Received ADS-B TC=31 message with non-existent version 3?

Question

On several dates, my ground station received some airborne operational status ADS-B messages (TC=31) where the version number was reported as 3. As far as I know, the only valid versions are 0, 1, and 2.

I don't think it was just a garbled reception because

1. I received more than 500 messages of the same type, all reporting 3 as the version number.
2. The messages came from three USAF aircraft of the same type, KC-10A Extenders.

Here is a sample message, in raw form: 8DAE022EF8000000206143635C24

The dates on which I saw the messages:

• 2018-04-13
• 2019-02-16

The aircraft associated with the messages:

• ICAO AE020E, registration 79-1948
• ICAO AE0212, registration 82-0193
• ICAO AE022E, registration 86-0037

What's the most likely reason behind these messages having this (AFAIK non-existent) version number? A test? A misconfigured piece of equipment (it strikes me as unlikely that the version number can be set to 3, but maybe I'm wrong). Is it a bug in the decoder I'm using?

[Edited 2019-02-16 to include other aircraft I've received the anomalous messages from.]

Answer 1

I've decoded the message with my own decoder. I don't claim it is bug free, but it is designed to follow the 1090ES MOPS (ADS-B Minimum Operational Performance Standards) very closely and gives warnings when messages deviate from the standard. I forced decoding conforming to version 2 of the MOPS, which is the latest standard.

HEX: 8DAE022EF8000000206143635C24
Aircraft Operational status message (airborne format)
    Capability Class:
        TCAS op.          false
        1090ES IN         false
        Reserved [13,14]      0
        ARV rep. cap.     false
        TS report cap.    false
        TC rep. cap. lvl      0
        UAT IN            false
        ADS-R rsvd flag   false
        Reserved              0
    Airborne Operational Mode (OM)
        TCAS RA active    false
        IDENT             false
        Reserved [ATC]    false
        Single Antenna    false
        SDA               0
        Reserved [33-40]  32
     MOPS version     3
     NIC supp. A      false
     NACp             1
     GVA              1
     SIL              0
     SIL supp.        true
     NIC baro         0
     Heading ref.     True North
     Reserved [56]    true

The 1090ES MOPS version is indeed indicated as version 3, which is not yet published. Also, the ME-bits 35 and 56 are set to 1. In version 2 of the 1090ES MOPS these are reserved bits, so they shall be 0.

Version 3 of the 1090ES MOPS is not yet published, so transponders should not use it.

Perhaps these messages are from an experimental transponder, which does not conform to the MOPS.

Answer 2

According to the FAA's Advisory Circular AC20-165B, numbers higher than 2 are allowed:

A.2.34 Version Number.

The applicable TSO Minimum Operational Performance Standard (MOPS) level is communicated through the version number, which is fixed at the time the ADS-B equipment is manufactured. Version 2 applies to ADS-B equipment that meets MOPS documents RTCA/DO-260B with corrigendum 1 or RTCA/DO-282B with corrigendum 1. ADS-B equipment outputting version 2 or higher is required by § 91.227.

Version 3 ADS-B Out avionics meet the requirements defined in DO-260C or DO-282C; although since you are referring to operational status messages my guess is that you are talking about 1090ES messages (i.e. compliant with DO-260C).