Hz (no NIC/NAC/SIL change) / 1.25 Hz (change in NIC/NAC/SIL)