The aircraft operational status message is designed to provide various information on an aircraft. It is transmitted with Type Code 31 (TC=31). The structures of this message type differ significantly over different ADS-B versions. The message has been defined in all ADS-B versions. But in practice, it is not implemented in ADS-B version 0. From version 1 onward, the operational status includes more information, such as ADS-B version, accuracy, and integrity indicators.
In version 0, the structure of the message is shown in Table 1.1. Two main blocks contain the status information, which includes operational capabilities (16 bits) and operational status (16 bits). Each block contains four parameters, and each parameter is encoded with 4 bits.
| FIELD | MSG | ME | BITS | |
|---|---|---|---|---|
| Type code = 31 (Binary: 11111) | TC | 33–37 | 1–5 | 5 |
| Sub-type code = 0 (Binary: 000) | ST | 38–40 | 6–8 | 3 |
| Enroute operational capabilities | CC4 | 41–44 | 9–12 | 4 |
| Terminal area operational capabilities | CC3 | 45–48 | 13–16 | 4 |
| Approach/landing operational capabilities | CC2 | 49–52 | 17–21 | 4 |
| Surface operational capabilities | CC1 | 53–56 | 22–24 | 4 |
| Enroute operational status | OM4 | 57–60 | 25–28 | 4 |
| Terminal area operational status | OM3 | 61–64 | 29–32 | 4 |
| Approach/landing operational status | OM2 | 65–68 | 33–36 | 4 |
| Surface operational status | OM1 | 69–72 | 36–39 | 4 |
| Reserved | 73–88 | 41–56 | 16 |
However, even though all the parameter fields are defined, all CC and bits are reserved in the third and fourth blocks according to [ICAO 2008], which eventually made the definitions unusable for version 0 transponders. As version 0 transponders do not transmit any operation status messages, the absence of TC=31 messages can help identify transponder version 0, which is discussed in Chapter [chap:adsb-basic].
From version 1 onward, the operational status report is implemented and broadcast by the transponder. Table 1.2 lists the structure of operational message in version 1.
| FIELD | MSG | ME | BITS | |
|---|---|---|---|---|
| Type code = 31 (Binary: 11111) | TC | 33–37 | 1–5 | 5 |
| Sub-type code | ST | 38–40 | 6–8 | 3 |
| 0: airborne, 1: surface, 2–7: reserved | ||||
| Capacity class codes | CC | 41–56 | 9–24 | 16 |
| ST=0: Airborne capacity class codes (16 bits) | ||||
| ST=1: Surface capacity codes (12 bits) | ||||
| + Length/width code (4 bits) | ||||
| Operational mode codes | OM | 57–72 | 25–40 | 16 |
| ADS-B version number | Ver | 73–75 | 41–43 | 3 |
| 0: Comply with DOC 9871, Appendix A | ||||
| 1: Comply with DOC 9871, Appendix B | ||||
| 2–7: Reserved | ||||
| NIC supplement | NICs | 76 | 44 | 1 |
| Navigational accuracy category - position | NACp | 77–80 | 45–48 | 4 |
| ST=0: Barometric altitude quality | BAQ | 81–82 | 49–50 | 2 |
| ST=1: Reserved | ||||
| Surveillance integrity level | SIL | 83–84 | 51–52 | 2 |
| ST=0: Barometric altitude integrity | 85 | 53 | 1 | |
| ST=1: Track angle or heading | ||||
| Horizontal reference direction | HRD | 86 | 54 | 1 |
| Reserved | 87–88 | 55–56 | 2 |
Bits 73 to 86, which are unused in the previous version, are now defined with new parameters. These parameters include ADS-B version number and various indicators related to the accuracy of the state measurements broadcast by the aircraft in other types of ADS-B messages.
It is worth noting that the definition of sub-types for some fields allow us to differentiate between airborne status messages and surface status messages.
Compared to the previous update, the changes in the operational status report from version 1 to 2 are minimal. Several fields are renamed and a SIL supplement bit is added to bit-87. The structure is shown in Table 1.3.
| FIELD | MSG | ME | BITS | |
|---|---|---|---|---|
| Type code = 31 (Binary: 11111) | TC | 33–37 | 1–5 | 5 |
| Sub-type code | ST | 38–40 | 6–8 | 3 |
| 0: Airborne status message | ||||
| 1: Surface status message | ||||
| Capacity class codes | CC | 41–56 | 9–24 | 16 |
| ST=0: Airborne capacity class codes (16 bits) | ||||
| ST=1: Surface capacity codes (12 bits) | ||||
| + Length/width code (4 bits) | ||||
| Operational mode codes | OM | 57–72 | 25–40 | 16 |
| ST=0: Airborne operational mode codes | ||||
| ST=1: Surface operational mode codes | ||||
| ADS-B version number | Ver | 73–75 | 41–43 | 3 |
| 0: Comply with DOC 9871, Appendix A | ||||
| 1: Comply with DOC 9871, Appendix B | ||||
| 2: Comply with DOC 9871, Appendix C | ||||
| 3–7: Reserved | ||||
| NIC supplement - A | NICa | 76 | 44 | |
| Navigational accuracy category - position | NACp | 77–80 | 45–48 | 4 |
| ST=0: Geometric vertical accuracy | GVA | 81–82 | 49–50 | 2 |
| ST=1: Reserved | ||||
| Source integrity level | SIL | 83–84 | 51–52 | 2 |
| ST=0: Barometric altitude integrity | 85 | 53 | 1 | |
| ST=1: Track angle or heading | ||||
| Horizontal reference direction | HRD | 86 | 54 | 1 |
| SIL supplement | SILs | 87 | 55 | 1 |
| Reserved | 88 | 56 | 1 |