The 1090 Megahertz Riddle (second edition)

A Guide to Decoding Mode S and ADS-B Signals
By: Junzi Sun (junzis.com)

Aircraft operation status

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.

Version 0

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.

Aircraft operational status (Version 0)
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 , 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].

Version 1

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.

Aircraft operational status (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.

Version 2

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.

Aircraft operational status (Version 2)
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

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